Looper、Handler、MessageQueue原理及源码解析

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Handler在开发中还是很常用的，通常我们主要用来在子线程中更新UI，其实深入点的话就是涉要另外及Looper、MessageQueue。当然我们也可以让他工作在非UI主线程，怎么用呐？看下边小栗子。

下边我们写个简单的小例子来引入要讲的源码分析。

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private class LooperThread extends Thread {
    @Override
    public void run() {
        Looper.prepare();
        mHandler = new Handler(new Handler.Callback() {
            @Override
            public boolean handleMessage(Message msg) {
                //TODO do somethings
                return false;
            }
        });
        Looper.loop();
    }
}

那么上边的例子Handler工作在哪歌线程中呐？？答案是在LooperThread线程，而非主线程。看看以下Handler构造器源码就能明白。

Looper.java

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public Handler(Callback callback) {
    this(callback, false);
}

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();//①获取当前的Looper
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;//②获取当前的MassageQueue
    mCallback = callback;//③当前的Callback
    mAsynchronous = async;
}

那么为什么在UI主线程中使用Handler没有Looper.prepare和Looper.loop()呐？？答案就是，在创建Activity时候，系统已经为我们调用了Looper.prepareMainLooper()，所以这个时候在我们UI主线程中直接new Handler()时，Handler就工作在主线程，所以我们就可以用来更新UI界面。

ActivityThread.java

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public static void main(String[] args) {
    ......
    Looper.prepareMainLooper();//

    ActivityThread thread = new ActivityThread();
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }

    if (false) {
        Looper.myLooper().setMessageLogging(new
                LogPrinter(Log.DEBUG, "ActivityThread"));
    }

    // End of event ActivityThreadMain.
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    Looper.loop();

    ......
}

Looper.java

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public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

Looper、Handler准备好了，接下来我们要sendMessage()了，最终会调用到queue.enqueueMessage(msg, uptimeMillis)，将Message入队。。。

Handler.java

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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);
}

最终Looper.loop()会从MessageQueue中轮询Message,然后dispatchMessage()回调到Handler的Callback中dispatchMessage()方法。。。

Looper.java

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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;
       ......
       for (;;) {
           Message msg = queue.next(); // might block
           ......
           try {
               msg.target.dispatchMessage(msg);//target即是上边sendMessage()时的handler
           } finally {
               if (traceTag != 0) {
                   Trace.traceEnd(traceTag);
               }
           }
           ......
       }
   }

这样完成了一一个线程的整个的Looper、Handler、MessageQueue流程。。。。