1.文章介绍
在这么多Android应用开发的时候，Handler一遍又一遍的出现在自己写的代码中，Handler的使用也让程序代码的流程清晰易懂。但是作为一名CODER，我不能仅仅满足熟练的使用Handler来完成消息异步事件的处理。一直以来，我都想写一篇文章来说一说Handler机制的实现，恰好最近总是接触AHandler，所以索性把两种实现放在一起来分析，有对比的同时还能加深不同层次的理解。
这是之前我想写又没有写本篇文章的原因，大概也有人跟我是同样的感觉吧。
问题1，从哪开始讲？
问题2，有什么收获？
问题3，怎么讲？
2.干货
本篇文章在Android SDK源码基础上，会分别从JAVA源码和C++源码来分析Handler和AHandler。
首先看看Android Java层的Handler使用，为了方便测试写了一个很简单的DEMO：
package com.example.apitestdemo;
import io.vov.vitamio.utils.Log;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
public class HandlerTest extends Activity {
public static final String TAG = HandlerTest.class.getSimpleName();
public static final int MSG_BASE = 0;
Handler mainHandler = new Handler() {
@Override
public void handleMessage(android.os.Message msg) {
switch (msg.what) {
case MSG_BASE:
Log.d(TAG, "get message , now we can do something");
break;
default:
break;
}
};
};
@Override
protected void onCreate(Bundle arg0) {
// TODO Auto-generated method stub
super.onCreate(arg0);
setContentView(R.layout.activity_main);
Message msg = mainHandler.obtainMessage();
msg.what = MSG_BASE;
mainHandler.sendMessageDelayed(msg, 5000);
Log.d(TAG, "for test , we print here");
}
}
这是Android上面很简单的使用Handler来实现消息分发的DEMO，为了体现Handler的异步机制，特意添加了两个打印，输出如下：
06-19 11:13:48.857 D/HandlerTest ( 2001): for test , we print here
06-19 11:13:53.857 D/HandlerTest ( 2001): get message , now we can do something
在应用层使用Handler就是这么简单(在这个DEMO中Handler绑定了当前Activity的线程，由这个线程维护消息队列)。
来看看Handler的完整源码：
/*
* Copyright (C) 2006 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.os;
import android.util.Log;
import android.util.Printer;
import java.lang.reflect.Modifier;
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread's {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread's message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it -- from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
* <p>Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The <em>post</em> versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the <em>sendMessage</em> versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler's {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
* <p>When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
* <p>When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same <em>post</em> or <em>sendMessage</em> methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler's message queue and processed when appropriate.
*/
public class Handler {
/*
* Set this flag to true to detect anonymous, local or member classes
* that extend this Handler class and that are not static. These kind
* of classes can potentially create leaks.
*/
private static final boolean FIND_POTENTIAL_LEAKS = false;
private static final String TAG = "Handler";
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
/**
* Use the {@link Looper} for the current thread
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(boolean async) {
this(null, async);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages. Also set whether the handler
* should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
/**
* Returns a string representing the name of the specified message.
* The default implementation will either return the class name of the
* message callback if any, or the hexadecimal representation of the
* message "what" field.
*
* @param message The message whose name is being queried
*/
public String getMessageName(Message message) {
if (message.callback != null) {
return message.callback.getClass().getName();
}
return "0x" + Integer.toHexString(message.what);
}
/**
* Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
* creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
* If you don't want that facility, just call Message.obtain() instead.
*/
public final Message obtainMessage()
{
return Message.obtain(this);
}
/**
* Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what)
{
return Message.obtain(this, what);
}
/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what and obj members
* of the returned Message.
*
* @param what Value to assign to the returned Message.what field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, Object obj)
{
return Message.obtain(this, what, obj);
}
/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what, arg1 and arg2 members of the returned
* Message.
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, int arg1, int arg2)
{
return Message.obtain(this, what, arg1, arg2);
}
/**
*
* Same as {@link #obtainMessage()}, except that it also sets the what, obj, arg1,and arg2 values on the
* returned Message.
* @param what Value to assign to the returned Message.what field.
* @param arg1 Value to assign to the returned Message.arg1 field.
* @param arg2 Value to assign to the returned Message.arg2 field.
* @param obj Value to assign to the returned Message.obj field.
* @return A Message from the global message pool.
*/
public final Message obtainMessage(int what, int arg1, int arg2, Object obj)
{
return Message.obtain(this, what, arg1, arg2, obj);
}
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
/**
* Causes the Runnable r to be added to the message queue, to be run
* at a specific time given by <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* The runnable will be run on the thread to which this handler is attached.
*
* @param r The Runnable that will be executed.
* @param uptimeMillis The absolute time at which the callback should run,
* using the {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the Runnable will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean postAtTime(Runnable r, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}
/**
* Causes the Runnable r to be added to the message queue, to be run
* at a specific time given by <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* The runnable will be run on the thread to which this handler is attached.
*
* @param r The Runnable that will be executed.
* @param uptimeMillis The absolute time at which the callback should run,
* using the {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the Runnable will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*
* @see android.os.SystemClock#uptimeMillis
*/
public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}
/**
* Causes the Runnable r to be added to the message queue, to be run
* after the specified amount of time elapses.
* The runnable will be run on the thread to which this handler
* is attached.
*
* @param r The Runnable that will be executed.
* @param delayMillis The delay (in milliseconds) until the Runnable
* will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the Runnable will be processed --
* if the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
/**
* Posts a message to an object that implements Runnable.
* Causes the Runnable r to executed on the next iteration through the
* message queue. The runnable will be run on the thread to which this
* handler is attached.
* <b>This method is only for use in very special circumstances -- it
* can easily starve the message queue, cause ordering problems, or have
* other unexpected side-effects.</b>
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
}
/**
* Runs the specified task synchronously.
* <p>
* If the current thread is the same as the handler thread, then the runnable
* runs immediately without being enqueued. Otherwise, posts the runnable
* to the handler and waits for it to complete before returning.
* </p><p>
* This method is dangerous! Improper use can result in deadlocks.
* Never call this method while any locks are held or use it in a
* possibly re-entrant manner.
* </p><p>
* This method is occasionally useful in situations where a background thread
* must synchronously await completion of a task that must run on the
* handler's thread. However, this problem is often a symptom of bad design.
* Consider improving the design (if possible) before resorting to this method.
* </p><p>
* One example of where you might want to use this method is when you just
* set up a Handler thread and need to perform some initialization steps on
* it before continuing execution.
* </p><p>
* If timeout occurs then this method returns <code>false</code> but the runnable
* will remain posted on the handler and may already be in progress or
* complete at a later time.
* </p><p>
* When using this method, be sure to use {@link Looper#quitSafely} when
* quitting the looper. Otherwise {@link #runWithScissors} may hang indefinitely.
* (TODO: We should fix this by making MessageQueue aware of blocking runnables.)
* </p>
*
* @param r The Runnable that will be executed synchronously.
* @param timeout The timeout in milliseconds, or 0 to wait indefinitely.
*
* @return Returns true if the Runnable was successfully executed.
* Returns false on failure, usually because the
* looper processing the message queue is exiting.
*
* @hide This method is prone to abuse and should probably not be in the API.
