Android消息机制不完全解析（上）

    Handler和Message是Android开发者常用的两个API，我一直对于它的内部实现比较好奇，所以用空闲的时间，阅读了一下他们的源码。

   相关的Java Class：

• android.os.Message
• android.os.MessageQueue
• android.os.Looper
• android.os.Handler

    相关的C++ Class：

• android.NativeMessageQueue
• android.Looper
• android.LooperCallback
• android.SimpleLooperCallback
• android.Message
• android.MessageHandler

首先，来看看这些类之间的关系：

首先，让我们从相对简单的java实现开始看起：

Message

    Message类可以说是最简单的，主要提供了一些成员，用以保存消息数据。

    public int what;//用以表示消息类别

    public int arg1;//消息数据

    public int arg2;//消息数据

    public Object obj;//消息数据
    

    /*package*/ long when;//消息应该被处理的时间

    /*package*/ Bundle data;//消息数据

    /*package*/ Handler target;//处理这个消息的handler

    /*package*/ Runnable callback;//回调函数

    // sometimes we store linked lists of these things
    /*package*/ Message next;//形成链表，保存Message实例

值得一提的是，Android提供了一个简单，但是有用的消息池，对于Message这种使用频繁的类型，可以有效的减少内存申请和释放的次数，提高性能。

    private static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;

    private static final int MAX_POOL_SIZE = 50;

    /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {//消息池不为空，则从消息池中获取实例
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }

    /**
     * Return a Message instance to the global pool.  You MUST NOT touch
     * the Message after calling this function -- it has effectively been
     * freed.
     */
    public void recycle() {
        clearForRecycle();

        synchronized (sPoolSync) {
            if (sPoolSize < MAX_POOL_SIZE) {//消息池大小未满，则放入消息池
                next = sPool;
                sPool = this;
                sPoolSize++;
            }
        }
    }
    /*package*/ void clearForRecycle() {
        flags = 0;
        what = 0;
        arg1 = 0;
        arg2 = 0;
        obj = null;
        replyTo = null;
        when = 0;
        target = null;
        callback = null;
        data = null;
    }

小结：

1. Message的核心在于它的数据域，Handler根据这些内容来识别和处理消息
2. 应该使用Message.obtain（或者Handler.obtainMessage)函数获取message实例

Handler

首先看看构造函数：

    public interface Callback {
        public boolean handleMessage(Message msg);
    }

    public Handler() {
        this(null, 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();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback; //使用Callback可以拦截Handler处理消息，之后会在dispatchMessage函数中，大展身手
        mAsynchronous = async;//设置handler的消息为异步消息，暂时先无视这个变量
    }

Handler的构造函数最主要的就是初始化成员变量：mLooper和mQueue。 这边需要注意的一个问题是：Looper.myLooper（）不能返回null，否则抛出RuntimeExeception。稍后详解Looper.myLooper();函数在何种情况下会抛出异常。

Handler.obtainMessage系列的函数都会调用Message类中对应的静态方法，从消息池中获取一个可用的消息实例。典型实现如下：

    public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

Handler.post系列和send系列函数最终都会调用enqueueMessage函数，把message入列，不同之处在于post系列函数会以Runable参数构建一个Message实例。

     private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;//一会我们会看到callback非空的message和callback为空的mesage在处理时的差异
        return m;
    }

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

    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 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);
    }

    //最终都会调用这个函数，把message入列
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);//Handler的mAsynchronous属性，决定了msg是否为asynchronous，稍后在MessageQueue.next函数中，可以看到asynchronous对于消息处理的影响        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

除了这些之外，Handler还提供了hasMessage系列和removeMessages系列函数用以管理Handler对应的MessageQueue中的消息。

接下来主角登场，Handler.dispatchMessage:

    private static void handleCallback(Message message) {
        message.callback.run();
    }
    /**
     * Subclasses must implement this to receive messages.
     */
    public void handleMessage(Message msg) {
    }

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {//message的callback不为null，则执行
            handleCallback(msg);
        } else {
            if (mCallback != null) {//如果Hanlder的mCallback成员不为null，则调用
                if (mCallback.handleMessage(msg)) {//如果handleMessage返回值为true，则拦截消息
                    return;
                }
            }
            handleMessage(msg);//处理消息
        }
    }

注释应该比较清楚，不多说。 小结：

1. Handler类最为核心的函数是enqueueMessage和dispatcherMessage,前者把待处理的消息放入MessageQueue，而Looper调用后者来处理从MessageQueue获取的消息。
2. callback不为null（通过post系列函数添加到消息队列中）的message无法被拦截，而callback为null的函数可以被Handler的mCallback拦截

Looper

    同样从构造函数看起：

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//每个Looper有一个MessageQueue
        mRun = true;
        mThread = Thread.currentThread();
    }

     ** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);//后台线程的looper都允许退出
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");//每个线程只能有一个Looper
        }
        sThreadLocal.set(new Looper(quitAllowed));//把实例保存到TLS（Thread Local Save），仅有每个线程访问自己的Looper
    }

    /**
     * Initialize the current thread as a looper, marking it as an
     * application's main looper. The main looper for your application
     * is created by the Android environment, so you should never need
     * to call this function yourself.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);//主线程的lopper不可以退出
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

因为是私有的构造函数，所以理论上来说只能通过prepare和prepareMainLooper两个函数来实例化Looper，但是google的注释也说的很清楚：prepareMainLooper（）应该由系统调用（有兴趣的同学可以去看看AtivityThread类的main函数），所以，应用开发者可以使用的只剩下prepare函数。
    好了，Looper的实例是构造出来，但是如何获取构造出来的实例呢？

    /** Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }
    /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static Looper myLooper() {
        return sThreadLocal.get();
    }

现在，我们应该知道如何防止Handler实例化的时候，抛出RuntimeException：在守护线程中实例化Handler之前，需要先调用Looper.perpare函数来构造Looper实例。

     然后，重头戏来了： 

    /**
     * Quits the looper.
     *
     * Causes the {@link #loop} method to terminate as soon as possible.
     */
    public void quit() {
        mQueue.quit();
    }

    /**
     * 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) {//调用looper之前，需要先调用perpare，否则您懂的...
            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();//不太明白这个函数，但不是重点可以无视
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block 获取一个下一个消息，如果当前没有要处理的消息，则block，之后我们会看到这个API的实现
            if (msg == null) {//调用了MessgeQueu的quit函数后，MessageQueue.next会返回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) {//借助logging我们可以打印Looper中处理的消息
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);//调用handler处理消息

            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();//选择性无视
            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.recycle();//回收消息到消息池
        }
    }

Looper.loop()函数是Looper类的核心函数，主要循环进行两个操作：

1. 从MessageQueue中获取一个消息，当前没有消息需要处理时，则block
2. 调用message的Handler（target）处理消息

    基本上，我们可以把Looper理解为一个死循环，Looper开始work以后，线程就进入了以消息为驱动的工作模型。

    小结：

1. 每个线程最多可以有一个Looper。
2. 每个Looper有且仅有一个MessageQueue
3. 每个Handler关联一个MessageQueue，由该MessageQueue关联的Looper执行（调用Hanlder.dispatchMessage)
4. 每个MessageQueue可以关联任意多个Handler
5. Looper API的调用顺序：Looper.prepare >> Looper.loop >> Looper.quit
6. Looper的核心函数是Looper.loop，一般loop不会返回，直到线程退出，所以需要线程完成某个work时，请发送消息给Message（或者说Handler）

MessageQueue

    MessageQueue类是唯一包含native函数的类，我们先大致看一下，稍后C++的部分在详细解释:

    private native void nativeInit();    //初始化
    private native void nativeDestroy(); //销毁
    private native void nativePollOnce(int ptr, int timeoutMillis); //等待timeoutMillis指定的时间
    private native void nativeWake(int ptr);//唤醒nativePollOnce的等待

然后，我们再从构造函数看起：

    Message mMessages;//数据域mMessages的类型虽然是Message，但是因为Message.next数据域的原因，其实mMessage是链表的第一个元素

    MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        nativeInit();//初始化nativeMessageQueue
    }

对应的，在销毁的时候：

    @Override
    protected void finalize() throws Throwable {
        try {
            nativeDestroy();//销毁nativeMessageQueue
        } finally {
            super.finalize();
        }
    }

    此外，MessageQueue提供了一组函数（e.g. hasMessage, removeMessage)来查询和移除待处理的消息，我们在前面的Handler类上看到的对应函数的实现就是调用这组函数。

    接下来，看看enqueueMessage函数，Handler函数就是调用这个函数把message放到MessageQueue中：

    final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {//检查msg是否在使用中，一会我们可以看到MessageQueue.next()在返回前通过Message.makeInUse函数设置msg为使用状态，而我们之前看到过Looper.loop中通过调用调用Message.recycle()，把Message重置为未使用的状态。
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {//msg必须知道由那个Handler负责处理它
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {//如果已经调用MessageQueue.quit，那么不再接收新的Message
                RuntimeException e = new RuntimeException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w("MessageQueue", e.getMessage(), e);
                return false;
            }

            msg.when = when;
            Message p = mMessages;
            if (p == null || when == 0 || when < p.when) {//插到列表头
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;//当前MessageQueue处于block状态，所以需要唤醒
            } 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();//当且仅当MessageQueue因为Sync Barrier而block，并且msg为异步消息时，唤醒。 关于msg.isAsyncChronous(),请回去看看Handler.enqueueMessage函数和构造函数
                Message prev;
                for (;;) {// 根据when的大小顺序，插入到合适的位置
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {//如果在插入位置以前，发现异步消息，则不需要唤醒
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }
        }
        if (needWake) {
            nativeWake(mPtr);//唤醒nativeMessageQueue
        }
        return true;
    }

    final void quit() {
        if (!mQuitAllowed) {//UI线程的Looper消息队列不可退出
            throw new RuntimeException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuiting) {
                return;
            }
            mQuiting = true;
        }
        nativeWake(mPtr);//唤醒nativeMessageQueue
    }

