Looper、Message、MessageQueue、Handler异步消息处理机制源码分析

1.Looper

Looper主要是prepare()和loop()两个方法。

prepare()方法主要是创建Looper和MessageQueue对象，并把Looper对象和线程绑定。

loop()方法主要是从线程绑定的Looper对象中获取MessageQueue消息队列，在死循环中从MessageQueue中循环取消息交给target对象处理。

public static final void prepare() {
// 获取绑定的Looper对象，如果不为空则抛异常。
// 这也就说明了Looper.prepare()方法不能被调用两次，同时也保证了一个线程中只有一个Looper对象
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
// 把Looper对象绑定到当前线程中
sThreadLocal.set(new Looper(true));
}

private Looper(boolean quitAllowed) {
// 创建MessageQueue成员变量
mQueue = new MessageQueue(quitAllowed);
mRun = true;
mThread = Thread.currentThread();
}

public static void loop() {
// 获取绑定的Looper对象
final Looper me = myLooper();
// 如果me为null则抛异常，说明loop()方法必须在prepare()方法之后调用
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
// 获取MessageQueue消息队列
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();

// 死循环
for (;;) {
// 取出一条消息
Message msg = queue.next(); // might block
// 如果消息为null则阻塞
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);
}

// 把消息交给target处理，这个target就是Handler
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);
}

// 释放消息占据的资源
msg.recycle();
}
}

public static Looper myLooper() {
return sThreadLocal.get();
}

2.Handler

Handler主要负责发送和处理消息。

Handler构造方法中获取当前线程的Looper实例，再通过该实例获取到MessageQueue消息队列。这样Handler就与Looper中的MessageQueue关联上了。

跟踪sendMessage()方法：

public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}

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) {
// 把Handler对象赋值给target
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

public void dispatchMessage(Message msg) {
// msg.callback是Runnable对象
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

3.Handler post

有时候为了方便，我们会直接写下面的代码：

mHandler.post(new Runnable()
{
@Override
public void run()
{
Log.e("TAG", Thread.currentThread().getName());
mText.setText("hehe");
}
});

在run方法中可以写更新UI的代码，其实这个Runnable并没有创建什么线程，而是发送了一条消息，源码如下：

public final boolean post(Runnable r)
{
return  sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}

可以看到，在getPostMessage方法中，得到了一个Message对象，然后将Runnable对象作为callback属性赋值给了msg。sendMessageDelayed方法最后调用了enqueueMessage方法，给msg.target赋值为handler，最终加入MessageQueue。

dispatchMessage方法中由于msg.callback不为null，故会执行handleCallback方法。在该方法中会执行Runnable中的run方法。

private static void handleCallback(Message msg) {
msg.callback.run();
}