* If we ever do make it part of the API, we might want to rename it to something
* less funny like runUnsafe().
*/
public final boolean runWithScissors(final Runnable r, long timeout) {
if (r == null) {
throw new IllegalArgumentException("runnable must not be null");
}
if (timeout < 0) {
throw new IllegalArgumentException("timeout must be non-negative");
}
if (Looper.myLooper() == mLooper) {
r.run();
return true;
}
BlockingRunnable br = new BlockingRunnable(r);
return br.postAndWait(this, timeout);
}
/**
* Remove any pending posts of Runnable r that are in the message queue.
*/
public final void removeCallbacks(Runnable r)
{
mQueue.removeMessages(this, r, null);
}
/**
* Remove any pending posts of Runnable <var>r</var> with Object
* <var>token</var> that are in the message queue. If <var>token</var> is null,
* all callbacks will be removed.
*/
public final void removeCallbacks(Runnable r, Object token)
{
mQueue.removeMessages(this, r, token);
}
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
/**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
/**
* Sends a Message containing only the what value, to be delivered
* at a specific time.
* @see #sendMessageAtTime(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
/**
* Enqueue a message at the front of the message queue, to be processed on
* the next iteration of the message loop. You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
* <b>This method is only for use in very special circumstances -- it
* can easily starve the message queue, cause ordering problems, or have
* other unexpected side-effects.</b>
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessageAtFrontOfQueue(Message msg) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, 0);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
/**
* Remove any pending posts of messages with code 'what' that are in the
* message queue.
*/
public final void removeMessages(int what) {
mQueue.removeMessages(this, what, null);
}
/**
* Remove any pending posts of messages with code 'what' and whose obj is
* 'object' that are in the message queue. If <var>object</var> is null,
* all messages will be removed.
*/
public final void removeMessages(int what, Object object) {
mQueue.removeMessages(this, what, object);
}
/**
* Remove any pending posts of callbacks and sent messages whose
* <var>obj</var> is <var>token</var>. If <var>token</var> is null,
* all callbacks and messages will be removed.
*/
public final void removeCallbacksAndMessages(Object token) {
mQueue.removeCallbacksAndMessages(this, token);
}
/**
* Check if there are any pending posts of messages with code 'what' in
* the message queue.
*/
public final boolean hasMessages(int what) {
return mQueue.hasMessages(this, what, null);
}
/**
* Check if there are any pending posts of messages with code 'what' and
* whose obj is 'object' in the message queue.
*/
public final boolean hasMessages(int what, Object object) {
return mQueue.hasMessages(this, what, object);
}
/**
* Check if there are any pending posts of messages with callback r in
* the message queue.
*
* @hide
*/
public final boolean hasCallbacks(Runnable r) {
return mQueue.hasMessages(this, r, null);
}
// if we can get rid of this method, the handler need not remember its loop
// we could instead export a getMessageQueue() method...
public final Looper getLooper() {
return mLooper;
}
public final void dump(Printer pw, String prefix) {
pw.println(prefix + this + " @ " + SystemClock.uptimeMillis());
if (mLooper == null) {
pw.println(prefix + "looper uninitialized");
} else {
mLooper.dump(pw, prefix + " ");
}
}
@Override
public String toString() {
return "Handler (" + getClass().getName() + ") {"
+ Integer.toHexString(System.identityHashCode(this))
+ "}";
}
final IMessenger getIMessenger() {
synchronized (mQueue) {
if (mMessenger != null) {
return mMessenger;
}
mMessenger = new MessengerImpl();
return mMessenger;
}
}
private final class MessengerImpl extends IMessenger.Stub {
public void send(Message msg) {
Handler.this.sendMessage(msg);
}
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r;
return m;
}
private static void handleCallback(Message message) {
message.callback.run();
}
final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;
private static final class BlockingRunnable implements Runnable {
private final Runnable mTask;
private boolean mDone;
public BlockingRunnable(Runnable task) {
mTask = task;
}
@Override
public void run() {
try {
mTask.run();
} finally {
synchronized (this) {
mDone = true;
notifyAll();
}
}
}
public boolean postAndWait(Handler handler, long timeout) {
if (!handler.post(this)) {
return false;
}
synchronized (this) {
if (timeout > 0) {
final long expirationTime = SystemClock.uptimeMillis() + timeout;
while (!mDone) {
long delay = expirationTime - SystemClock.uptimeMillis();
if (delay <= 0) {
return false; // timeout
}
try {
wait(delay);
} catch (InterruptedException ex) {
}
}
} else {
while (!mDone) {
try {
wait();
} catch (InterruptedException ex) {
}
}
}
}
return true;
}
}
}
按文章开篇的DEMO来分析，首先创建了一个Handler对象：
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //注意 Handler中的mQueue其实就是Looper中的mQueue，清楚这点对后面的分析有帮助
mCallback = callback;
mAsynchronous = async;
}
这里创建了一个Looper，先看看Looper的完整源码：
/*
* Copyright (C) 2006 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.os;
import android.util.Log;
import android.util.Printer;
import android.util.PrefixPrinter;
/**
* Class used to run a message loop for a thread. Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
*
* <p>Most interaction with a message loop is through the
* {@link Handler} class.
*
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <pre>
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
* }</pre>
*/
public final class Looper {
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper; // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;
private Printer mLogging;
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
/** Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycle();
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
/**
* Control logging of messages as they are processed by this Looper. If
* enabled, a log message will be written to <var>printer</var>
* at the beginning and ending of each message dispatch, identifying the
* target Handler and message contents.
*
* @param printer A Printer object that will receive log messages, or
* null to disable message logging.
*/
public void setMessageLogging(Printer printer) {
mLogging = printer;
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static MessageQueue myQueue() {
return myLooper().mQueue;
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
/**
* Returns true if the current thread is this looper's thread.
* @hide
*/
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
/**
* Quits the looper.
* <p>
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
/**
* Quits the looper safely.
* <p>
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p>
*/
public void quitSafely() {
mQueue.quit(true);
}
/**
* Posts a synchronization barrier to the Looper's message queue.
*
* Message processing occurs as usual until the message queue encounters the
* synchronization barrier that has been posted. When the barrier is encountered,
* later synchronous messages in the queue are stalled (prevented from being executed)
* until the barrier is released by calling {@link #removeSyncBarrier} and specifying
* the token that identifies the synchronization barrier.
*
* This method is used to immediately postpone execution of all subsequently posted
* synchronous messages until a condition is met that releases the barrier.
* Asynchronous messages (see {@link Message#isAsynchronous} are exempt from the barrier
* and continue to be processed as usual.
*
* This call must be always matched by a call to {@link #removeSyncBarrier} with
* the same token to ensure that the message queue resumes normal operation.
* Otherwise the application will probably hang!
*
* @return A token that uniquely identifies the barrier. This token must be
* passed to {@link #removeSyncBarrier} to release the barrier.
*
* @hide
*/
public int postSyncBarrier() {
return mQueue.enqueueSyncBarrier(SystemClock.uptimeMillis());
}
/**
* Removes a synchronization barrier.
*
* @param token The synchronization barrier token that was returned by
* {@link #postSyncBarrier}.
*
* @throws IllegalStateException if the barrier was not found.
