Android消息机制

一、概述
Android的消息机制主要是指handler的运行机制，它需要与Looper和MessageQueue配合。Handler是message的发送者和处理者，多用于更新UI。MessageQueue是负责存放message的单向链表。Looper负责开启一个MessageQueue，并循环遍历其中的消息。

二、实例分析

Handler handler = new Handler( );

在主线程中此代码没有问题，但是如果运行在子线程，就会报错：Can’t create handler inside thread that has not called Looper.prepare()，我们看下handler的源码：

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;    }

这里我们找到原因了，因为Looper为空，需要创建自己线程的Looper：

 Looper.prepare(); Handler handler = new Handler(); Looper.loop();

prepare的源码如下：

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为这个线程new 了一个Looper，之后还需要让Looper去遍历消息队列， loop()代码如下：

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();        }}

for循环取queue.next()中数据，如果没有数据，就阻塞，直到next()为空。这样有人会问，为什么在主线程中不需要创建Looper。这是因为在主线程中默认已创建了一个Looper用于处理主线程的消息循环。每个线程都有自己的Looper，我们来看一下Looper的创建：

static final ThreadLocat<Looper> sThreadLocal = new ThreadLocal<Looper>();

ThreadLocal是一个线程内部的数据存储类，所有线程访问一个对象，但是每个线程都有自己的一份数据，这就是它的奇妙之处。我们看一下ThreadLocal的get方法：

public void set(T value){    Thread currentThread = Thread.currentThread();    Values values = Values(currentThread);    if(values == null){        values = initializeValues(currentThread);    }    values.put(this, values);}

可以看到ThreadLocal通过内部的成员，设置自己的值，每个线程都会存一份副本，这样可以很好的解决线程并发的问题，而且比synchronized有更好的并发量。ThreadLoacl请查看博客：
http://blog.csdn.net/lufeng20/article/details/24314381

三、总结
Looper提供了quit和quitSafely两种方法来退出，quit会直接退出，quitSafely会把消息队列中数据执行完毕才会退出。自己退出Looper是个好习惯，防止线程一直处于等待状态。
如果在子线程用handler，可以使用HandlerThread，已经创建好了Looper和消息队列，不用自己手写。而且避免了多次创建Thread线程导致的性能问题，关于HandlerThread请查看博客：
http://blog.csdn.net/feiduclear_up/article/details/46840523