在Android中每个应用的UI线程是被保护的，不能在UI线程中进行耗时的操作，其他的子线程也不能直接进行UI操作。为了达到这个目的Android设计了handler Looper这个系统框架，Android 推荐把耗时的操作放到子线程中去执行，子线程执行完毕后如果想通知UI更新，可以通过给主线程的Handler发送Message的方式来予以实现，说起Handler就不得不提到它的另外两个伙伴：Looper和MessageQueue,它们三者之间的关系如下：
Handler: 消息处理者(发送消息和处理消息, 准确的说是发送消息到MessageQueue, 处理Looper从MessageQueue中抽取的消息)。
MessageQueue: 消息队列（承载消息的容器，存放Handler发送的消息）。
Looper: 管理者（管理消息队列，不断的从MessageQueue中抽取消息交给Handler进行处理)。
以子线程向主线程发送一条消息为例，当在一条子线程中调用mHandler.sendMessage(msg)时，它的执行过程如下：
1. Handler会依次调用 sendMessageDelayed() 、 sendMessageAtTime() 、 enqueueMessage() 方法将消息压入MessageQueue。
1.1 sendMessage
/** * Pushes a message onto the end of the message queue after all pending messages * before the current time. It will be received in {@link #handleMessage}, * in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }
1.2 sendMessageDelayed
/** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }
1.3 sendMessageAtTime
/** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) <var>uptimeMillis</var>. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ 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); }
这里的mQueue其实是Handler已经持有的一个成员变量，在Handler的构造方法中通过Looper对象身上获取到的，Looper对象本身就含有一个MessageQueue。
/** * Use the provided {@link Looper} instead of the default one and take a callback * interface in which to handle messages. Also set whether the handler * should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * * @param looper The looper, must not be null. * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }
1.4 enqueueMessage
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
在enqueueMessage方法中通过调用MessageQueue的enqueueMessage()方法将消息压入消息队列。
2. 以上是入队的操作，接下来主线程通过Looper.loop()方法不断的从消息队列中获取消息，并交给其指定的Handler进行处理：
这里有个问题，主线程的Looper是什么时候启动的？我们知道，Google设计Looper的目的就是为了在一个线程里运行一个消息循环，但是Looper需要调用 prepare() 方法才能运行起来，调用 loop() 方法开始处理消息。那么这两个方法在哪里开始调用的？
一般说来，Android程序的入口点可以认为是android.app.ActivityThread类的main()方法（Android 2.3）:
public static final void main(String[] args) { SamplingProfilerIntegration.start(); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); if (sMainThreadHandler == null) { sMainThreadHandler = new Handler(); } ActivityThread thread = new ActivityThread(); thread.attach(false); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); if (Process.supportsProcesses()) { throw new RuntimeException("Main thread loop unexpectedly exited"); } thread.detach(); String name = (thread.mInitialApplication != null) ? thread.mInitialApplication.getPackageName() : "<unknown>"; Slog.i(TAG, "Main thread of " + name + " is now exiting"); }
果然，在这个方法里找到了prepare()和loop()方法的调用，这说明程序在启动的时候就自动开始了Looper的运转。接下来就要看出队的操作了，Looper通过调用loop()方法开始处理消息，来到Looper.java:
2.1 loop()
public static final void loop() { Looper me = myLooper(); MessageQueue queue = me.mQueue; while (true) { Message msg = queue.next(); // might block if (msg != null) { if (msg.target == null) { return; } if (me.mLogging!= null) me.mLogging.println( ">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what ); msg.target.dispatchMessage(msg); if (me.mLogging!= null) me.mLogging.println( "<<<<< Finished to " + msg.target + " " + msg.callback); msg.recycle(); } } }
可以看到，它内部起了一个死循环，并不断的从队列中获取Message, 然后通过调用 msg.target.dispatchMessage(msg); 方法将消息分发出去， 这里的target就是之前指定处理该消息的Handler, 从这里也可以看出，一个界面可以有很多个Handler，因为最终消息都会被分发到它所指定的那个Handler上面去。
2.2 dispatchMessage
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
这里首先对Message的callback做了判断，如果它自身的callback不为空，就调用handleCallback处理该消息，那什么时候Message的callback不为空呢？其实是在调用Handler的post(Runnable r)方法发送消息的时候：
/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ 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; }
那这个mCallback是个什么东西呢，它其实是Handler的构造方法中传入的用于处理具体msg的一个类：
/** * Use the provided {@link Looper} instead of the default one and take a callback * interface in which to handle messages. Also set whether the handler * should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * * @param looper The looper, must not be null. * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }
接下来就很简单了，如果mCallback不为null，就调用了mCallback的handleMessage来处理消息，如果mCallback为null，就直接调用Handler的handleMessage来处理消息，注意到，Handle本身的handleMessage()方法其实是个空方法：
/** * Subclasses must implement this to receive messages. */ public void handleMessage(Message msg) { }
我们一般在使用Handler在处理Message的时候都会将其重写。
总结：整个执行流程就是：
1.Handler先将指定的Message压入到Looper对象的MessageQueue中；
2.Looper对象会调用loop()方法不断的从MessageQueue中获取消息；
3.当拿到一条消息后，会根据该Message的target选择合适的Handler并调用其handleMessage()方法对消息进行处理。
参考链接：