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。

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