Handler Looper MessageQueue 之间的关系

handler在安卓开发中常用于更新界面ui，以及其他在主线程中的操作。内部结构大概图为：

1、handler持有一个Looper对象，这个Looper对象可以是自定义子线程的Looper，也可以是默认MainLooper。Looper主要作用就是不断循环MessageQueue中的Message，回调Handler的dispatchMessage方法。源码：

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);
            }

//这里回调handler中的dispatchMessage方法
            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();
        }

2、Handler还持有MessageQueue对象的引用，用于sendMessage时，将消息添加到MessageQueue中。这样Looper就可以获取到最新的message。Handler中的MessageQueue其实就是Looper的MessageQueue成员属性。

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

所以一个大致的流程就是：

1、Looper在在不断的获取MessageQueue消息，调用handler的dispatchMessage方法

2、Handler.postMessage就是往Looper的MessageQueue中添加Message，并且设置message.target为Handler本身