*
* @hide
*/
public void removeSyncBarrier(int token) {
mQueue.removeSyncBarrier(token);
}
/**
* Return the Thread associated with this Looper.
*/
public Thread getThread() {
return mThread;
}
/** @hide */
public MessageQueue getQueue() {
return mQueue;
}
/**
* Return whether this looper's thread is currently idle, waiting for new work
* to do. This is intrinsically racy, since its state can change before you get
* the result back.
* @hide
*/
public boolean isIdling() {
return mQueue.isIdling();
}
public void dump(Printer pw, String prefix) {
pw.println(prefix + toString());
mQueue.dump(pw, prefix + " ");
}
public String toString() {
return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
+ ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
}
}
那么在创建Handler时Looper.myLooper()是什么：
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
从上面的源码看出Looper只有prepare后sThreadLocal才会有有效的Looper对象：
这里结合开篇的DEMO了解下Looper是如何产生的：
/**ActivityManagerService.java*/
private final void startProcessLocked(ProcessRecord app,
String hostingType, String hostingNameStr) {
if (app.pid > 0 && app.pid != MY_PID) {
synchronized (mPidsSelfLocked) {
mPidsSelfLocked.remove(app.pid);
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
}
app.setPid(0);
}
if (DEBUG_PROCESSES && mProcessesOnHold.contains(app)) Slog.v(TAG,
"startProcessLocked removing on hold: " + app);
mProcessesOnHold.remove(app);
updateCpuStats();
try {
int uid = app.uid;
int[] gids = null;
int mountExternal = Zygote.MOUNT_EXTERNAL_NONE;
if (!app.isolated) {
int[] permGids = null;
try {
final PackageManager pm = mContext.getPackageManager();
permGids = pm.getPackageGids(app.info.packageName);
if (Environment.isExternalStorageEmulated()) {
if (pm.checkPermission(
android.Manifest.permission.ACCESS_ALL_EXTERNAL_STORAGE,
app.info.packageName) == PERMISSION_GRANTED) {
mountExternal = Zygote.MOUNT_EXTERNAL_MULTIUSER_ALL;
} else {
mountExternal = Zygote.MOUNT_EXTERNAL_MULTIUSER;
}
}
} catch (PackageManager.NameNotFoundException e) {
Slog.w(TAG, "Unable to retrieve gids", e);
}
/*
* Add shared application GID so applications can share some
* resources like shared libraries
*/
if (permGids == null) {
gids = new int[1];
} else {
gids = new int[permGids.length + 1];
System.arraycopy(permGids, 0, gids, 1, permGids.length);
}
gids[0] = UserHandle.getSharedAppGid(UserHandle.getAppId(uid));
}
if (mFactoryTest != SystemServer.FACTORY_TEST_OFF) {
if (mFactoryTest == SystemServer.FACTORY_TEST_LOW_LEVEL
&& mTopComponent != null
&& app.processName.equals(mTopComponent.getPackageName())) {
uid = 0;
}
if (mFactoryTest == SystemServer.FACTORY_TEST_HIGH_LEVEL
&& (app.info.flags&ApplicationInfo.FLAG_FACTORY_TEST) != 0) {
uid = 0;
}
}
int debugFlags = 0;
if ((app.info.flags & ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
debugFlags |= Zygote.DEBUG_ENABLE_DEBUGGER;
// Also turn on CheckJNI for debuggable apps. It's quite
// awkward to turn on otherwise.
debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
}
// Run the app in safe mode if its manifest requests so or the
// system is booted in safe mode.
if ((app.info.flags & ApplicationInfo.FLAG_VM_SAFE_MODE) != 0 ||
Zygote.systemInSafeMode == true) {
debugFlags |= Zygote.DEBUG_ENABLE_SAFEMODE;
}
if ("1".equals(SystemProperties.get("debug.checkjni"))) {
debugFlags |= Zygote.DEBUG_ENABLE_CHECKJNI;
}
if ("1".equals(SystemProperties.get("debug.jni.logging"))) {
debugFlags |= Zygote.DEBUG_ENABLE_JNI_LOGGING;
}
if ("1".equals(SystemProperties.get("debug.assert"))) {
debugFlags |= Zygote.DEBUG_ENABLE_ASSERT;
}
// Start the process. It will either succeed and return a result containing
// the PID of the new process, or else throw a RuntimeException.
Process.ProcessStartResult startResult = Process.start("android.app.ActivityThread",
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, null);
BatteryStatsImpl bs = mBatteryStatsService.getActiveStatistics();
synchronized (bs) {
if (bs.isOnBattery()) {
bs.getProcessStatsLocked(app.uid, app.processName).incStartsLocked();
}
}
EventLog.writeEvent(EventLogTags.AM_PROC_START,
UserHandle.getUserId(uid), startResult.pid, uid,
app.processName, hostingType,
hostingNameStr != null ? hostingNameStr : "");
if (app.persistent) {
Watchdog.getInstance().processStarted(app.processName, startResult.pid);
}
StringBuilder buf = mStringBuilder;
buf.setLength(0);
buf.append("Start proc ");
buf.append(app.processName);
buf.append(" for ");
buf.append(hostingType);
if (hostingNameStr != null) {
buf.append(" ");
buf.append(hostingNameStr);
}
buf.append(": pid=");
buf.append(startResult.pid);
buf.append(" uid=");
buf.append(uid);
buf.append(" gids={");
if (gids != null) {
for (int gi=0; gi<gids.length; gi++) {
if (gi != 0) buf.append(", ");
buf.append(gids[gi]);
}
}
buf.append("}");
Slog.i(TAG, buf.toString());
app.setPid(startResult.pid);
app.usingWrapper = startResult.usingWrapper;
app.removed = false;
synchronized (mPidsSelfLocked) {
this.mPidsSelfLocked.put(startResult.pid, app);
Message msg = mHandler.obtainMessage(PROC_START_TIMEOUT_MSG);
msg.obj = app;
mHandler.sendMessageDelayed(msg, startResult.usingWrapper
? PROC_START_TIMEOUT_WITH_WRAPPER : PROC_START_TIMEOUT);
}
} catch (RuntimeException e) {
// XXX do better error recovery.
app.setPid(0);
Slog.e(TAG, "Failure starting process " + app.processName, e);
}
}
在启动一个APK时，ActivityManagerService调用startProcessLocked方法(调用流程读者自己可以百度下)，在startProcessLocked中：
Process.ProcessStartResult startResult = Process.start("android.app.ActivityThread",
app.processName, uid, uid, gids, debugFlags, mountExternal,
app.info.targetSdkVersion, app.info.seinfo, null);
通过反射调用了ActivityThread的main入口:
public static void main(String[] args) {
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Security.addProvider(new AndroidKeyStoreProvider());
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
那么启动一个APK，就会默认创建一个依附于主进程的Looper(prepareMainLooper)，最终会开始loop轮询MessageQueue队列中的消息：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycle();
}
}
所以DEMO中Handler的Looper就是主进程的MainLooper，发送消息时的准备：
Message msg = mainHandler.obtainMessage();
msg.what = MSG_BASE;
mainHandler.sendMessageDelayed(msg, 5000);
来看看obtainMessage:
/**Handler.java*/
public final Message obtainMessage()
{
return Message.obtain(this);
}
Message的作用是数据Parcelable流化，以下是完整源码：
/*
* Copyright (C) 2006 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.os;
import android.util.TimeUtils;
/**
*
* Defines a message containing a description and arbitrary data object that can be
* sent to a {@link Handler}. This object contains two extra int fields and an
* extra object field that allow you to not do allocations in many cases.
