淘宝(阿里百川)手机客户端开发日记第九篇 Looper详解

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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