Android Looper和Handler分析
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;
- }
- }
- }
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