android异步处理，分析Handle消息机制

首先看下handle比较常用的用法

• 子线程使用主线程的handle发送一条消息到主线程，这个消息会被加到MessageQueue里。
• 这样就有了loop的概念。looper会不停的从MessageQueue中取消息，发现消息后会调用msg的dispatchMessage方法，就可以在handleMessage的回调中接收到来处理这个消息进行UI操作。

首先看下handle的构造方法

    public Handler(Callback callback, boolean async) {        if (FIND_POTENTIAL_LEAKS) {            final Class<? extends Handler> klass = getClass();            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&                    (klass.getModifiers() & Modifier.STATIC) == 0) {                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +                    klass.getCanonicalName());            }        }        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 = callback;        mAsynchronous = async;    }

Looper.myLooper();获取当前线程的looper实例
mQueue = mLooper.mQueue;拿到looper里的消息队列
callback 是looper发现消息队列里有消息的时候调用dispatchMessage然后回调handleMessage

接下来看looper

    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));    }

prepare方法用来保证一个线程只能有一个looper实例。将looper放到sThreadLocal中，那sThreadLocal又是什么呢。ThreadLocal类为每一个线程都维护了自己独有的变量拷贝。每个线程都拥有了自己独立的一个变量，那就没有任何必要对这些线程进行同步，它们也能最大限度的由CPU调度，并发执行。所以sThreadLocal是一个存放looper的变量，looper.myLooper()也就是从sThreadLocal取的。

接着看loop方法

 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();        }    }

首先取出当前线程的looper然后拿到消息队列开始无限循环，进行遍历发现有消息就进行处理调用msg.target.dispatchMessage(msg); 也就是handle的dispatchMessage方法，没有消息就return。
那Looper是怎样关联MessageQueue呢。

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

looper的构造方法中会创建一个消息队列。

博主水平有限，如有错误的地方，希望大家能指出。谢谢。