Android异步线程，Looper, MessageQueue

Looper类

Looper类有一个静态成员变量mThreadLocal，该类提供线程局部存储（Thread Local Storage），也就是内部存储的变量的作用域是线程，内部使用了Thread.getCurrentThread作为key来获取变量。

调用Looper类的静态方法prepare()，可以为当前线程创建Looper对象，并且把Looper对象放到线程局部存储里，MessageQueue是Looper的成员，他们是聚合关系。

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

Looper的静态方法loop()，来使当前线程进入处理Message的循环。

    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.recycleUnchecked();        }    }

代码中在用死循环来处理MessageQueue中的Message，其中msg.target就是sendMessage的handler，所以target.dispatchMessage就是以下代码

public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);        }    }

调用了handleMessage，也就是我们平常所实现的handler的handleMessage方法。