Android 消息机制

要理解原理， read the fucking source

主要涉及到的类：Thread，Handler，Looper，MessageQueue，Message

这几个类的关系图：

发送消息流程图：

1、从HandlerThread入手。

     HandlerThread是android系统提供的类。继承Thread，是一个线程，调用start()方法后。将运行run()方法。

     public void run() {
        mTid = Process.myTid();
        Looper.prepare();// #1
        synchronized (this) {
            mLooper = Looper.myLooper();// #2
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();// #3
        mTid = -1;
    }

　第1个标示中，是调用了Looper的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));// #4
    }

   在代码段的第4个标示中。将Looper实例存放在线程局部变量ThreadLocal中，将Looper和当前线程绑定。

   在代码段的第2个标示中，获取了与当前线程绑定的Looper实例，当前线程就拥有了Looper实例。

   在代码段的第3个标示中，调用了Looper的loop()方法。该方法是一个堵塞方法，loop()方法代码：

    /**
     * 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　＃５
            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);＃６

            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();
        }
    }

　for(;;)是一个死循环，在代码段的标示５中，官方凝视queue.next()方法是一个堵塞方法，也就是会一直等待消息。  

　代码段的标示６中调用handler的dispatchMessage方法。发送消息，这里的handler是怎样和Message关联上的？这里做个埋笔。往下分析。

　也许有人会问，Message取完之后。在哪里唤醒该线程，然后继续循环获取Message呢？没错，是在调用Handler的sendMessage后。向MessageQueue中插入消息的时候唤醒，调用本地方法nativeWake(mPtr)。

 

２、開始发送消息

将Handler和Looper关联的代码：

handlerThread.start();
mLogicHandler = new LogicHandler(handlerThread.getLooper());

Handler构造方法：

public Handler(Looper looper, Callback callback, boolean async) {
     mLooper = looper;
     mQueue = looper.mQueue;
     mCallback = callback;
     mAsynchronous = async;
}

Handler就拥有了Looper和MessageQueue了。

Handler中的发送消息方法：

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);
}

enqueueMessage方法：

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;#7
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);#8
}

代码段中的标示７。将当前handler实例赋值给message的targer，这是上面埋点的解答。

代码段中的标示８。将会调用MessageQueue的。

MessageQueue的enqueueMessage方法

final boolean enqueueMessage(Message msg, long when) {
        if (msg.isInUse()) {
            throw new AndroidRuntimeException(msg + " This message is already in use.");
        }
        if (msg.target == null) {
            throw new AndroidRuntimeException("Message must have a target.");
        }

        boolean needWake;
        synchronized (this) {
            if (mQuiting) {
                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) {
                // 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;
            }
        }
        if (needWake) {
            nativeWake(mPtr);// #９
        }
        return true;
    }

在代码片段标示5中。调用了nativeWake(mPtr)方法，该方法是本地方法，用于唤醒Thread线程。

參考资料：

android的消息队列机制