概述

Handler无疑是Android的异步消息处理机制的核心，这遍文章将通过分析Handler的源码来系统的解析Android异步消息机制。

Handler的使用

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {

            switch (msg.arg1){
                case 1:
                    if (BuildConfig.DEBUG) Log.d(TAG, msg.getData().getString("firstName"));
                    break;
                case 2:
                    if (BuildConfig.DEBUG) Log.d(TAG, msg.getData().getString("lastName"));
                    break;
                default:
                    break;
            }
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                Message msg1 = new Message();
                msg1.arg1 = 1;
                Bundle bundle1 = new Bundle();
                bundle1.putString("firstName", "xiao");
                msg1.setData(bundle1);
                mHandler.sendMessage(msg1);

                Message msg2 = new Message();
                msg2.arg1 = 2;
                Bundle bundle2 = new Bundle();
                bundle2.putString("lastName", "zhang");
                msg2.setData(bundle2);
                mHandler.sendMessage(msg2);
            }
        }).start();
    }
}

打印结果：

image.png

问题来了：

1.消息是怎样传递到handleMessage()方法里面的。
2.多个消息是如何保证正确的顺序。

首先看一下Handler()

    public Handler() {
        this(null, false);
    }

    public Handler(Callback callback, boolean async) {
       //省略部分代码
        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;
    }

跟一下Looper.myLooper()

    /**   
     * 返回当前线程绑定的Looper,如果没有绑定，则返回null
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

继续跟一下Looper.prepare()

     /** 为当前线程初始化一个Looper
      *在调用prepare()之后，可以创建一个绑定了该Looper的Handler
      * 通过loop()方法，开启循环
      * 通过quit()方法，结束循环
      */
    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

1."Can't create handler inside thread that has not called Looper.prepare()"，表示在初始化Handler之前必须先调用Looper .prepare()。但是通常在使用Handler的时候并没有去调用Looper.prepare()，这是因为在ActivityThread（app启动的时候会初始化）中已经调用了，如果想看一下，可以看ActivityThread类的main()方法，这里点到为止，只要记住一点：在非主线程中初始化Handler,必须先调用Looper.prepare()。
2."Only one Looper may be created per thread"，表示Looper和线程是一对一的。

接下来我们看一下Handler的sendMessageAtTime(Message msg, long uptimeMillis),sendMessage最终调用的都是该方法

    /**
     * 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);
    }

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

通过sendMessage将消息存入queue中，并且通过一个时间参数uptimeMillis来表示延时。

跟一下enqueueMessage(msg,uptimeMillis)

    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();
/*************************重点关注*****************************/
            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;
            }
/*************************重点关注*****************************/

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

两个“重点关注”中间的代码表示通过时间参数when(也就是之前传入的uptimeMillis)，将msg按照时间顺序插入到queue中。
接下来，我们通过Looper.loop()看一下msg是怎么被取出的。

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    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 (;;) {//--------------------------->(1)
            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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);//---------------------------------->(2)
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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();
        }
    }

(1) 表示死循环，queue.next()是一个阻塞的方法，一直等待消息。
(2) msg.target表示对应的Handler。

跟一下Handler.dispatchMessage(Message msg)

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {//msg.callback对应public static Message obtain(Handler h, Runnable callback){...}
            handleCallback(msg);
        } else {
            if (mCallback != null) {//mCallback对应public Handler(Looper looper, Callback callback) {...}
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

如果Msg有初始化一个callback则执行Msg.callback.handleCallback(Message msg)，否则进行下面的判断
如果Handler有初始化一个callback则执行Handler.callback.handleCallback(Message msg)，否则执行Handler.handleMessage(Message msg)。

以上就是从Handler.sendMessage(Message msg)到Handlerd.handleMessage(Message msg)的全部过程。流程图如下

Handler (1).png

发送异步消息的其它方式：

1. Handler的post(Runnable r)方法

    public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

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

    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);
    }

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

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

最终调用的MessageQueue.enqueueMessage(msg, uptimeMillis)。

2. View的post()方法

    public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }

        // Postpone the runnable until we know on which thread it needs to run.
        // Assume that the runnable will be successfully placed after attach.
        getRunQueue().post(action);
        return true;
    }

最终是调用上面的Handler.post(runnable r)。

3. Activity的runOnUiThread()方法

    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

如果当前线程是UI线程，直接运行。如果当前线程不是UI线程，则通过Handler.post(Runnable r)发送到UI线程。

至此，Handler的解析全部完成：）