Android訊息機制不完全解析（上） .

轉自：http://blog.csdn.net/a220315410/article/details/9857225?utm_source=tuicool&utm_medium=referral

Handler和Message是Android開發者常用的兩個API，我一直對於它的內部實現比較好奇，所以用空閒的時間，閱讀了一下他們的原始碼。

   相關的Java Class：

    相關的C++ Class：

    Message類可以說是最簡單的，主要提供了一些成員，用以儲存訊息資料。

    public int what;//用以表示訊息類別

    public int arg1;//訊息資料

    public int arg2;//訊息資料

    public Object obj;//訊息資料
    

    /*package*/ long when;//訊息應該被處理的時間
    
    /*package*/ Bundle data;//訊息資料
    
    /*package*/ Handler target;//處理這個訊息的handler
    
    /*package*/ Runnable callback;//回撥函式
    
    // sometimes we store linked lists of these things
    /*package*/ Message next;//形成連結串列，儲存Message例項

    值得一提的是，Android提供了一個簡單，但是有用的訊息池，對於Message這種使用頻繁的型別，可以有效的減少記憶體申請和釋放的次數，提高效能。

    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) {//訊息池不為空，則從訊息池中獲取例項
                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) {//訊息池大小未滿，則放入訊息池
                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;
    }

    小結：

1. Message的核心在於它的資料域，Handler根據這些內容來識別和處理訊息
2. 應該使用Message.obtain（或者Handler.obtainMessage)函式獲取message例項

    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可以攔截Handler處理訊息，之後會在dispatchMessage函式中，大展身手
        mAsynchronous = async;//設定handler的訊息為非同步訊息，暫時先無視這個變數
    }

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

     private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;//一會我們會看到callback非空的message和callback為空的mesage在處理時的差異
        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);
    }

    //最終都會呼叫這個函式，把message入列
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);//Handler的<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">mAsynchronous屬性，決定了msg是否為asynchronous，稍後在MessageQueue.next函式中，可以看到</span><span style="FONT-FAMILY: Arial, Helvetica, sans-serif">asynchronous對於訊息處理的影響</span><span style="FONT-FAMILY: Arial, Helvetica, sans-serif">
</span>        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

    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不為null，則執行
            handleCallback(msg);
        } else {
            if (mCallback != null) {//如果Hanlder的mCallback成員不為null，則呼叫
                if (mCallback.handleMessage(msg)) {//如果handleMessage返回值為true，則攔截訊息
                    return;
                }
            }
            handleMessage(msg);//處理訊息
        }
    }

    同樣從建構函式看起：

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//每個Looper有一個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);//後臺執行緒的looper都允許退出
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");//每個執行緒只能有一個Looper
        }
        sThreadLocal.set(new Looper(quitAllowed));//把例項儲存到TLS（Thread Local Save），僅有每個執行緒訪問自己的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);//主執行緒的lopper不可以退出
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

    因為是私有的建構函式，所以理論上來說只能通過prepare和prepareMainLooper兩個函式來例項化Looper，但是google的註釋也說的很清楚：prepareMainLooper（）應該由系統呼叫（有興趣的同學可以去看看AtivityThread類的main函式），所以，應用開發者可以使用的只剩下prepare函式。
    好了，Looper的例項是構造出來，但是如何獲取構造出來的例項呢？

    /** 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();
    }

     現在，我們應該知道如何防止Handler例項化的時候，丟擲RuntimeException：在守護執行緒中例項化Handler之前，需要先呼叫Looper.perpare函式來構造Looper例項。

     然後，重頭戲來了： 

    /**
     * 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) {//呼叫looper之前，需要先呼叫perpare，否則您懂的...
            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();//不太明白這個函式，但不是重點可以無視
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block 獲取一個下一個訊息，如果當前沒有要處理的訊息，則block，之後我們會看到這個API的實現
            if (msg == null) {//呼叫了MessgeQueu的quit函式後，MessageQueue.next會返回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) {//藉助logging我們可以列印Looper中處理的訊息
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);//呼叫handler處理訊息

            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();//選擇性無視
            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();//回收訊息到訊息池
        }
    }

    Looper.loop()函式是Looper類的核心函式，主要迴圈進行兩個操作：

1. 從MessageQueue中獲取一個訊息，當前沒有訊息需要處理時，則block
2. 呼叫message的Handler（target）處理訊息

    基本上，我們可以把Looper理解為一個死迴圈，Looper開始work以後，執行緒就進入了以訊息為驅動的工作模型。

    小結：

1. 每個執行緒最多可以有一個Looper。
2. 每個Looper有且僅有一個MessageQueue
3. 每個Handler關聯一個MessageQueue，由該MessageQueue關聯的Looper執行（呼叫Hanlder.dispatchMessage)
4. 每個MessageQueue可以關聯任意多個Handler
5. Looper API的呼叫順序：Looper.prepare >> Looper.loop >> Looper.quit
6. Looper的核心函式是Looper.loop，一般loop不會返回，直到執行緒退出，所以需要執行緒完成某個work時，請傳送訊息給Message（或者說Handler）

MessageQueue

    