关于sync barrier，再补充点解释： sync barrier是起到了一个阻塞器的作用，它可以阻塞when>它（即执行时间比它晚）的同步消息的执行，但不影响异步消息。sync barrier的特征是targe为null，所以它只能被remove，无法被执行。MessageQueue提供了下面两个函数来控制MessageQueue中的sync barrier（如何觉得sync barrier和异步消息难以理解的话，选择性无视就好，因为它们不妨碍我们理解Android消息机制的原理）：

    final int enqueueSyncBarrier(long when) {
        // Enqueue a new sync barrier token.
        // We don't need to wake the queue because the purpose of a barrier is to stall it.
        synchronized (this) {
            final int token = mNextBarrierToken++;
            final Message msg = Message.obtain();
            msg.arg1 = token;

            Message prev = null;
            Message p = mMessages;
            if (when != 0) {
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
            }
            if (prev != null) { // invariant: p == prev.next
                msg.next = p;
                prev.next = msg;
            } else {
                msg.next = p;
                mMessages = msg;
            }
            return token;
        }
    }

    final void removeSyncBarrier(int token) {
        // Remove a sync barrier token from the queue.
        // If the queue is no longer stalled by a barrier then wake it.
        final boolean needWake;
        synchronized (this) {
            Message prev = null;
            Message p = mMessages;
            while (p != null && (p.target != null || p.arg1 != token)) {
                prev = p;
                p = p.next;
            }
            if (p == null) {
                throw new IllegalStateException("The specified message queue synchronization "
                        + " barrier token has not been posted or has already been removed.");
            }
            if (prev != null) {
                prev.next = p.next;
                needWake = false;
            } else {
                mMessages = p.next;
                needWake = mMessages == null || mMessages.target != null;//其实我觉得这边应该是needWake = mMessages != null && mMessages.target != null
            }
            p.recycle();
        }
        if (needWake) {
            nativeWake(mPtr);//有需要的话，唤醒nativeMessageQueue
        }
    }

    重头戏又来了：

  final Message next() {
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;

        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();//不太理解，选择性无视
            }
            nativePollOnce(mPtr, nextPollTimeoutMillis);//等待nativeMessageQueue返回，最多等待nextPollTimeoutMillis毫秒

            synchronized (this) {
                if (mQuiting) {//如果要退出，则返回null
                    return null;
                }

                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {//下一个消息为sync barrier
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());//因为存在sync barrier，仅有异步消息可以执行，所以寻在最近的异步消息
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);//消息还没到执行的时间，所以我们继续等待msg.when - now毫秒
                    } else {
                        // Got a message.
                        mBlocked = false;//开始处理消息了，所以不再是blocked状态
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;//从链表中间移除message
                        } else {
                            mMessages = msg.next;//从链表头移除message
                        }
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        msg.markInUse();//标记msg正在使用
                        return msg;//返回到Looper.loop函数
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;//没有消息可以处理，所以无限制的等待
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {// 目前无消息可以处理，可以执行IdleHandler
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf("MessageQueue", "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;//Looper.looper调用一次MessageQueue.next(),只允许调用一轮IdleHandler

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;//因为执行IdleHandler的过程中，可能有新的消息到来，所以把等待时间设置为0
        }
    }

为了方便大家理解Message的工作原理，先简单描述nativeWake，和natePollonce的作用：

1. nativePollOnce(mPtr, nextPollTimeoutMillis);暂时无视mPtr参数，阻塞等待nextPollTimeoutMillis毫秒的时间返回，与Object.wait(long timeout)相似
2. nativeWake(mPtr)；暂时无视mPtr参数，唤醒等待的nativePollOnce函数返回的线程，从这个角度解释nativePollOnce函数应该是最多等待nextPollTimeoutMillis毫秒

    小结：

1. MessageQueue作为一个容器，保存了所有待执行的消息。
2. MessageQueue中的Message包含三种类型：普通的同步消息，Sync barrier（target = null），异步消息（isAsynchronous（） = true）。
3. MessageQueue的核心函数为enqueueMessage和next,前者用于向容器内添加Message，而Looper通过后者从MessageQueue中获取消息，并实现无消息情况下的等待。
4. MessageQueue把Android消息机制的Java实现和C++实现联系起来。

    本来我是想一口气把java实现和C++实现都写完的，但是，无奈最近工作和个人事务都比较多，稍后为大家奉上C++实现的解析。