*
* <p class="note">While the constructor of Message is public, the best way to get
* one of these is to call {@link #obtain Message.obtain()} or one of the
* {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
* them from a pool of recycled objects.</p>
*/
public final class Message implements Parcelable {
/**
* User-defined message code so that the recipient can identify
* what this message is about. Each {@link Handler} has its own name-space
* for message codes, so you do not need to worry about yours conflicting
* with other handlers.
*/
public int what;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg1;
/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg2;
/**
* An arbitrary object to send to the recipient. When using
* {@link Messenger} to send the message across processes this can only
* be non-null if it contains a Parcelable of a framework class (not one
* implemented by the application). For other data transfer use
* {@link #setData}.
*
* <p>Note that Parcelable objects here are not supported prior to
* the {@link android.os.Build.VERSION_CODES#FROYO} release.
*/
public Object obj;
/**
* Optional Messenger where replies to this message can be sent. The
* semantics of exactly how this is used are up to the sender and
* receiver.
*/
public Messenger replyTo;
/** If set message is in use */
/*package*/ static final int FLAG_IN_USE = 1 << 0;
/** If set message is asynchronous */
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
/** Flags to clear in the copyFrom method */
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message();
}
/**
* Same as {@link #obtain()}, but copies the values of an existing
* message (including its target) into the new one.
* @param orig Original message to copy.
* @return A Message object from the global pool.
*/
public static Message obtain(Message orig) {
Message m = obtain();
m.what = orig.what;
m.arg1 = orig.arg1;
m.arg2 = orig.arg2;
m.obj = orig.obj;
m.replyTo = orig.replyTo;
if (orig.data != null) {
m.data = new Bundle(orig.data);
}
m.target = orig.target;
m.callback = orig.callback;
return m;
}
/**
* Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
/**
* Same as {@link #obtain(Handler)}, but assigns a callback Runnable on
* the Message that is returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @param callback Runnable that will execute when the message is handled.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
/**
* Same as {@link #obtain()}, but sets the values for both <em>target</em> and
* <em>what</em> members on the Message.
* @param h Value to assign to the <em>target</em> member.
* @param what Value to assign to the <em>what</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what) {
Message m = obtain();
m.target = h;
m.what = what;
return m;
}
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, and <em>obj</em>
* members.
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param obj The <em>object</em> method to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.obj = obj;
return m;
}
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, and <em>arg2</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, int arg1, int arg2) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
return m;
}
/**
* Same as {@link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @param obj The <em>obj</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what,
int arg1, int arg2, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
m.obj = obj;
return m;
}
/**
* Return a Message instance to the global pool. You MUST NOT touch
* the Message after calling this function -- it has effectively been
* freed.
*/
public void recycle() {
clearForRecycle();
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
/**
* Make this message like o. Performs a shallow copy of the data field.
* Does not copy the linked list fields, nor the timestamp or
* target/callback of the original message.
*/
public void copyFrom(Message o) {
this.flags = o.flags & ~FLAGS_TO_CLEAR_ON_COPY_FROM;
this.what = o.what;
this.arg1 = o.arg1;
this.arg2 = o.arg2;
this.obj = o.obj;
this.replyTo = o.replyTo;
if (o.data != null) {
this.data = (Bundle) o.data.clone();
} else {
this.data = null;
}
}
/**
* Return the targeted delivery time of this message, in milliseconds.
*/
public long getWhen() {
return when;
}
public void setTarget(Handler target) {
this.target = target;
}
/**
* Retrieve the a {@link android.os.Handler Handler} implementation that
* will receive this message. The object must implement
* {@link android.os.Handler#handleMessage(android.os.Message)
* Handler.handleMessage()}. Each Handler has its own name-space for
* message codes, so you do not need to
* worry about yours conflicting with other handlers.
*/
public Handler getTarget() {
return target;
}
/**
* Retrieve callback object that will execute when this message is handled.
* This object must implement Runnable. This is called by
* the <em>target</em> {@link Handler} that is receiving this Message to
* dispatch it. If
* not set, the message will be dispatched to the receiving Handler's
* {@link Handler#handleMessage(Message Handler.handleMessage())}.
*/
public Runnable getCallback() {
return callback;
}
/**
* Obtains a Bundle of arbitrary data associated with this
* event, lazily creating it if necessary. Set this value by calling
* {@link #setData(Bundle)}. Note that when transferring data across
* processes via {@link Messenger}, you will need to set your ClassLoader
* on the Bundle via {@link Bundle#setClassLoader(ClassLoader)
* Bundle.setClassLoader()} so that it can instantiate your objects when
* you retrieve them.
* @see #peekData()
* @see #setData(Bundle)
*/
public Bundle getData() {
if (data == null) {
data = new Bundle();
}
return data;
}
/**
* Like getData(), but does not lazily create the Bundle. A null
* is returned if the Bundle does not already exist. See
* {@link #getData} for further information on this.
* @see #getData()
* @see #setData(Bundle)
*/
public Bundle peekData() {
return data;
}
/**
* Sets a Bundle of arbitrary data values. Use arg1 and arg1 members
* as a lower cost way to send a few simple integer values, if you can.
* @see #getData()
* @see #peekData()
*/
public void setData(Bundle data) {
this.data = data;
}
/**
* Sends this Message to the Handler specified by {@link #getTarget}.
* Throws a null pointer exception if this field has not been set.
*/
public void sendToTarget() {
target.sendMessage(this);
}
/**
* Returns true if the message is asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @return True if the message is asynchronous.
*
* @see #setAsynchronous(boolean)
* @see MessageQueue#enqueueSyncBarrier(long)
* @see MessageQueue#removeSyncBarrier(int)
*
* @hide
*/
public boolean isAsynchronous() {
return (flags & FLAG_ASYNCHRONOUS) != 0;
}
/**
* Sets whether the message is asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async True if the message is asynchronous.
