Looper: Looper,Handler,MessageQueue三者之间的联系

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：

1. 郭神-Android异步消息处理机制完全解析，带你从源码的角度彻底理解

2.Android源代码在线

3.Android中为什么主线程不会因为Looper.loop()方法造成阻塞