Android Looper和Handler分析

第一次接触android应用程序（这里指的是JAVA层的UI程序，也难怪了，Google放出的API就只支持JAVA应用程序了），很难搞明白内部是如何实现的。但是，从原理上分析，应该是有一个消息循环，一个消息队列，然后主线程不断得从消息队列中取得消息并处理之。

然而，google封装得太厉害了，所以一时半会还是搞不清楚到底是怎么做的。本文将分析android内的looper，这个是用来封装消息循环和消息队列的一个类，handler其实可以看做是一个工具类，用来向消息队列中插入消息的。好比是Windows API的SendMessage中的HANDLE，这个handle是窗口句柄。

//Looper类分析//没找到合适的分析代码的办法，只能这么来了。每个重要行的上面都会加上注释//功能方面的代码会在代码前加上一段分析public class Looper {   //static变量，判断是否打印调试信息。    private static final boolean DEBUG = false;    private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV;    // sThreadLocal.get() will return null unless you've called prepare().//线程本地存储功能的封装，TLS，thread local storage,什么意思呢？因为存储要么在栈上，例如函数内定义的内部变量。要么在堆上，例如new或者malloc出来的东西//但是现在的系统比如Linux和windows都提供了线程本地存储空间，也就是这个存储空间是和线程相关的，一个线程内有一个内部存储空间，这样的话我把线程相关的东西就存储到//这个线程的TLS中，就不用放在堆上而进行同步操作了。    private static final ThreadLocal sThreadLocal = new ThreadLocal();//消息队列，MessageQueue，看名字就知道是个queue..    final MessageQueue mQueue;    volatile boolean mRun;//和本looper相关的那个线程，初始化为null    Thread mThread;    private Printer mLogging = null;//static变量，代表一个UI Process（也可能是service吧，这里默认就是UI）的主线程    private static Looper mMainLooper = null;         /** Initialize the current thread as a looper.      * This gives you a chance to create handlers that then reference      * this looper, before actually starting the loop. Be sure to call      * {@link #loop()} after calling this method, and end it by calling      * {@link #quit()}.      *///往TLS中设上这个Looper对象的，如果这个线程已经设过了ｌｏｏｐｅｒ的话就会报错//这说明，一个线程只能设一个looper    public static final void prepare() {        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }        sThreadLocal.set(new Looper());    }        /** Initialize the current thread as a looper, marking it as an application's main      *  looper. The main looper for your application is created by the Android environment,     *  so you should never need to call this function yourself.     * {@link #prepare()}     */ //由framework设置的UI程序的主消息循环，注意，这个主消息循环是不会主动退出的//        public static final void prepareMainLooper() {        prepare();        setMainLooper(myLooper());//判断主消息循环是否能退出....//通过quit函数向looper发出退出申请        if (Process.supportsProcesses()) {            myLooper().mQueue.mQuitAllowed = false;        }    }    private synchronized static void setMainLooper(Looper looper) {        mMainLooper = looper;    }        /** Returns the application's main looper, which lives in the main thread of the application.     */    public synchronized static final Looper getMainLooper() {        return mMainLooper;    }    /**     *  Run the message queue in this thread. Be sure to call     * {@link #quit()} to end the loop.     *///消息循环，整个程序就在这里while了。//这个是static函数喔！    public static final void loop() {        Looper me = myLooper();//从该线程中取出对应的looper对象        MessageQueue queue = me.mQueue;//取消息队列对象...        while (true) {            Message msg = queue.next(); // might block取消息队列中的一个待处理消息..            //if (!me.mRun) {//是否需要退出？mRun是个volatile变量，跨线程同步的，应该是有地方设置它。            //    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);                if (me.mLogging!= null) me.mLogging.println(                        "<<<<< Finished to    " + msg.target + " "                        + msg.callback);                msg.recycle();            }        }    }    /**     * Return the Looper object associated with the current thread.  Returns     * null if the calling thread is not associated with a Looper.     *///返回和线程相关的looper    public static final Looper myLooper() {        return (Looper)sThreadLocal.get();    }    /**     * Control logging of messages as they are processed by this Looper.  If     * enabled, a log message will be written to <var>printer</var>      * at the beginning and ending of each message dispatch, identifying the     * target Handler and message contents.     *      * @param printer A Printer object that will receive log messages, or     * null to disable message logging.     *///设置调试输出对象，looper循环的时候会打印相关信息，用来调试用最好了。    public void setMessageLogging(Printer printer) {        mLogging = printer;    }        /**     * Return the {@link MessageQueue} object associated with the current     * thread.  This must be called from a thread running a Looper, or a     * NullPointerException will be thrown.     */    public static final MessageQueue myQueue() {        return myLooper().mQueue;    }//创建一个新的looper对象，//内部分配一个消息队列，设置mRun为true    private Looper() {        mQueue = new MessageQueue();        mRun = true;        mThread = Thread.currentThread();    }    public void quit() {        Message msg = Message.obtain();        // NOTE: By enqueueing directly into the message queue, the        // message is left with a null target.  This is how we know it is        // a quit message.        mQueue.enqueueMessage(msg, 0);    }    /**     * Return the Thread associated with this Looper.     */    public Thread getThread() {        return mThread;    }    //后面就简单了，打印，异常定义等。    public void dump(Printer pw, String prefix) {        pw.println(prefix + this);        pw.println(prefix + "mRun=" + mRun);        pw.println(prefix + "mThread=" + mThread);        pw.println(prefix + "mQueue=" + ((mQueue != null) ? mQueue : "(null"));        if (mQueue != null) {            synchronized (mQueue) {                Message msg = mQueue.mMessages;                int n = 0;                while (msg != null) {                    pw.println(prefix + "  Message " + n + ": " + msg);                    n++;                    msg = msg.next;                }                pw.println(prefix + "(Total messages: " + n + ")");            }        }    }    public String toString() {        return "Looper{"            + Integer.toHexString(System.identityHashCode(this))            + "}";    }    static class HandlerException extends Exception {        HandlerException(Message message, Throwable cause) {            super(createMessage(cause), cause);        }        static String createMessage(Throwable cause) {            String causeMsg = cause.getMessage();            if (causeMsg == null) {                causeMsg = cause.toString();            }            return causeMsg;        }    }}

