Handler主要用于线程切换，一个典型的应用场景是：子线程通过Handler更新主线程UI
本文将从源码上来介绍Handler的实现原理
CSDN地址：http://blog.csdn.net/myterabithia/article/details/58603639
Handler的工作流程
先看一张图：
主要流程如下：
- 构造Message对象
- 通过Handler将Message发送到MessageQueue
- Looper从MessageQueue里取出Message对象
- Looper调用Message对象里保存的Handler对象的dispatchMessage方法将Message的处理移交给Handler
那么Looper和MessageQueue是哪来的呢？看一下Handler的构造方法：
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();//获取Handler所在线程的Looper。
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;//获取Looper里的MessageQueue
mCallback = callback;
mAsynchronous = async;
}
如果mLooper为空则直接抛异常了，所以如果不是在主线程创建Handler之前一定要在子线程里调用Looper.prepare()准备好一个Looper。Looper.prepare()会调用Looper的构造方法创建一个Looper，在Looper的构造方法中又创建了一个MessageQueue。
下面通过源码来看这几个步骤是如何实现的
1.构造Message对象
或者，直接
Message message = new Message();
各种方式在使用效果上最后的差别不大，任选其一即可。
2.通过Handler将Message发送到MessageQueue
//Handler
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//注意这一句，将target对象指向当前handler
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);//将Message入消息队列
}
左右的发送消息的方法最终都调用到了这个方法，顾名思义，这个方法将Message对象添加到MessageQueue队列，在入队之前，将message的target对象赋值为当前handler对象，最后会通过这个target对象来处理这个message。
//MessageQueue
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
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;
}
可以看到，MessageQueue其实是一个单链表，所以很多操作都是单链表的操作。如果p==null（当前队列为空）或者when==0（通过sendMessageAtFrontOfQueue方法发送的消息）或者when<p.when的时就将此消息插入到队的头部，否则按时间先后顺序入队，这里的时间是什么时间呢？看下面代码，其实是SystemClock.uptimeMillis() + delayMillis，也就是当前开机时间的毫秒数加上我们设置的延时。
//Handler
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
3.Looper从MessageQueue里取出Message对象
当创建好Looper后，会调用Looper.loop()方法不断的从MessageQueue里读取Message，如果是主线程，那么Looper.loop()方法在系统创建进程的时候就已经调用过了，如果在子线程则需要自己调用。
//Looper
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;
...省略
for (;;) {
//从队列里取出消息
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...省略
try {
//将msg的处理移交给Handler
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...省略
}
}
Message msg = queue.next(); // might block 通过一个无限的for循环中通过MessageQueue.next()读消息，如果队列没有消息则阻塞。在next方法中，如果队首的消息执行时间还没到，就设置一个等待时间，如果到了就从链表里取出来，然后返回。
//MessageQueue
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
...
for(;;){
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
...
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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
...
}
}
}
4.调用Handler的dispatchMessage方法将Message的处理移交给Handler
在Loopermsg.target.dispatchMessage(msg)让Handler去处理，这里的target就是在调用Handler.的enqueueMessage方法时赋值得，忘记了可以去步骤2里再看一下。
至此，处理流程又回到了Handler的dispatchMessage方法里，逻辑很简单，一个细节要注意，如果mCallback不为空，是不会调用handleMessage，这里mCallback是在创建Handler的时候就传进来的，所以使用Handler处理消息，要么在创建Handler的时候传入一个Callback，要么重写handleMessage方法。
//Handler
/**
* 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);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