*
* @see #isAsynchronous()
* @see MessageQueue#enqueueSyncBarrier(long)
* @see MessageQueue#removeSyncBarrier(int)
*
* @hide
*/
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
/*package*/ void clearForRecycle() {
flags = 0;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
when = 0;
target = null;
callback = null;
data = null;
}
/*package*/ boolean isInUse() {
return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
}
/*package*/ void markInUse() {
flags |= FLAG_IN_USE;
}
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}
@Override
public String toString() {
return toString(SystemClock.uptimeMillis());
}
String toString(long now) {
StringBuilder b = new StringBuilder();
b.append("{ when=");
TimeUtils.formatDuration(when - now, b);
if (target != null) {
if (callback != null) {
b.append(" callback=");
b.append(callback.getClass().getName());
} else {
b.append(" what=");
b.append(what);
}
if (arg1 != 0) {
b.append(" arg1=");
b.append(arg1);
}
if (arg2 != 0) {
b.append(" arg2=");
b.append(arg2);
}
if (obj != null) {
b.append(" obj=");
b.append(obj);
}
b.append(" target=");
b.append(target.getClass().getName());
} else {
b.append(" barrier=");
b.append(arg1);
}
b.append(" }");
return b.toString();
}
public static final Parcelable.Creator<Message> CREATOR
= new Parcelable.Creator<Message>() {
public Message createFromParcel(Parcel source) {
Message msg = Message.obtain();
msg.readFromParcel(source);
return msg;
}
public Message[] newArray(int size) {
return new Message[size];
}
};
public int describeContents() {
return 0;
}
public void writeToParcel(Parcel dest, int flags) {
if (callback != null) {
throw new RuntimeException(
"Can't marshal callbacks across processes.");
}
dest.writeInt(what);
dest.writeInt(arg1);
dest.writeInt(arg2);
if (obj != null) {
try {
Parcelable p = (Parcelable)obj;
dest.writeInt(1);
dest.writeParcelable(p, flags);
} catch (ClassCastException e) {
throw new RuntimeException(
"Can't marshal non-Parcelable objects across processes.");
}
} else {
dest.writeInt(0);
}
dest.writeLong(when);
dest.writeBundle(data);
Messenger.writeMessengerOrNullToParcel(replyTo, dest);
}
private void readFromParcel(Parcel source) {
what = source.readInt();
arg1 = source.readInt();
arg2 = source.readInt();
if (source.readInt() != 0) {
obj = source.readParcelable(getClass().getClassLoader());
}
when = source.readLong();
data = source.readBundle();
replyTo = Messenger.readMessengerOrNullFromParcel(source);
}
}
/**Message.java*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
sPoolSize--;
return m;
}
}
return new Message();
}
从上面代码可以看到，Message就是一个消息池，当池里面还有可用的消息对象时，pull出这个消息重复使用，避免资源浪费，否则会新建一个Message对象。
那么sendMessageDelayed了解一下：
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
还记得Looper中的mQueue吗，发消息时在sendMessageAtTime把mQueue作为参数间接调用了MessageQueue的enqueueMessage接口实现排队处理，MessageQueue完整源码如下：
/*
* Copyright (C) 2006 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.os;
import android.util.AndroidRuntimeException;
import android.util.Log;
import android.util.Printer;
import java.util.ArrayList;
/**
* Low-level class holding the list of messages to be dispatched by a
* {@link Looper}. Messages are not added directly to a MessageQueue,
* but rather through {@link Handler} objects associated with the Looper.
*
* <p>You can retrieve the MessageQueue for the current thread with
* {@link Looper#myQueue() Looper.myQueue()}.
*/
public final class MessageQueue {
// True if the message queue can be quit.
private final boolean mQuitAllowed;
@SuppressWarnings("unused")
private int mPtr; // used by native code
Message mMessages;
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
private IdleHandler[] mPendingIdleHandlers;
private boolean mQuitting;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;
// The next barrier token.
// Barriers are indicated by messages with a null target whose arg1 field carries the token.
private int mNextBarrierToken;
private native static int nativeInit();
private native static void nativeDestroy(int ptr);
private native static void nativePollOnce(int ptr, int timeoutMillis);
private native static void nativeWake(int ptr);
private native static boolean nativeIsIdling(int ptr);
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more. Return true to keep your idle handler active, false
* to have it removed. This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}
/**
* Add a new {@link IdleHandler} to this message queue. This may be
* removed automatically for you by returning false from
* {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is
* invoked, or explicitly removing it with {@link #removeIdleHandler}.
*
* <p>This method is safe to call from any thread.
*
* @param handler The IdleHandler to be added.
*/
public void addIdleHandler(IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
/**
* Remove an {@link IdleHandler} from the queue that was previously added
* with {@link #addIdleHandler}. If the given object is not currently
* in the idle list, nothing is done.
*
* @param handler The IdleHandler to be removed.
*/
public void removeIdleHandler(IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
@Override
protected void finalize() throws Throwable {
try {
dispose();
} finally {
super.finalize();
}
}
// Disposes of the underlying message queue.
// Must only be called on the looper thread or the finalizer.
private void dispose() {
if (mPtr != 0) {
nativeDestroy(mPtr);
mPtr = 0;
}
}
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
int enqueueSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.when = when;
msg.arg1 = token;
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycle();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
synchronized (this) {
if (mQuitting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
boolean hasMessages(Handler h, int what, Object object) {
if (h == null) {
return false;
}
synchronized (this) {
Message p = mMessages;
while (p != null) {
if (p.target == h && p.what == what && (object == null || p.obj == object)) {
return true;
}
p = p.next;
}
return false;
}
}
boolean hasMessages(Handler h, Runnable r, Object object) {
if (h == null) {
return false;
}
synchronized (this) {
Message p = mMessages;
while (p != null) {
if (p.target == h && p.callback == r && (object == null || p.obj == object)) {
return true;
}
p = p.next;
}
return false;
}
}
boolean isIdling() {
synchronized (this) {
return isIdlingLocked();
}
}
private boolean isIdlingLocked() {
// If the loop is quitting then it must not be idling.
// We can assume mPtr != 0 when mQuitting is false.
return !mQuitting && nativeIsIdling(mPtr);
}
void removeMessages(Handler h, int what, Object object) {
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h && p.what == what
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.what == what
&& (object == null || n.obj == object)) {
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
}
}
void removeMessages(Handler h, Runnable r, Object object) {
