android消息处理机制--Looper

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From:http://blog.youkuaiyun.com/faith_boys/article/details/8100197

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WindowsAPI的SendMessage中的HANDLE，这个handle是窗口句柄。

//Looper类分析
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sendMessageDelayed
public 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sendMessageAtTime
public 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);
}
}
..........
}