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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