if (h == null || r == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h && p.callback == r
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.callback == r
&& (object == null || n.obj == object)) {
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
}
}
void removeCallbacksAndMessages(Handler h, Object object) {
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h
&& (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycle();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && (object == null || n.obj == object)) {
Message nn = n.next;
n.recycle();
p.next = nn;
continue;
}
}
p = n;
}
}
}
private void removeAllMessagesLocked() {
Message p = mMessages;
while (p != null) {
Message n = p.next;
p.recycle();
p = n;
}
mMessages = null;
}
private void removeAllFutureMessagesLocked() {
final long now = SystemClock.uptimeMillis();
Message p = mMessages;
if (p != null) {
if (p.when > now) {
removeAllMessagesLocked();
} else {
Message n;
for (;;) {
n = p.next;
if (n == null) {
return;
}
if (n.when > now) {
break;
}
p = n;
}
p.next = null;
do {
p = n;
n = p.next;
p.recycle();
} while (n != null);
}
}
}
void dump(Printer pw, String prefix) {
synchronized (this) {
long now = SystemClock.uptimeMillis();
int n = 0;
for (Message msg = mMessages; msg != null; msg = msg.next) {
pw.println(prefix + "Message " + n + ": " + msg.toString(now));
n++;
}
pw.println(prefix + "(Total messages: " + n + ", idling=" + isIdlingLocked()
+ ", quitting=" + mQuitting + ")");
}
}
}
下面是enqueueMessage的实现，可以看到做了类似链表的操作：
boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
synchronized (this) {
if (mQuitting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
看到这里其实事情就明朗了，回顾下Looper中的实现：
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycle();
}
}
这里就很明显的去轮询Looper消息队列mQueue，队列中有消息则会调用msg.target.dispatchMessage(msg);，而msg.target就是在发消息时使用的Handler对象，最后回收msg。
再看看Handler中的dispatchMessage实现：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
public void handleMessage(Message msg) {
}
所以在消息处理完时，就会调用子类的handleMessage，因此在DEMO中就会收到Handler分发下来的MSG_BASE消息。
当然，Handler不止这一种使用方法，在本篇文章中我只是举例了一种非常简单的使用方法，其他使用方法希望读者可以自己参考本文去分析。
在Android Java源码层Handler 、 Looper 、Message就是这样构成了应用层面的消息处理机制。
在Android C++源码层也有类似的消息机制，AHandler，AMessage，ALooper一起构成了这样的一个消息机制框架，很少有人直接使用这套机制写个简单易懂的DEMO，为了和上面JAVA代码保持一致，我这里自己也写了一个DEMO：
/**SimpleTest.h*/
#include <media/stagefright/foundation/AHandler.h>
#include <media/stagefright/foundation/AMessage.h>
#include <media/stagefright/foundation/AString.h>
namespace android{
struct SimpleTest : public AHandler {
SimpleTest();
void testMsg();
protected:
virtual ~SimpleTest();
virtual void onMessageReceived(const sp<AMessage> &msg);
private:
enum {
kMsgBase,
kMsgTest,
};
DISALLOW_EVIL_CONSTRUCTORS(SimpleTest);
};
}
/**SimpleTest.cpp*/
//#define LOG_NDEBUG 0
#define LOG_TAG "SimpleTest"
#include <media/stagefright/foundation/ABuffer.h>
#include <media/stagefright/foundation/ADebug.h>
#include <media/stagefright/foundation/AMessage.h>
#include <utils/Log.h>
#include "SimpleTest.h"
namespace android {
SimpleTest::SimpleTest(){
}
SimpleTest::~SimpleTest(){
}
void SimpleTest::testMsg(){
ALOGE("testMsg post");
sp<AMessage> msg = new AMessage(kMsgTest, id());
msg->setString("info", "this is info");
msg->post(5000000); //delay 5s
}
void SimpleTest::onMessageReceived(const sp<AMessage> &msg){
switch (msg->what()) {
case kMsgTest:
{
ALOGE("onMessageReceived kMsgTest");
break;
}
default:
break;
}
}
}
/**main.cpp*/
/*
* Author:yangwu
* Date:2018/06/20
*/
//#define LOG_NDEBUG 0
#define LOG_TAG "HandlerTest"
#include <utils/Log.h>
#include <binder/IServiceManager.h>
#include <binder/ProcessState.h>
#include <media/stagefright/foundation/ALooper.h>
#include <media/stagefright/foundation/AMessage.h>
#include "SimpleTest.h"
int main(int agc,char **agv){
using namespace android;
ProcessState::self()->startThreadPool();
sp<ALooper> looper = new ALooper;
looper->setName("testLooper");
sp<SimpleTest> test = new SimpleTest;
looper->registerHandler(test);
looper->start(
false, // runOnCallingThread
false, // canCallJava
ANDROID_PRIORITY_FOREGROUND);
test->testMsg();
ALOGE("looper here");
while(1){
sleep(1);
}
return 0;
}
输出如下：
06-20 10:40:48.491 E/SimpleTest(17844): testMsg post
06-20 10:40:48.491 E/HandlerTest(17844): looper here
06-20 10:40:53.491 E/SimpleTest(17844): onMessageReceived kMsgTest
所以通过对比可以看到Android Java层Handler的handleMessage和Android C++层中AHandler的onMessageReceived都是回调函数，用于响应消息。
还是按照DEMO来分析实现：
sp<ALooper> looper = new ALooper;
looper->setName("testLooper");
sp<SimpleTest> test = new SimpleTest;
looper->registerHandler(test);
looper->start(
false, // runOnCallingThread
false, // canCallJava
ANDROID_PRIORITY_FOREGROUND);
首先来看看ALooper源码：
#ifndef A_LOOPER_H_
#define A_LOOPER_H_
#include <media/stagefright/foundation/ABase.h>
#include <media/stagefright/foundation/AString.h>
#include <utils/Errors.h>
#include <utils/KeyedVector.h>
#include <utils/List.h>
#include <utils/RefBase.h>
#include <utils/threads.h>
namespace android {
struct AHandler;
struct AMessage;
struct ALooper : public RefBase {
typedef int32_t event_id;
typedef int32_t handler_id;
ALooper();
// Takes effect in a subsequent call to start().
void setName(const char *name);
handler_id registerHandler(const sp<AHandler> &handler);
void unregisterHandler(handler_id handlerID);
status_t start(
bool runOnCallingThread = false,
bool canCallJava = false,
int32_t priority = PRIORITY_DEFAULT
);
status_t stop();
static int64_t GetNowUs();
protected:
virtual ~ALooper();
private:
friend struct ALooperRoster;
struct Event {
int64_t mWhenUs;
sp<AMessage> mMessage;
};
Mutex mLock;
Condition mQueueChangedCondition;
AString mName;
List<Event> mEventQueue;
struct LooperThread;
sp<LooperThread> mThread;
bool mRunningLocally;
void post(const sp<AMessage> &msg, int64_t delayUs);
bool loop();
DISALLOW_EVIL_CONSTRUCTORS(ALooper);
};
} // namespace android
#endif // A_LOOPER_H_
/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
//#define LOG_NDEBUG 0
#define LOG_TAG "ALooper"
#include <utils/Log.h>
#include <sys/time.h>
#include "ALooper.h"
#include "AHandler.h"
#include "ALooperRoster.h"
#include "AMessage.h"
namespace android {
ALooperRoster gLooperRoster;
struct ALooper::LooperThread : public Thread {
LooperThread(ALooper *looper, bool canCallJava)
: Thread(canCallJava),
mLooper(looper),
mThreadId(NULL) {
}
virtual status_t readyToRun() {
mThreadId = androidGetThreadId();
return Thread::readyToRun();
}
virtual bool threadLoop() {
return mLooper->loop();
}
bool isCurrentThread() const {
return mThreadId == androidGetThreadId();
}
protected:
virtual ~LooperThread() {}
private:
ALooper *mLooper;
android_thread_id_t mThreadId;
DISALLOW_EVIL_CONSTRUCTORS(LooperThread);
};
// static
int64_t ALooper::GetNowUs() {
return systemTime(SYSTEM_TIME_MONOTONIC) / 1000ll;
}
ALooper::ALooper()
: mRunningLocally(false) {
}
ALooper::~ALooper() {
stop();
// Since this looper is "dead" (or as good as dead by now),
// have ALooperRoster unregister any handlers still registered for it.
