什么是Handler(三)

1.Looper当中loop()方法的作用

2.什么是Message对象的Target

3.处理一个Message的方法

public static void loop() {  //静态方法
        final Looper me = myLooper(); //myLooper是根据已知的线程对象取出对应的Looper对象
        if (me == null) { //判断取出的Looper对象是否为空
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue; //取出Looper的消息队列

        // 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 (;;) {  //此循环是不停的从消息队列中取出消息，如果没有值就会阻塞
            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);//查看Message.java源码, 
            //一个Handler对应一个looper对象,一个looper对应一个MessageQueue对象,使用Handler生成Message, 所生成的Message对象的target属性, 就是该Handler对象
           //查看Handler的dispatchMessage方法, 得知用handlerMessage处理消息

            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();
        }
    }