那怎么往这个消息队列中发送消息呢？？调用looper的static函数myQueue可以获得消息队列，这样你就可用自己往里边插入消息了。不过这种方法比较麻烦，这个时候handler类就发挥作用了。先来看看handler的代码，就明白了。

class Handler{..........//handler默认构造函数public Handler() {//这个if是干嘛用的暂时还不明白，涉及到java的深层次的内容了应该        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());            }        }//获取本线程的looper对象//如果本线程还没有设置looper，这回抛异常        mLooper = Looper.myLooper();        if (mLooper == null) {            throw new RuntimeException(                "Can't create handler inside thread that has not called Looper.prepare()");        }//无耻啊，直接把looper的queue和自己的queue搞成一个了//这样的话，我通过handler的封装机制加消息的话，就相当于直接加到了looper的消息队列中去了        mQueue = mLooper.mQueue;        mCallback = null;    }//还有好几种构造函数，一个是带callback的，一个是带looper的//由外部设置looper    public Handler(Looper looper) {        mLooper = looper;        mQueue = looper.mQueue;        mCallback = null;    }// 带callback的，一个handler可以设置一个callback。如果有callback的话，//凡是发到通过这个handler发送的消息，都有callback处理，相当于一个总的集中处理//待会看dispatchMessage的时候再分析public Handler(Looper looper, Callback callback) {        mLooper = looper;        mQueue = looper.mQueue;        mCallback = callback;    }////通过handler发送消息//调用了内部的一个sendMessageDelayedpublic final boolean sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }//FT，又封装了一层，这回是调用sendMessageAtTime了//因为延时时间是基于当前调用时间的，所以需要获得绝对时间传递给sendMessageAtTimepublic final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }public boolean sendMessageAtTime(Message msg, long uptimeMillis)    {        boolean sent = false;        MessageQueue queue = mQueue;        if (queue != null) {//把消息的target设置为自己，然后加入到消息队列中//对于队列这种数据结构来说，操作比较简单了            msg.target = this;            sent = queue.enqueueMessage(msg, uptimeMillis);        }        else {            RuntimeException e = new RuntimeException(                this + " sendMessageAtTime() called with no mQueue");            Log.w("Looper", e.getMessage(), e);        }        return sent;    }//还记得looper中的那个消息循环处理吗//从消息队列中得到一个消息后，会调用它的target的dispatchMesage函数//message的target已经设置为handler了，所以//最后会转到handler的msg处理上来//这里有个处理流程的问题public void dispatchMessage(Message msg) {//如果msg本身设置了callback，则直接交给这个callback处理了        if (msg.callback != null) {            handleCallback(msg);        } else {//如果该handler的callback有的话，则交给这个callback处理了---相当于集中处理          if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }           }//否则交给派生处理,基类默认处理是什么都不干            handleMessage(msg);        }    }..........}

 讲了这么多，该怎么创建和使用一个带消息循环的线程呢？

//假设在onCreate中创建一个线程//不花时间考虑代码的完整和严谨性了，以讲述原理为主。....... onCreate(...){//难点是如何把android中的looper和java的thread弄到一起去。//而且还要把随时取得这个looper用来创建handler//最简单的办法就是从Thread派生一个class ThreadWithMessageHandle extends Thread{  //重载run函数  Looper myLooper = null;  run(){  Looper.prepare();//将Looper设置到这个线程中  myLooper = Looper.myLooper();  Looper.loop();开启消息循环} ThreadWithMessageHandle  threadWithMgs = new ThreadWithMessageHandle(); threadWithMsg.start(); Looper looper = threadWithMsg.myLooper;////这里有个问题.threadWithMgs中的myLooper可能此时为空//需要同步处理一下//或者像API文档中的那样，把handler定义到ThreadWithMessageHandle到去。//外线程获得这个handler的时候仍然要注意同步的问题，因为handler的创建是在run中的 Handler threadHandler = new Handler(looper); threadHandler.sendMessage(...)}}...