gLooperRoster.unregisterStaleHandlers();
}
void ALooper::setName(const char *name) {
mName = name;
}
ALooper::handler_id ALooper::registerHandler(const sp<AHandler> &handler) {
return gLooperRoster.registerHandler(this, handler);
}
void ALooper::unregisterHandler(handler_id handlerID) {
gLooperRoster.unregisterHandler(handlerID);
}
status_t ALooper::start(
bool runOnCallingThread, bool canCallJava, int32_t priority) {
if (runOnCallingThread) {
{
Mutex::Autolock autoLock(mLock);
if (mThread != NULL || mRunningLocally) {
return INVALID_OPERATION;
}
mRunningLocally = true;
}
do {
} while (loop());
return OK;
}
Mutex::Autolock autoLock(mLock);
if (mThread != NULL || mRunningLocally) {
return INVALID_OPERATION;
}
mThread = new LooperThread(this, canCallJava);
status_t err = mThread->run(
mName.empty() ? "ALooper" : mName.c_str(), priority);
if (err != OK) {
mThread.clear();
}
return err;
}
status_t ALooper::stop() {
sp<LooperThread> thread;
bool runningLocally;
{
Mutex::Autolock autoLock(mLock);
thread = mThread;
runningLocally = mRunningLocally;
mThread.clear();
mRunningLocally = false;
}
if (thread == NULL && !runningLocally) {
return INVALID_OPERATION;
}
if (thread != NULL) {
thread->requestExit();
}
mQueueChangedCondition.signal();
if (!runningLocally && !thread->isCurrentThread()) {
// If not running locally and this thread _is_ the looper thread,
// the loop() function will return and never be called again.
thread->requestExitAndWait();
}
return OK;
}
void ALooper::post(const sp<AMessage> &msg, int64_t delayUs) {
Mutex::Autolock autoLock(mLock);
int64_t whenUs;
if (delayUs > 0) {
whenUs = GetNowUs() + delayUs;
} else {
whenUs = GetNowUs();
}
List<Event>::iterator it = mEventQueue.begin();
while (it != mEventQueue.end() && (*it).mWhenUs <= whenUs) {
++it;
}
Event event;
event.mWhenUs = whenUs;
event.mMessage = msg;
if (it == mEventQueue.begin()) {
mQueueChangedCondition.signal();
}
mEventQueue.insert(it, event);
}
bool ALooper::loop() {
Event event;
{
Mutex::Autolock autoLock(mLock);
if (mThread == NULL && !mRunningLocally) {
return false;
}
if (mEventQueue.empty()) {
mQueueChangedCondition.wait(mLock);
return true;
}
int64_t whenUs = (*mEventQueue.begin()).mWhenUs;
int64_t nowUs = GetNowUs();
if (whenUs > nowUs) {
int64_t delayUs = whenUs - nowUs;
mQueueChangedCondition.waitRelative(mLock, delayUs * 1000ll);
return true;
}
event = *mEventQueue.begin();
mEventQueue.erase(mEventQueue.begin());
}
gLooperRoster.deliverMessage(event.mMessage);
// NOTE: It's important to note that at this point our "ALooper" object
// may no longer exist (its final reference may have gone away while
// delivering the message). We have made sure, however, that loop()
// won't be called again.
return true;
}
} // namespace android
可以看到Looper子类LooperThread是继承Thread的。
先是调用：
ALooper::ALooper()
: mRunningLocally(false) {
}
void ALooper::setName(const char *name) {
mName = name;
}
ALooper::handler_id ALooper::registerHandler(const sp<AHandler> &handler) {
return gLooperRoster.registerHandler(this, handler);
}
DEMO中SimpleTest是继承AHandler的子类，ALooper通过registerHandler函数和AHandler绑定起来。根据ALooper::registerHandler的实现，我们先看看gLooperRoster，源码如下：
#ifndef A_LOOPER_ROSTER_H_
#define A_LOOPER_ROSTER_H_
#include <media/stagefright/foundation/ALooper.h>
#include <utils/KeyedVector.h>
namespace android {
struct ALooperRoster {
ALooperRoster();
ALooper::handler_id registerHandler(
const sp<ALooper> looper, const sp<AHandler> &handler);
void unregisterHandler(ALooper::handler_id handlerID);
void unregisterStaleHandlers();
status_t postMessage(const sp<AMessage> &msg, int64_t delayUs = 0);
void deliverMessage(const sp<AMessage> &msg);
status_t postAndAwaitResponse(
const sp<AMessage> &msg, sp<AMessage> *response);
void postReply(uint32_t replyID, const sp<AMessage> &reply);
sp<ALooper> findLooper(ALooper::handler_id handlerID);
private:
struct HandlerInfo {
wp<ALooper> mLooper;
wp<AHandler> mHandler;
};
Mutex mLock;
KeyedVector<ALooper::handler_id, HandlerInfo> mHandlers;
ALooper::handler_id mNextHandlerID;
uint32_t mNextReplyID;
Condition mRepliesCondition;
KeyedVector<uint32_t, sp<AMessage> > mReplies;
status_t postMessage_l(const sp<AMessage> &msg, int64_t delayUs);
DISALLOW_EVIL_CONSTRUCTORS(ALooperRoster);
};
} // namespace android
#endif // A_LOOPER_ROSTER_H_
//#define LOG_NDEBUG 0
#define LOG_TAG "ALooperRoster"
#include <utils/Log.h>
#include "ALooperRoster.h"
#include "ADebug.h"
#include "AHandler.h"
#include "AMessage.h"
namespace android {
ALooperRoster::ALooperRoster()
: mNextHandlerID(1),
mNextReplyID(1) {
}
ALooper::handler_id ALooperRoster::registerHandler(
const sp<ALooper> looper, const sp<AHandler> &handler) {
Mutex::Autolock autoLock(mLock);
if (handler->id() != 0) {
CHECK(!"A handler must only be registered once.");
return INVALID_OPERATION;
}
HandlerInfo info;
info.mLooper = looper;
info.mHandler = handler;
ALooper::handler_id handlerID = mNextHandlerID++;
mHandlers.add(handlerID, info);
ALOGW("ALooper::handler_id ALooperRoster::registerHandler %d\n",handlerID);
handler->setID(handlerID);
return handlerID;
}
void ALooperRoster::unregisterHandler(ALooper::handler_id handlerID) {
Mutex::Autolock autoLock(mLock);
ALOGI("ALooperRoster unregisterHandler handlerID=%x\n",handlerID);
ssize_t index = mHandlers.indexOfKey(handlerID);
if (index < 0) {
return;
}
const HandlerInfo &info = mHandlers.valueAt(index);
sp<AHandler> handler = info.mHandler.promote();
if (handler != NULL) {
handler->setID(0);
}
mHandlers.removeItemsAt(index);
}
void ALooperRoster::unregisterStaleHandlers() {
Mutex::Autolock autoLock(mLock);
for (size_t i = mHandlers.size(); i-- > 0;) {
const HandlerInfo &info = mHandlers.valueAt(i);
sp<ALooper> looper = info.mLooper.promote();
if (looper == NULL) {
ALOGV("Unregistering stale handler %d", mHandlers.keyAt(i));
mHandlers.removeItemsAt(i);
}
}
}
status_t ALooperRoster::postMessage(
const sp<AMessage> &msg, int64_t delayUs) {
Mutex::Autolock autoLock(mLock);
return postMessage_l(msg, delayUs);
}
status_t ALooperRoster::postMessage_l(
