Android Handler消息派发机制源码分析

注：这里只是说一下sendmessage的一个过程，post就类似的
如果我们需要发送消息，会调用sendMessage方法

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

这个方法会调用如下的这个方法 

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
 delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

接下来设定延迟时间，然后继续调用sendMessageAtTime方法 

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);
}

这里获得了消息队列，检查队列是否存在，然后返回enqueMessage的方法的执行结果，这个结果是说明消息能否进入队列的一个布尔值 

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
 msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

这里是对消息进行入队处理，下面就是在MessageQueue中对消息进行入队 

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;
}

就是对传递过来的消息进行一些封装然后放到队列中，至此我们的sendMessage处理完毕，返回的结果是进队是否成功的布尔值，那么究竟消息之后是如何被处理的呢？
我们可以看到在Handler构造的时候记录了一个Looper对象，也记录了一个回掉函数 

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;
}

这里的myLooper方法返回的是当前线程关联的一个Looper对象

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

当Looper实例化了以后会执行自己的prepare方法然后执行loop方法，loop方法就是不断的读取消息队列中的消息然后执行相应的操作的方法，因为是在其他线程中执行的循环所以不会影响其他线程 

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.recycleUnchecked();
}
}

在循环中如果读取到了消息，就会执行dispatchMessage方法，然后分派完消息之后再执行一次recycleUnchecked方法来重用这个Message，我们看到dispatchMessage方法 

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

这里看到直接执行了一个handlerMessage方法，这个方法是一个回调方法，我们是必须实现的，否则Handler什么都不会做，为什么呢？还记得刚刚说构造Handler的时候我们记录了一个CallBack的回掉吗？Handler中的这个handlerMessage方法是一个空方法，如果我们重写了这个方法，在回调的时候就会执行我们先写下的代码，也就是接收到消息之后要做什么。 

public interface Callback {
public boolean handleMessage(Message msg);
}

public void handleMessage(Message msg) {
}

这里简单说下整个过程： 
当我们实例化一个Handler的子类并重写handleMessage方法之后，这个时候系统已经帮我们做了几个事情 
1.实例化了一个消息队列MessageQueue 
2.实例化了一个关联的Looper对象，并让Looper不断的读取消息队列
3.把我们重写的handleMessage方法记录为我们需要回调的方法 
当我们执行Handler的sendMessage方法的时候，系统会把我们传过去的Message对象添加到消息队列，这个时候如果Looper读取到了消息，就会把消息派发出去，然后回调handleMessage方法，执行我们设定的代码。