关于这几个技术术语，让我们从Android的实际使用中来渐渐理清它们。
1. Looper的初始化
Application被创建时,ActivityThread的main函数会被调用,其中初始化了一个Looper
Looper.prepareMainLooper();
其实现在Looper.java中:
/**
* 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); //进行初始化
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper(); //赋值
}
}
prepare(false)是对Looper进行初始化, 初始化之后调用将Looper对象赋值给sMainLooper. 所以我们只需要看prepare函数:
......
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal();
......
private static void prepare(boolean quitAllowed) {
if(sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
} else {
sThreadLocal.set(new Looper(quitAllowed)); //创建一个Looper对象,放入ThreadLocal中
}
}
从之前的描述可以看出,Looper是一个单例,所以其构造函数是private的:
private Looper(boolean quitAllowed) {
this.mQueue = new MessageQueue(quitAllowed);
this.mRun = true;
this.mThread = Thread.currentThread();
}
2. 初始化MessageQueue
上面的Looper的构造函数中又初始化了一个MessageQueue，这里又引出了一个重要的对象MessageQueue . 每个Looper都保持一个MessageQueue用来存放Message. 这里只需要记住Looper和MessageQueue的关系即可.下面是MessageQueue 的构造函数：
/**
* Low-level class holding the list of messages to be dispatched by a
* Looper. Messages are not added directly to a MessageQueue,
* but rather through Handler objects associated with the Looper.
*
* You can retrieve the MessageQueue for the current thread with
* Looper.myQueue().
*/
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
从构造函数上面的描述可以看到,Message是不能直接加入到MessageQueue中的, 必须通过Handler来添加. 这里你可能会有疑问，为什么Message不能直接加入到MessageQueue里面呢？带着这个问题我们继续向下看。
3. 创建ActivityThread对象
Looper初始化完成后, 回到ActivityThread的main函数, 这里还创建了一个ActivityThread对象,对App来说,ActivityThread是一个非常重要的类,
//初始化ActivityThread
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
ActivityThread的构造函数中初始化了ResourcesManager,保存在mResourcesManager中. thread.attach(false)这个比较重要,是进行一些状态的设置和初始化. 随后会初始化这个应用程序UI线程的Handle .下面是getHandler()
final Handler getHandler() {
return mH;
}
其中mH的定义如下:
private class H extends Handler {
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: {
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
} break;
case RELAUNCH_ACTIVITY: {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
ActivityClientRecord r = (ActivityClientRecord)msg.obj;
handleRelaunchActivity(r);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}
}
其handleMessage这个函数很长,仔细看就可以发现,这里调度了应用开发中各个组件的生命周期.包括activityRestart/activityPause/activityStop/activityDestroy/activityNewIntent等等.
4. 执行消息循环
随后main函数会调用Looper.loop(),其定义如下:
/**
* 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();
......
final MessageQueue queue = me.mQueue;
......
for (;;) {
Message msg = queue.next(); // might block
.......
msg.target.dispatchMessage(msg);
......
msg.recycleUnchecked();
}
}
可以看到其中调用了一个死循环for (;;),在循环开始调用
Message msg = queue.next(); // might block
来从MessageQueue中获取Message来进行处理.注意到MessageQueue的next函数会调用this.nativePollOnce(this.mPtr, nextPollTimeoutMillis); 这是一个阻塞方法. 会调用到Native层的Looper的一些方法. 当有消息的时候,就会返回一个Message.
这时候一个Activity的Handle/ Looper / MessageQueue都已经跑起来了.他们直接的关系也清楚了.
5. 自己创建的Handler与Looper和MessageQueue之间的关系
在Android开发中,我们一般会自己创建一个Handler,然后发送Message进行通信 (典型场景就是创建一个子线程, 在其中执行耗时较多的操作,然后发一个Message通知UI更新)
第一种写法:无参数的Handler
Handler defaultHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_HELLO:
Log.w(TAG, "defaultHandler ThreadName = "
+ defaultHandler.getLooper().getThread().getName());
}
}
};
......
//使用
new Thread(){
@Override
public void run() {
defaultHandler.sendEmptyMessage(MSG_HELLO);
}
}.start();
其打印结果如下：
这种写法创建的Handler,将自动与当前运行的线程相关联,也就是说这个自定义的Handler将与当前运行的线程使用同一个消息队列,并且可以处理该队列中的消息.如果不太理解这个可以查看Handler的构造函数:
public Handler(Callback callback, boolean async) {
......
mLooper = Looper.myLooper();
......
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
可以看到, Handler中的mLooper和mQueue就是之前ActivityThread中初始化好的.由于Looper是单例,也就是说mLooper和mQueue都只有一个, 而Handler的sendMessage最终会调用sendMessageAtTime:
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
......
return enqueueMessage(queue, msg, uptimeMillis);
}
看到这里,应该对自己创建的Handler和MessageQueue以及Looper直接的关系更清楚了.
第二种写法:参数包含Looper的Handler
Handler有好几个构造函数,其中如果参数中不包含Looper,则最终会进入第一种写法中的那个构造函数,如果参数中包含Looper,则会进入到三个参数的构造函数中:
/**
* Use the provided 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 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 Message#setAsynchronous(boolean) for
* each Message that is sent to it or Runnable that is posted to it.
*
* @hide
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
这种Handler初始化的时候必须创建一个Looper对象
Handler mHandler;
class CustomThread extends Thread {
@Override
public void run() {
Looper.prepare();
mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_HELLO:
Log.w(TAG, "defaultHandler ThreadName = "
+ defaultHandler.getLooper().getThread().getName());
break;
default:
break;
}
}
};
Looper.loop();
}
}
//使用
CustomThread mCustomThread = new CustomThread();
mCustomThread.start();
我给TextView加了一个点击事件，点击会打印对应的线程名：
appNameText.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mHandler.sendEmptyMessage(MSG_HELLO);
Log.w(TAG, "MainThreadName =" + getMainLooper().getThread().getName());
}
});
其结果如下：
通过上面两个例子,你应该对Thread/Looper/MessageQueue/Handler有更深入的了解了.