    MessageQueue類是唯一包含native函式的類，我們先大致看一下，稍後C++的部分在詳細解釋:

    private native void nativeInit();    //初始化
    private native void nativeDestroy(); //銷燬
    private native void nativePollOnce(int ptr, int timeoutMillis); //等待<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">timeoutMillis指定的時間</span>
    private native void nativeWake(int ptr);//喚醒<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">nativePollOnce的等待</span>

    然後，我們再從建構函式看起：

    
    Message mMessages;//資料域mMessages的型別雖然是Message，但是因為Message.next資料域的原因，其實mMessage是連結串列的第一個元素

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

    對應的，在銷燬的時候：

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

    

    此外，MessageQueue提供了一組函式（e.g. hasMessage, removeMessage)來查詢和移除待處理的訊息，我們在前面的Handler類上看到的對應函式的實現就是呼叫這組函式。

    接下來，看看enqueueMessage函式，Handler函式就是呼叫這個函式把message放到MessageQueue中：

    final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {//檢查msg是否在使用中，一會我們可以看到MessageQueue.next()在返回前通過Message.makeInUse函式設定msg為使用狀態，而我們之前看到過Looper.loop中通過呼叫呼叫Message.recycle()，把Message重置為未使用的狀態。
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {//msg必須知道由那個Handler負責處理它
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {//如果已經呼叫MessageQueue.quit，那麼不再接收新的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) {//插到列表頭
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;//當前MessageQueue處於block狀態，所以需要喚醒
            } 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();//當且僅當MessageQueue因為Sync Barrier而block，並且msg為非同步訊息時，喚醒。 關於msg.isAsyncChronous(),請回去看看Handler.enqueueMessage函式和建構函式
                Message prev;
                for (;;) {<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">// 根據when的大小順序，插入到合適的位置</span>
                    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，發現非同步訊息，則不需要喚醒
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }
        }
        if (needWake) {
            nativeWake(mPtr);//喚醒nativeMessageQueue
        }
        return true;
    }
 
    final void quit() {
        if (!mQuitAllowed) {//UI執行緒的Looper訊息佇列不可退出
            throw new RuntimeException("Main thread not allowed to quit.");
        }


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

    關於sync barrier，再補充點解釋： sync barrier是起到了一個阻塞器的作用，它可以阻塞when>它（即執行時間比它晚）的同步訊息的執行，但不影響非同步訊息。sync barrier的特徵是targe為null，所以它只能被remove，無法被執行。MessageQueue提供了下面兩個函式來控制MessageQueue中的sync barrier（如何覺得sync barrier和非同步訊息難以理解的話，選擇性無視就好，因為它們不妨礙我們理解Android訊息機制的原理）：

    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;//其實我覺得這邊應該是needWake = mMessages != null && mMessages.target != null
            }
            p.recycle();
        }
        if (needWake) {
            nativeWake(mPtr);//有需要的話，喚醒nativeMessageQueue
        }
    }

    重頭戲又來了：

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

        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();//不太理解，選擇性無視
            }
            nativePollOnce(mPtr, nextPollTimeoutMillis);//等待nativeMessageQueue返回，最多等待nextPollTimeoutMillis毫秒

            synchronized (this) {
                if (mQuiting) {//如果要退出，則返回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) {//下一個訊息為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());//因為存在sync barrier，僅有非同步訊息可以執行，所以尋在最近的非同步訊息
                }
                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);//訊息還沒到執行的時間，所以我們繼續等待msg.when - now毫秒
                    } else {
                        // Got a message.
                        mBlocked = false;//開始處理訊息了，所以不再是blocked狀態
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;//從連結串列中間移除message
                        } else {
                            mMessages = msg.next;//從連結串列頭移除message
                        }
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        msg.markInUse();//標記msg正在使用
                        return msg;//返回到Looper.loop函式
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;//沒有訊息可以處理，所以無限制的等待
                }

                // 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)) {<span style="FONT-FAMILY: Arial, Helvetica, sans-serif">// 目前無訊息可以處理，可以執行IdleHandler</span>
                    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呼叫一次MessageQueue.next(),只允許呼叫一輪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;//因為執行IdleHandler的過程中，可能有新的訊息到來，所以把等待時間設定為0
        }
    }

              為了方便大家理解Message的工作原理，先簡單描述nativeWake，和natePollonce的作用：

1. nativePollOnce(mPtr, nextPollTimeoutMillis);暫時無視mPtr引數，阻塞等待nextPollTimeoutMillis毫秒的時間返回，與Object.wait(long timeout)相似
2. nativeWake(mPtr)；暫時無視mPtr引數，喚醒等待的nativePollOnce函式返回的執行緒，從這個角度解釋nativePollOnce函式應該是最多等待nextPollTimeoutMillis毫秒

    小結：

1. MessageQueue作為一個容器，儲存了所有待執行的訊息。
2. MessageQueue中的Message包含三種型別：普通的同步訊息，Sync barrier（target = null），非同步訊息（isAsynchronous（） = true）。
3. MessageQueue的核心函式為enqueueMessage和next,前者用於向容器內新增Message，而Looper通過後者從MessageQueue中獲取訊息，並實現無訊息情況下的等待。
4. MessageQueue把Android訊息機制的Java實現和C++實現聯絡起來。

    本來我是想一口氣把java實現和C++實現都寫完的，但是，無奈最近工作和個人事務都比較多，稍後為大家奉上C++實現的解析。