android消息机制

主要有四大类Message, Looper, MessageQueue, Handler

1. 

一个线程有一个Looper，每个Looper都有一个MessageQueue.

2.

MessageQueue是Message的一个容器，是个链表

3. 

Message的结构有几个比较有意思的变量

Runnable callback;  

Handler target;

4.

Looper的关键函数就是一个循环，不停去获取MessageQueue中的Message，执行msg.target.dispatchMessage

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
     ...
        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

          ...

            msg.target.dispatchMessage(msg);

          ...

            msg.recycle();
        }
    }

    /**
     * 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);
        }
    }