什么是Handler(三)

 1 public static void loop() {  //静态方法
 2         final Looper me = myLooper(); //myLooper是根据已知的线程对象取出对应的Looper对象
 3         if (me == null) { //判断取出的Looper对象是否为空
 4             throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");
 5         }
 6         final MessageQueue queue = me.mQueue; //取出Looper的消息队列
 7 
 8         // Make sure the identity of this thread is that of the local process,
 9         // and keep track of what that identity token actually is.
10         Binder.clearCallingIdentity();
11         final long ident = Binder.clearCallingIdentity();
12 
13         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
14             Message msg = queue.next(); // might block 
15             if (msg == null) {
16                 // No message indicates that the message queue is quitting.
17                 return;
18             }
19 
20             // This must be in a local variable, in case a UI event sets the logger
21             Printer logging = me.mLogging;
22             if (logging != null) {
23                 logging.println(">>>>> Dispatching to " + msg.target + " " +
24                         msg.callback + ": " + msg.what);
25             }
26 
27             msg.target.dispatchMessage(msg);//查看Message.java源码, 
28             //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
29            //查看Handler的dispatchMessage方法, 得知用handlerMessage处理消息
30 
31             if (logging != null) {
32                 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
33             }
34 
35             // Make sure that during the course of dispatching the
36             // identity of the thread wasn‘t corrupted.
37             final long newIdent = Binder.clearCallingIdentity();
38             if (ident != newIdent) {
39                 Log.wtf(TAG, "Thread identity changed from 0x"
40                         + Long.toHexString(ident) + " to 0x"
41                         + Long.toHexString(newIdent) + " while dispatching to "
42                         + msg.target.getClass().getName() + " "
43                         + msg.callback + " what=" + msg.what);
44             }
45 
46             msg.recycle();
47         }
48     }