Android 的消息機制主要由 Handler,Looper,MessageQueue,Message 等組成,而 Handler 的運行主要依賴后三者;

源碼分析

Handler 的消息處理主要有五個部分, Message, Handler, MessageQueue, Looper, Threadlocal.

在了解到這些基本的概念以后我們就來深入的看一看 Handler 的工作機制.

MessageQueue 的工作原理

MessageQueue 消息隊列是通過一個單鏈表的數(shù)據(jù)結(jié)構(gòu)來維護消息鏈表.下面最主要看 enqueueMessage 方法和 next() 方法.

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();
            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;
            }

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

可以看出來,這個方法主要是根據(jù)時間順序向表單中插入一條消息.
那么 next() 又是用來干什么呢?
我們繼續(xù)看下去:

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // 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) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                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);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // 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)) {
                    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(TAG, "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;

            // 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;
        }
    }

在 next 中有一個關(guān)鍵方法 for (;;) 對就是這個死循環(huán),慢慢梳理代碼我們不難看出如果有消息返回就從鏈表中移除.沒有消息的時候就會一直柱塞在這里

Looper

在一個 Android 啟動應(yīng)用的時候,會創(chuàng)建一個主線程, 也就是UI線程.而這個主線程 ActivityThread 中的一個靜態(tài)的 main 方法. 這個 main 方法也就是我們應(yīng)用程序的入口點. 我們來簡單的看一下這個 main 方法.

public static void main(String[] args) {

    ......

    Looper.prepareMainLooper();

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

    ......

    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

從上面的方法中我們可以看出來,使用了 Looper.prepareMainLooper() 方法為主線程創(chuàng)建了 Looper 以及 MessageQueue, 并通過Looper.loop() 來開心主線程的消息循環(huán).

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

這個方法中調(diào)用了 prepare(false);方法和 myLooper(); 方法

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));
}

在這里 sThreadLocal 對象保存了一個 Looper 對象,以防止被調(diào)用兩次.sThreadLocal 對象是 ThreadLocal 類型，因此保證了每個線程中只有一個 Looper 對象。
那么 Looper 到底干了什么?

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

果不其然,在 Looper 的構(gòu)造函數(shù)中創(chuàng)建一個 MessageQueue 對象和保存了當前的線程.從之前的代碼中可以看出一個線程只能有一個 Looper 對象,而 MessageQueue 又是在 Looper 構(gòu)造函數(shù)中創(chuàng)建出來的,因此每一個線程也只有一個 MessageQueue 對象;
還有一個 prepare 的重載方法;

public static void prepare() {
    prepare(true);
}

prepare() 僅僅是對 prepare(boolean quitAllowed) 的封裝,這里就是解釋了主線程不需要調(diào)用 Looper.prepare() 方法了.因為主線程在啟動的時候已經(jīng)很貼心的幫我們調(diào)用了啊!

然后在說說我們在 Looper.perpareMainLooper() 方法中的 myLooper()

/**
 * 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();
}

這串英文是什么意思呢? 讓我們請出我們可愛谷歌娘:返回與當前線程關(guān)聯(lián)的Looper對象。 如果調(diào)用線程未與Looper關(guān)聯(lián)，則返回null。
有沒有很清楚一下就明白了.就是取出我們當前線程中的 Looper 對象,保存在 sMainLooper 瞧瞧這命名方式多簡單易懂;

我是不是忘記了什么?
哦!對了還有一個方法: Looper.loop() main 還調(diào)用了這個方法是用來干什么的呢?

