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