安卓android.os.Looper.java分析

looper对象是要是一个消息处理的循环，使用的场景是，附着在一个线程里面进行消息处理。常用的方法也是准备消息队列，进入消息处理循环，操作消息队列。

准备方法：prepare

使用looper的第一步，必须调用prepare方法，创建出looper对象
有几种prepare方法，looper创建出来后，通过sThreadLocal保存起来。

一种可以指定looper是否退出

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

一种允许推出的looper

    public static void prepare() {        prepare(true);    }

一种不允许推出的looper

public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

构造方法

构造方法中，初始化一个消息队列。这个是loop主要打交道的对象。

private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

loop方法，主要干活的方法

这个方法在状态机分析时，已经遇到过，死循环，主要是从消息队列中获取消息，然后处理。当消息队列中返回没有消息时，意味着队列已经销毁，循环推出。

public static void loop() {        final Looper me = myLooper();        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }        final MessageQueue queue = me.mQueue;        // Make sure the identity of this thread is that of the local process,        // and keep track of what that identity token actually is.        Binder.clearCallingIdentity();        final long ident = Binder.clearCallingIdentity();        for (;;) {            Message msg = queue.next(); // might block            if (msg == null) {                // No message indicates that the message queue is quitting.                return;            }            // This must be in a local variable, in case a UI event sets the logger            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            msg.target.dispatchMessage(msg);            if (logging != null) {                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);            }            // Make sure that during the course of dispatching the            // identity of the thread wasn't corrupted.            final long newIdent = Binder.clearCallingIdentity();            if (ident != newIdent) {                Log.wtf(TAG, "Thread identity changed from 0x"                        + Long.toHexString(ident) + " to 0x"                        + Long.toHexString(newIdent) + " while dispatching to "                        + msg.target.getClass().getName() + " "                        + msg.callback + " what=" + msg.what);            }            msg.recycle();        }    }

*其他方法*

大多数looper类的方法都是给予messagequeue的方法来来封装，无需再重复分析。