public final class Looper；
官方的API:
Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.
Most interaction with a message loop is through the Handler class.
这个类用来运行为一个在线程中消息循环，线程默认是不和消息循环关联的。为了创建在Thread中可以对消息循环，调用prepare() 函数在线程中，然后调用loop() 来处理消息直到循环停止。
大部分和消息循环交互是通过Handler 这个类。
官方给出的文档，说明的不是很详细，如果我们深入去理解它，我们只能去挖掘它的内部源代码来分析。应为android是开源的嘛！首先我们先看下Looper这个类的内部代码：
public class Looper {
private static final ThreadLocal sThreadLocal = new ThreadLocal();
//消息队列
final MessageQueue mQueue;
// 当前线程
Thread mThread; private Looper() { mQueue = new MessageQueue(); mThread = Thread.currentThread();
}
public static final void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
} sThreadLocal.set(new Looper());
}
public static final Looper myLooper() { return (Looper)sThreadLocal.get(); }
public static final void loop() { Looper me = myLooper(); //得到当前线程Looper MessageQueue queue = me.mQueue; //得到当前looper对应的消息队列 Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); // 开始循环 while (true) { Message msg = queue.next(); if (msg != null) { if (msg.target == null) { // message的target为0，结束循环 return; } msg.target.dispatchMessage(msg); // 将消息交由message的target(其实就是handler)处理 msg.recycle(); } } } public void quit() { // 结束循环消息 Message msg = Message.obtain(); mQueue.enqueueMessage(msg, 0); } }
public class Handler {
final MessageQueue mQueue; final Looper mLooper; final Callback mCallback; //回调
public Handler() { mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = null; } public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); //立即执行 }
private final Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }
public boolean sendMessageAtTime(Message msg, long uptimeMillis) { boolean sent = false; MessageQueue queue = mQueue; if (queue != null) { msg.target = this; sent = queue.enqueueMessage(msg, uptimeMillis); } else { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); } return sent; }
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } private final void handleCallback(Message message) { message.callback.run(); //执行线程run方法 }
public void handleMessage(Message msg) { //处理消息，由客户端自己去实现调用 }
}
我们从Looper类，和Handler类的源代码，得到很多一些重要的结论：
Looper自己有一个消息队列，这个消息的添加由Handler来发送，然后，Looper自己不断的循环，取出消息，再有Handler的handleMessage方法来处理，但是handleMessage这个方法，是一个空实现，由程序员根据自己的业务逻辑来对消息进行处理。
Looper在android中被广泛的使用到；比如，在一个Activity里，大家知道activity 其实就是一个UI线程，在activity创建期间，它也创建了消息队列和消息循环Looper。这个大家可以到ActivityThread.java中能够找到对应的信息。
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