Android进阶-消息机制

Handler的主要作用是将一个任务切换到某个指定的线程去执行。

Android的消息机制主要涉及三个类:Handler, Looper, MessageQueue：

现在假设一个情景：

有两个线程，线程1和线程2，在线程1中调用Looper.prepare(), 创建一个Handler对象handler，调用Looper.loop()。

在线程2中调用handler.sendMessage()发送消息，那最终这个消息会在线程1中被handler.handleMassage()处理。我们画出内存模型帮助理解：

从根据这个调用顺序来看看这个模型：

1.Looper在线程1中调用prepare()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));
    }

这里的sTreadLocal是Looper的static对象：

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

ThreadLocal对象在不同的线程中会指向不同的Looper对象（java编程思想中有介绍）或者说在不同线程调用sThreadLocal.get()返回的是个该线程关联的Looper对象，使用get(), set()访问。prepare()为线程1创建一个Looper对象，来看看构造方法：

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

Looper内部封装了一个MessageQueue对象。

2.在线程1中创建handler对象：

    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;
    }

从第11行看出Handler中封装了线程1的Looper对象，和消息队列，也就是说handler对象可以在线程2中访问线程1的消息队列，从而向里面添加新消息（后面将看到），我们从内存模型也可以理解这点。但是这个Handler可以被任意线程访问，这就是关键所在。

3.在线程1中调用Looper.loop()：

/**
     * 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) {
            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.recycleUnchecked();
        }
    }

loop（）获取当前线程的Looper对象，开启无限循环调用MessageQueue.next不断获取Message，msg.target.dispatchMessage(msg)；msg.target就是handler对象（下面将看到），我们看看该方法：

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

第10行调用handlerMessage()方法，一定是在线程1中执行。

4.在线程2中调用handler.sendMessage()：

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

该方法最终调用：

    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);
    }

第9行方法：

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

第2行将handler对象保存到msg.target；

第6行，因为handler有线程1MessageQueue对象的引用，这里向该消息队列增加消息。

5.当线程切换到线程1时，loop循环不断获取message对象，调用msg.target.dispatchMessage(msg)；在线程1中处理该消息。这样就实现了，在线程2中调用sendMessage()，在线程1中处理该消息的效果了，是不是很巧妙。