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 (;;) {
            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
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            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.recycle();
        }
}

快看是 next() 方法,是不是好像在剛剛的 MessageQueue 方法中看到了?
對就是這樣,從這個方法里進入一個無限循環(huán)，不斷的從 MessageQueue 的 next 方法獲取消息,而next方法是一個阻塞操作，當沒有消息的時候就一直在阻塞,當有消息通過 msg.target.dispatchMessage(msg); 這里的msg.target其實就是發(fā)送給這條消息的Handler對象。那么我們就去看看 Handle 又干了什么;

Handler

構(gòu)造方法

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

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

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

怎么都要傳遞 Looper 我們沒有怎么辦,那就看看不需要傳遞 Looper 的構(gòu)造函數(shù)

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;
    mAsynchronous = async;
}

什么? 當 Looper 為 null 竟然拋出了 Can't create handler inside thread that has not called Looper.prepare() 異常,這樣我們知道為什么在子線程中使用 Handler 時要手動調(diào)用 Looper.prepare() 方法了,原來是用來創(chuàng)建一個 Looper 對象.那么主線程為什么不用呢?因為服務(wù)周到的主線程已經(jīng)在創(chuàng)建的時候就自己調(diào)用了 Looper.prepare() 方法了.

Handler 的工作主要事實包含發(fā)送和接收過程.其中 post 和 send 的一系列方法主要是用倆發(fā)送消息.但是 post 其實最終也會通過 ssend 的一系列方法來實現(xiàn)的. 而 send 的一系列方法最終會通過 sendMessageAtTime 方法來實現(xiàn).來我們看看 send 的一系列方法:

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }  

    public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
    }

    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);
    }

    public final boolean sendMessageAtFrontOfQueue(Message msg) {
        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, 0);
    }

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

這里我們可以看出,handler 發(fā)送的消息其實就是將消息插入消息隊列,在 Looper 的 loop 方法中從 MessageQueue 中取出并調(diào)用 msg.target.dispatchMessage(msg) ;而這個其實就是就是在調(diào)用 Handler 的 dispatchMessage(msg) :

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

先最一個非空判斷,非空時調(diào)用了 handleCallback(msg); 來處理消息,那么 msg.callback 是什么東西啊? 其實這里的 msg.callback 就是一個 Runnable對象, 也就是 Handler 發(fā)送過來的 post 對象.先看看 post 的對象的方法;

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

 public final boolean postAtTime(Runnable r, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}


public final boolean postDelayed(Runnable r, long delayMillis)
{
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}


public final boolean postAtFrontOfQueue(Runnable r)
{
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

好了我們在看看 msg.callback 非空時 handleCallback(msg) 是做了什么:

private static void handleCallback(Message message) {
    message.callback.run();
}

emmmmm... 果然就是很簡單的回調(diào)了 Runnable 對象的 run 方法. 其實吧我們?nèi)タ纯?Activity 中的 runOnUiThread 和 View 中的 postDelayed 方法也是使用了同樣的原理，我們先看看 runOnUiThread 方法:

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

View 的 postDelayed 方法:

public boolean postDelayed(Runnable action, long delayMillis) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.postDelayed(action, delayMillis);
    }
    // Assume that post will succeed later
    ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);
    return true;
}

實質(zhì)上都是在 UI 線程中執(zhí)行 Runnable 中的 run 方法.

在回來看看 msg.callback 為空的時候會對 mCallback 進行非空判斷,而 mCallback 又使用一個接口的引用:

/**
 * Callback interface you can use when instantiating a Handler to avoid
 * having to implement your own subclass of Handler.
 *
 * @param msg A {@link android.os.Message Message} object
 * @return True if no further handling is desired
 */
public interface Callback {
    public boolean handleMessage(Message msg);
}

原來 CallBack 其實就是另一種使用 Handler 的方式啊, 看來既可以派生子類重寫 handleMessage() 的方法也可以通過設(shè)置 CallBack 來實現(xiàn).

總結(jié)

首先在主線程創(chuàng)建一個 Handler 對象 ，并重寫 handleMessage（）方法。然后當在子線程中需要進行更新 UI 的操作，我們就創(chuàng)建一個 Message 對象，并通過 handler 發(fā)送這條消息出去。之后這條消息被加入到 MessageQueue 隊列中等待被處理，通過 Looper 對象會一直嘗試從 MessageQueue 中取出待處理的消息，最后分發(fā)會 Handler 的 handlerMessage() 方法中。

最后來一張我話的流程圖:

Handler流程.png

有沒有很美觀.
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