const sp<AMessage> &msg, int64_t delayUs) {
ssize_t index = mHandlers.indexOfKey(msg->target());
if (index < 0) {
ALOGW("failed to post message '%s'. Target handler not registered.",
msg->debugString().c_str());
ALOGW("failed to post message 'msg->target() %d\n",msg->target());
return -ENOENT;
}
const HandlerInfo &info = mHandlers.valueAt(index);
sp<ALooper> looper = info.mLooper.promote();
if (looper == NULL) {
ALOGW("failed to post message. "
"Target handler %d still registered, but object gone.",
msg->target());
mHandlers.removeItemsAt(index);
return -ENOENT;
}
looper->post(msg, delayUs);
return OK;
}
void ALooperRoster::deliverMessage(const sp<AMessage> &msg) {
sp<AHandler> handler;
{
Mutex::Autolock autoLock(mLock);
ssize_t index = mHandlers.indexOfKey(msg->target());
if (index < 0) {
ALOGW("failed to deliver message. Target handler not registered.");
return;
}
const HandlerInfo &info = mHandlers.valueAt(index);
handler = info.mHandler.promote();
if (handler == NULL) {
ALOGW("failed to deliver message. "
"Target handler %d registered, but object gone.",
msg->target());
mHandlers.removeItemsAt(index);
return;
}
}
handler->onMessageReceived(msg);
}
sp<ALooper> ALooperRoster::findLooper(ALooper::handler_id handlerID) {
Mutex::Autolock autoLock(mLock);
ssize_t index = mHandlers.indexOfKey(handlerID);
if (index < 0) {
return NULL;
}
sp<ALooper> looper = mHandlers.valueAt(index).mLooper.promote();
if (looper == NULL) {
mHandlers.removeItemsAt(index);
return NULL;
}
return looper;
}
status_t ALooperRoster::postAndAwaitResponse(
const sp<AMessage> &msg, sp<AMessage> *response) {
Mutex::Autolock autoLock(mLock);
uint32_t replyID = mNextReplyID++;
msg->setInt32("replyID", replyID);
status_t err = postMessage_l(msg, 0 /* delayUs */);
if (err != OK) {
response->clear();
return err;
}
ssize_t index;
while ((index = mReplies.indexOfKey(replyID)) < 0) {
mRepliesCondition.wait(mLock);
}
*response = mReplies.valueAt(index);
mReplies.removeItemsAt(index);
return OK;
}
void ALooperRoster::postReply(uint32_t replyID, const sp<AMessage> &reply) {
Mutex::Autolock autoLock(mLock);
CHECK(mReplies.indexOfKey(replyID) < 0);
mReplies.add(replyID, reply);
mRepliesCondition.broadcast();
}
} // namespace android
其中ALooperRoster ::registerHandler函数把ALooper(这里传递的是this)和AHandler(这里是子类SimpleTest)关联在一起，把looper和handler添加到mHandlers池中，下面再来重点分析AHandler源码：
#ifndef A_HANDLER_H_
#define A_HANDLER_H_
#include <media/stagefright/foundation/ALooper.h>
#include <utils/RefBase.h>
namespace android {
struct AMessage;
struct AHandler : public RefBase {
AHandler()
: mID(0) {
}
ALooper::handler_id id() const {
return mID;
}
sp<ALooper> looper();
protected:
virtual void onMessageReceived(const sp<AMessage> &msg) = 0;
private:
friend struct ALooperRoster;
ALooper::handler_id mID;
void setID(ALooper::handler_id id) {
mID = id;
}
DISALLOW_EVIL_CONSTRUCTORS(AHandler);
};
} // namespace android
#endif // A_HANDLER_H_
//#define LOG_NDEBUG 0
#define LOG_TAG "AHandler"
#include <utils/Log.h>
#include <media/stagefright/foundation/AHandler.h>
#include <media/stagefright/foundation/ALooperRoster.h>
namespace android {
sp<ALooper> AHandler::looper() {
extern ALooperRoster gLooperRoster;
return gLooperRoster.findLooper(id());
}
} // namespace android
AHandler实现就很简单了，其实就是让ALooperRoster来背锅￣へ￣，看下面这张图就清楚多了：
ALooper和AHandler创建并关联后，还需要启动ALooper:
looper->start(
false, // runOnCallingThread
false, // canCallJava
ANDROID_PRIORITY_FOREGROUND);
源码如下：
status_t ALooper::start(
bool runOnCallingThread, bool canCallJava, int32_t priority) {
if (runOnCallingThread) {
{
Mutex::Autolock autoLock(mLock);
if (mThread != NULL || mRunningLocally) {
return INVALID_OPERATION;
}
mRunningLocally = true;
}
do {
} while (loop());
return OK;
}
Mutex::Autolock autoLock(mLock);
if (mThread != NULL || mRunningLocally) {
return INVALID_OPERATION;
}
mThread = new LooperThread(this, canCallJava);
status_t err = mThread->run(
mName.empty() ? "ALooper" : mName.c_str(), priority);
if (err != OK) {
mThread.clear();
}
return err;
}
当runOnCallingThread为true时表示在调用进程中直接loop：
bool ALooper::loop() {
Event event;
{
Mutex::Autolock autoLock(mLock);
if (mThread == NULL && !mRunningLocally) {
return false;
}
if (mEventQueue.empty()) {
mQueueChangedCondition.wait(mLock);
return true;
}
int64_t whenUs = (*mEventQueue.begin()).mWhenUs;
int64_t nowUs = GetNowUs();
if (whenUs > nowUs) {
int64_t delayUs = whenUs - nowUs;
mQueueChangedCondition.waitRelative(mLock, delayUs * 1000ll);
return true;
}
event = *mEventQueue.begin();
mEventQueue.erase(mEventQueue.begin());
}
gLooperRoster.deliverMessage(event.mMessage);
// NOTE: It's important to note that at this point our "ALooper" object
// may no longer exist (its final reference may have gone away while
// delivering the message). We have made sure, however, that loop()
// won't be called again.
return true;
}
当runOnCallingThread为false时，创建LooperThread并run起来，把looper作为传参：
virtual status_t readyToRun() {
mThreadId = androidGetThreadId();
return Thread::readyToRun();
}
virtual bool threadLoop() {
return mLooper->loop();
}
在线程的threadLoop中，清晰的看到了ALooper::loop的调用，根据loop函数的实现可以知道最终通过LooperRoster::deliverMessage来分发消息：
void ALooperRoster::deliverMessage(const sp<AMessage> &msg) {
sp<AHandler> handler;
{
Mutex::Autolock autoLock(mLock);
ssize_t index = mHandlers.indexOfKey(msg->target());
if (index < 0) {
ALOGW("failed to deliver message. Target handler not registered.");
return;
}
const HandlerInfo &info = mHandlers.valueAt(index);
handler = info.mHandler.promote();
if (handler == NULL) {
ALOGW("failed to deliver message. "
"Target handler %d registered, but object gone.",
msg->target());
mHandlers.removeItemsAt(index);
return;
}
}
handler->onMessageReceived(msg);
}
从上面的源码中看到，mHandlers池里面去除消息对应的AHandler对象，然后调用对应的onMessageReceived函数发送消息，因此在DEMO中的SimpleTest实现的onMessageReceived函数就能收到对应的kMsgTest消息。
虽然AHandler是Android在libstagefright中实现的，但从这里看起来，完全可以封装起来作为一个共享库。这和本文无关，后面我打算精简出来放到git上。
3.结束语
到这里，本篇文章就该结束了，我在Android源码基础上分别从JAVA层和C++层来分析了Handler/AHandler消息机制，可以看到两种方式设计模式基本是一样的，这种优秀的设计模式在Android源码中到处可见，也希望自己乃至读者能在其他业务中灵活的运用这种消息机制。
