android消息机制 Message， Looper，Handler

其原理为：消息线程（默认是主线程，也就是UI线程），维护一个消息循环和一个消息队列，工作线程向消息队列中添加消息，消息循环从队列中取出消息并处理。

其中Looper类负责消息循环和消息队列，Message类是消息的载体，Handler类负责消息队列中添加消息和消息的处理。

Looper的prepare接口负责为调用线程创建一个Looper对象，下面是android源码，汉字部分为本文添加的注释：

public static final void prepare() {    if (sThreadLocal.get() != null) {  //其中sThreadLocal为ThreadLocal对象，不熟悉ThreadLocal的同学可以google下        throw new RuntimeException("Only one Looper may be created per thread");    }    sThreadLocal.set(new Looper());}

Looper的loop接口负责开启消息循环：

/** *  Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */public static final void loop() {    Looper me = myLooper();    MessageQueue queue = me.mQueue;//loop中维护的消息队列    while (true) {        Message msg = queue.next(); // 从消息队列中取消息        //if (!me.mRun) {        //    break;        //}        if (msg != null) {            if (msg.target == null) {                // No target is a magic identifier for the quit message.                return;            }            if (me.mLogging!= null) me.mLogging.println(                    ">>>>> Dispatching to " + msg.target + " "                    + msg.callback + ": " + msg.what                    );            msg.target.dispatchMessage(msg);  //消息处理            if (me.mLogging!= null) me.mLogging.println(                    "<<<<< Finished to    " + msg.target + " "                    + msg.callback);            msg.recycle();        }    }}

Handler中维护一个对Looper对象的引用，并直接获取它的消息队列，以便向消息队列中添加消息：

/** * Default constructor associates this handler with the queue for the * current thread. * * If there isn't one, this handler won't be able to receive messages. */public Handler() {    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();   //获取线程的Looper对象    if (mLooper == null) {        throw new RuntimeException(            "Can't create handler inside thread that has not called Looper.prepare()");    }    mQueue = mLooper.mQueue;    //直接获取Looper对象的消息队列    mCallback = null;}

Looper.loop方法中有一句代码msg.target.dispatchMessage(msg)（见上文）即是调用Handler的dispatchMessage方法：

/** * Handle system messages here. */public void dispatchMessage(Message msg) {    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        handleMessage(msg);   //调用子类的重写方法，故继承Handler时要重写该方法    }}

清楚了消息机制后，就不难理解消息机制的用法了：

MyHandler extends Handler{    public void handleMessage(final Message msg){        switch(msg.what){            case 1:            //做一些具体处理，比如说更新UI等             break;        }    }}MyHandler mHandler=new MyHandler();Message msg = Message.obtain(mHandler,1);msg.sendToTarget();