Android消息处理机制详解
首先需要介绍几个非常重要的角色:Looper、Handler、HandlerThread、MessageQueue、Message
Looper帮助一个线程维护一个消息队列,每一个线程都可以拥有一个Looper对象。
Handler负责把消息放入线程的消息队列中以及分发消息。
HandlerThread本身是一个Thread,并且内部封装了一个Looper对象,所以不用我们去关心Looper的创建和释放问题。
Message本身是一个Parcelable对象,MessageQueue用来存储一些待分发的Message,内部实现了一个Message链表的结构。
在一个子线程中创建Handler时,必须初始化该线程的Looper对象,因为普通的Thread默认是没有消息队列的。程序启动的时候,系统会为主线程(UI线程)创建一个Looper对象和消息队列,我们可以通过Looper.getMainLooper()获得。如果不使用HandlerThread,则通常我们在子线程中创建一个Handler的代码如下所示:
class MyThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
/* 处理接收到的消息 */
}
};
Looper.loop();
}
}
可以使用HandlerThread替代上面的方式,代码如下:
[font=Courier New]class[/font] MyHandler extends Handler { public MyHandler(Looper looper) {
super(looper);
}
public void handleMessage(Message msg) {
/** 处理接收到的消息*/
}
}
class UpdateRunnable implements Runnable {
public void run() {
Message msg = mHandler.obtainMessage();
msg.what = 10000;
msg.sendToTarget();
}
}
//在需要调用的地方,加入下面代码即可
HandlerThread handlerThread = new HandlerThread("MyHandlerThread");
handlerThread.start();
mHandler = new MyHandler(handlerThread.getLooper());
//add to MessageQueue
mHandler.post(new UpdateRunnable());
这两种方式原理是一样的,HandlerThread在run方法中也调用了Looper.prepare()和Looper.loop()方法。
那这两个方法做了些什么呢?下面根据sdk源码来分析。
public static void prepare() { prepare(true); }
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread"); }
sThreadLocal.set(new Looper(quitAllowed));
}
可以看到,调用Looper.prepare()时,为当前线程创建了一个Looper对象,继续跟踪,看创建Looper对象的时候又做了些什么呢?
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mRun = true;
mThread = Thread.currentThread();
}
为当前绑定的线程创建了一个消息队列,下面继续看Looper.loop()内部干了些啥呢?
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is. Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) { // No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + "" + msg.callback + " what=" + msg.what);
}
msg.recycle();
}
}
从源码可以看出,在一个无限for循环中遍历消息队列,然后调用Handler进行消息分发处理,分发之后调用recycle()把Message对象回收到Message Pool中(最大值为50个,若消息池中已经有50个Message,则丢弃不保存),实现Message对象的复用,调用handler.obtainMessage()时就是从消息池取得Message对象。当消息队列返回null时就退出循环。
调用next函数时,也是执行一个无限for循环,直接获取消息链表指向的下一个节点,判断该消息指定的执行时间是否已到,若已到则返回Message对象,若没有满足条件的Message,则继续等待直到线程结束,具体方法可以查询MessageQueue.java源码。
现在Looper对象和消息队列已经创建好了,看看Handler是怎么分发消息的。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
handler的处理比较简单,先判断Message中有没有指定的callback对象(Runnable),有的话就调用callback的run方法,没有则调用我们自己创建Handler对象时实现的handleMessage(Message msg)方法,就这样实现了消息的分发。还有一个疑问没有解决,就是handler如何把消息添加到消息队列呢?继续跟踪Handler的sendMessage函数。
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
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) {
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;
}
enqueueMessage这个函数就是我们要找的,它负责把Message对象添加到了MessageQueue管理的Message链表里。
原文链接:http://codingnow.cn/android/582.html