android Looper类
Android中的Looper类，该类为一个线程维护一个消息队列，用于android线程中进行消息处理。消息并不是直接加入到MessageQueue，而是通过与Looper对象关联MessageQueue.IdleHandler对象添加的。
调用Looper.myQueue方法可以获取当前线程的MessageQueue，MessageQueue通常附属于某一个创建它的线程。
Looper类用来为一个线程开启一个消息循环。默认情况下android中新诞生的线程是没有开启消息循环的。主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环。Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。 Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。
通常是通过Handler对象来与Looper进行交互的。Handler可看做是Looper的一个接口，用来向指定的Looper发送消息及定义处理方法。默认情况下Handler会与其被定义时所在线程的Looper绑定，比如，Handler在主线程中定义，那么它是与主线程的Looper绑定。mainHandler = new Handler() 等价于new Handler（Looper.myLooper()）.
Looper.myLooper()：获取当前进程的looper对象，类似的 Looper.getMainLooper() 用于获取主线程的Looper对象。
在非主线程中直接new Handler() 会报如下的错误: E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() 原因是非主线程中默认没有创建Looper对象，需要先调用Looper.prepare()启用Looper。
Looper.loop(); 让Looper开始工作，从消息队列里取消息，处理消息。
注意：写在Looper.loop()之后的代码不会被执行，这个函数内部应该是一个循环，当调用mHandler.getLooper().quit()后，loop才会中止，其后的代码才能得以运行。
01.public class Looper {
02. private static final boolean DEBUG = false;
03. private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV;
04.
05. // sThreadLocal.get() will return null unless you've called prepare().
06. private static final ThreadLocal sThreadLocal = new ThreadLocal();
07.
08. final MessageQueue mQueue;
09. volatile boolean mRun;
10. Thread mThread;
11. private Printer mLogging = null;
12. private static Looper mMainLooper = null;
13. /** Initialize the current thread as a looper.
14. * This gives you a chance to create handlers that then reference
15. * this looper, before actually starting the loop. Be sure to call
16. * {@link #loop()} after calling this method, and end it by calling
17. * {@link #quit()}.
18. */
19. public static final void prepare() {
20. if (sThreadLocal.get() != null) {
21. throw new RuntimeException("Only one Looper may be created per thread");
22. }
23. sThreadLocal.set(new Looper());
24.}
25./** Initialize the current thread as a looper, marking it as an application's main
26. * looper. The main looper for your application is created by the Android environment,
27. * so you should never need to call this function yourself.
28. * {@link #prepare()}
29. */
30.
31. public static final void prepareMainLooper() {
32. prepare();
33. setMainLooper(myLooper());
34. if (Process.supportsProcesses()) {
35. myLooper().mQueue.mQuitAllowed = false;
36. }
37. }
38.
39.
40. private synchronized static void setMainLooper(Looper looper) {
41. mMainLooper = looper;
42. }
43.
44. /** Returns the application's main looper, which lives in the main thread of the application.
45. */
46. public synchronized static final Looper getMainLooper() {
47. return mMainLooper;
48. }
49.public static final void loop() {
50. Looper me = myLooper();
51. MessageQueue queue = me.mQueue;
52. while (true) {
53. Message msg = queue.next(); // might block
54. if (msg != null) {
55. if (msg.target == null) {
56. return;
57. }
58. if (me.mLogging!= null) me.mLogging.println(
59. ">>>>> Dispatching to " + msg.target + " "
60. + msg.callback + ": " + msg.what
61. );
62. msg.target.dispatchMessage(msg);
63. if (me.mLogging!= null) me.mLogging.println(
64. "<<<<< Finished to " + msg.target + " "
65. + msg.callback);
66. msg.recycle();
67. }
68. }
69. }
70./**
71. * Return the Looper object associated with the current thread. Returns
72. * null if the calling thread is not associated with a Looper.
73. */
74. public static final Looper myLooper() {
75. return (Looper)sThreadLocal.get();
76. }