Android技术21：Android异步消息处理线程

Android异步消息处理线程，该线程一直处于无限循环之中，每次从Message Queue中读取消息，然后回调消息处理的方法，Handler的HandlerMessage中处理消息。如果消息队列为空，该线程就挂，等待消息队列中有消息进来，就唤醒线程。

1.Android异步线程内部结构

在线程内部有一个或者多个Handler对象，外部程序通过Handler对象向线程发送异步消息，消息经过Handler传递到Message Queue对象中，每个线程内部只包含一个一个消息队列对象，线程主执行函数从消息队列中读取消息，并回调Handler对象方法handlerMessage()。

2.Thread Local Storage

程序员通过Looper类的静态方法prepare()为线程创建消息队列对象。

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

1   private static void prepare(boolean quitAllowed) {
2         if (sThreadLocal.get() != null) {
3             throw new RuntimeException("Only one Looper may be created per thread");
4         }
5         sThreadLocal.set(new Looper(quitAllowed));
6     }

线程局部对象TLS,该对象是通过sThreadLocal的set方法设置进去。

3.Looper

Looper对象作用：一是创建消息队列，二是让当前线程提供静态方法loop()进入循环，并从消息队列中读取消息。

1     private Looper(boolean quitAllowed) {
2         mQueue = new MessageQueue(quitAllowed);
3         mRun = true;
4         mThread = Thread.currentThread();
5     }

当需要把一个线程变为异步消息处理线程时，应该在Thread类的run()函数中先调用Looper.prepare()为线程创建一个消息队列对象Message Queue.然后调用Looper.loop()使当前线程进入消息处理循环。

 1   public static void loop() {
 2         final Looper me = myLooper();
 3         if (me == null) {
 4             throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");
 5         }
 6         final MessageQueue queue = me.mQueue;
 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 (;;) {
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);
28
29             if (logging != null) {
30                 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
31             }
32
33             // Make sure that during the course of dispatching the
34             // identity of the thread wasn‘t corrupted.
35             final long newIdent = Binder.clearCallingIdentity();
36             if (ident != newIdent) {
37                 Log.wtf(TAG, "Thread identity changed from 0x"
38                         + Long.toHexString(ident) + " to 0x"
39                         + Long.toHexString(newIdent) + " while dispatching to "
40                         + msg.target.getClass().getName() + " "
41                         + msg.callback + " what=" + msg.what);
42             }
43
44             msg.recycle();
45         }

4.Message Queue

从上面看出通过queue.next(),读取一条消息，但是在Framework中并没有消息队列对象，Message Queue两个主要函数读取消息和添加消息，分别为next(),enquenceMessage().但是真正实现并不是在Framework层而是通过JNI在C代码中实现。

1     private native static int nativeInit();
2     private native static void nativeDestroy(int ptr);
3     private native static void nativePollOnce(int ptr, int timeoutMillis);
4     private native static void nativeWake(int ptr);

5.Handler

在构造Handler对象前，必须经过执行过Looper.prepare(),在Looper.loop()函数中，不同的Message对应不同的Handler对象，从而回调不同的handlerMessage()函数。

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