Android Looper跟Handler分析(转载)

?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; } }}

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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); } }..........}

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?讲了这么多，该怎么创建和使用一个带消息循环的线程呢？

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//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(...)}}...

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好了，handler和looper的分析就都这了，其实原理挺简单的。

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原文：http://blog.csdn.net/innost/article/details/6055793