Android開發 面試必問的Handler訊息機制

最近專案提測了也閑了下來看到Handler就想起面試必問，Handler機制相信大家每個人面試的時候都被問到吧，就來總結一下看看，話不多說先看流體圖：

這個流體圖應該已經把整個Handler訊息機制的流程都涵蓋了，應該算是很直觀了吧，首先最外層我寫了Thread.currentThread()，這說明了一個執行緒里有且僅有一個Looper，所以大家應該注意如果在子執行緒中使用Handler應該要如下寫法：

    @Override
    public void run() {
        Looper.prepare();
        handler = new MyHandler();
        Looper.loop();
    }

    class MyHandler extends Handler {
        @Override
        public void handleMessage(Message message) {
        ...
        }
    }

如果有同學問為什么要這么寫，那么我們就來看看詳細的原始碼決議一下吧，首先看一下Looper.prepare()：

   static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); 

   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));
    }

這里如果有疑問說ThreadLocal是什么東西，其實ThreadLocal是執行緒用來存盤資料的Map，咱們看一下set方法原始碼就知道了：

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

這里是把Looper物件存盤到當前執行緒里面了，當然在創建Looper的同時，還在Looper類中創建了MessageQueue訊息佇列用來存盤Handler發送的Message物件的，不信我們就看一下Looper的建構式吧：

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

Looper到此已經算是準備好了，那么我們再看看Handler創建物件的時候多做了哪些準備呢，那我們就看一下Handler建構式：

    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 " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

如果你還對Looper.myLooper()有疑問，那我們再來看看原始碼：

    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

其實這sThreadLocal.get();就是從當前執行緒中取出之前存的map中的Looper，好我們再看一下get方法原始碼：

    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

現在Handler也準備好了，就可以開始作業了，主要是由Looper.loop()無限回圈讀取MessageQueue中的message物件并且調msg.target.dispatchMessage(msg)方法：

    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
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

        ...
            try {
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
        ...
        }
    }

這里msg.target.dispatchMessage(msg)其實就是handler.dispatchMessage(msg)，我們看一下handler.sendMessage(msg)發送訊息方法的原始碼就知道了：

    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);
    }

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

可以看到msg.target=this就是handler自身，queue.enqueueMessage(msg,upTimeMills)就是向MessageQueue堆疊中存盤msg資料，既然發送訊息原始碼咱們已經看了，那么咱們就來看看handler.dispatchMessage(msg)接收訊息吧：

    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

    @Override
    public void handleMessage(Message message) {

    }

已經到了handlerMessage就不需要我過多講解了吧這就是大家創建Handler時重寫的handlerMessage方法用來接收Massage訊息的，好了整個Handler訊息機制的流程已經全部講解完畢，如果還有什么需要了解的請在下面評論區留言吧，