Handler是Android中消息传递机制，可以将工作线程所创建的消息传递到主线程中去处理，进行UI更新或者其他需要在主线程进行处理的工作。总的来说，就是线程之间的通讯。
Handler原理解析
了解Handler原理之前，我们需要先了解几个相关的类
- Hander (主要作用发送和处理消息和Runnable)
- Message (线程间消息传递的载体)
- MessageQueue(消息队列，存放消息)
- Looper (循环读取消息队列，发送到指定Handler处理)
Handler的基本使用
//在主线程创建一个Handler对象
//并重载handleMessage方法
Handler mHandler = new Handler(){
@Override
public void handleMessage(Message){
//需要在mHandler的线程处理的业务逻辑
//可以根据Message的参数处理相应业务，例如UI更新
textView.setText("通过sendMessage更新UI")
}
}
//然后在子线程中发送消息
//通过sendMessage方法发送
new Thread(new Runnable(){
@Override
public void run(){
//主线的Handler
Message message = new Message();
message.what = 1;
message.obj = "可以传递对象，这里是传递字符串对象";
//通过sendMessage发送消息对象
mHandler.sendMessage(message)
}
}).start();
//或者可以传递一个Runnable对象
new Thread(new Runnable(){
@Override
public void run(){
mHandler.post(new Runnable(){
@Override
public void run(){
//需要在mHandler的线程处理的业务逻辑
//如更新UI
textView.setText("通过post更新UI")
}
})
}
})
除了上面2种方法，Handler发送消息很多种
//发送Runnable
post(Runnable)
postAtTime(Runnable,long)
postAtTime(Runnable,Object,long)
postDelayed(Runnable,long)
postDelayed(Runnable,Object,long)
//发送Message
sendMessage(Message)
sendMessageDelayed(Message,long)
sendMessageAtTime(Message,long)
sendMessageAtFrontOfQueue(Message)
sendEmptyMessage(int)
sendEmptyMessageDelayed(int,long)
sendEmptyMessageAtTime(int,long)
//我们先分析比较简单的sendMessage和post
public final boolean sendMessage(Message msg){
return sendMessageDelayed(msg, 0);
}
//调用成员方法sendMessageDelayed，延时为0
public final boolean sendMessageDelayed(Message msg, long delayMillis){
if (delayMillis < 0) {
delayMillis = 0;
}
//所以插入消息队列的时候就是（当前时间+0）
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//uptimeMellis就是消息传递到Handler的时间
public boolean sendMessageAtTime(Message msg, long uptimeMillis){
MessageQueue queue = mQueue;
//省略部分代码...
//最终通过enqueueMessage把消息插入到消息队列
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
//把当前Handler对象引用保存在Message.target，后续通过这个target来传递到指定Handler去处理消息
msg.target = this;
//设置Message是否异步Message
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//插入到消息队列
return queue.enqueueMessage(msg, uptimeMillis);
}
//MessageQueue.java#enqueueMessage
boolean enqueueMessage(Message msg, long when) {
//省略部分代码...
synchronized (this) {
//省略部分代码...
msg.markInUse();//标记Message为使用中
msg.when = when;//执行时间
Message p = mMessages;//消息队列链头部
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;
//遍历消息队列，寻找插入Message的位置
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
//插入到消息队列
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
//唤醒
nativeWake(mPtr);
}
}
return true;
}
//sendMessage基本流程就如上所示
//sendMessage -> sendMessageDelayed -> sendMessageAtTime -> enqueueMessage
//接着看post流程
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
//调用getPostMessage把Runnable封装为Message
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
//最后流程还是sendMessage流程一致
通过上面的代码，了解了Handler如何发送消息的方法，那么线程之间消息是如何传递的呢？
为什么在其他线程通过Hander发送消息后，会到Handler线程进行处理？
首先先看创建Hander实例时候处理
Handler mHandler = new Handler()//创建了Handler实例
//Handler.java
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
...
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException("Can't create handler inside thread "
+ Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
...
}
//Handler构造方法主要工作：
//获取Looper对象
//从Looper对象获取mQueue(消息队列)
Looper对象获取
//Looper.java
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
public static @Nullable Looper myLooper() {
//通过ThreadLocal.get获取Looper对象
return sThreadLocal.get();
}
ThreadLoal变量，线程局部变量，简单理解为线程私有变量，这里就不展开描述；
myLooper方法中通过sThreadLocal.get()获取Looper，那Looper是在什么时候set进去的？通过sThreadLocal.set关键字搜索，在Looper的prepare方法内找到
//一般线程初始化Looper
public static void prepare() {
prepare(true);
}
//主线程初始化Looper
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
//真正初始化Looper方法
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//创建Looper对象，并将set到ThreadLocal
sThreadLocal.set(new Looper(quitAllowed));
}
//Looper构造方法
private Looper(boolean quitAllowed) {
//创建MessageQueue
mQueue = new MessageQueue(quitAllowed);
//记录线程ID
mThread = Thread.currentThread();
}
所以要创建Handler对象，必定要先调用Looper对象的prepare或者prepareMainLooper方法，否则在Handler构造方法就会跑出异常，因为Looper.myLooper返回的对象为null
public Handler(Callback callback, boolean async) {
···
mLooper = Looper.myLooper();
if (mLooper == null) {
//获取到Looper对象为空，抛出异常
//报错说明未调用Looper.prepare()
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
···
}
那Looper.prepare方法在什么时候调用？了解过Android源码的可能知道，在ActivityThread的main方法中，调用了Looper.prepareMainLooper，而且当前线程也就是我们所说的主线程
ActivityThread.java
public static void main(String[] args) {
···
//初始化Looper对象
Looper.prepareMainLooper();
···
//开始循环读取消息
Looper.loop();
···
}
既然我们要循环从消息队列获取消息进行发送，那就可能到开启循环，Looper.loop方法的作用就是循环从消息队列读取消息并且发送。通过源码看看Looper.loop方法是如何循环读取消息
Looper.java
public static void loop() {
//获取当前线程的Looper对象
final Looper me = myLooper();
···
//获取Looper的消息队列
final MessageQueue queue = me.mQueue;
···
//开始循环获取消息
for (;;) {
//获取消息队列，可能会阻塞
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
···
try {
//分发到指定Handler去处理消息
//msg.target就是前面sendMessage中设置的Handler引用
msg.target.dispatchMessage(msg);
···
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
···
//回收Message对象
msg.recycleUnchecked();
}
}
知道了Message如何分发到指定Handler，跟踪dispatchMessage看看最后如何传递到Hander的handleMessage方法
//Handler.java#dispatchMessage
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//判断Runnable对象存在
handleCallback(msg);
} else {
//可以在构造方法传入Callback，作用和handlerMessage一样
//一般都直接重写handleMessage方法
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//最终调用handlerMessage，把消息传递到我们实例话的Handler对象，一般我们都会重写handleMessage方法
handleMessage(msg);
}
}
总结
Handler消息传递机制使用之前，需要先初始化Looper绑定当前线程，再循环从消息队列获取消息分发到指定的Handler去处理，以上只分析了Handerl机制的一般用法，还有其他如异步消息，IdleHandlder的用法还未涉及，后续逐步进行分析
