android 消息机制
一、Android应用程序的主线程主要用于更新UI界面，并且主线程不能做耗时操作，否则会引起ANR；这种情况下需要开一个子线程来进行耗时操作，动作完成之后，子线程发消息给主线程通知其更新UI显示，常见方法有：
- Activity.runOnUiThread(Runnable);
- View.post(Runnable);
- View.postDelayed(Runnable, long);
- Handler消息机制。
注:经过看源码，会发现Activity.runOnUiThread(Runnable),View.post(Runnable),View.postDelayed(Runnable, long)最终本质上会调用到Handler发送消息的方法，如下代码：
Activity.runOnUiThread(Runnable);
/**
* Runs the specified action on the UI thread. If the current thread is the UI
* thread, then the action is executed immediately. If the current thread is
* not the UI thread, the action is posted to the event queue of the UI thread.
*
* @param action the action to run on the UI thread
*/
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}
View.Post(Runnable),View.PostDelayed(Runnable); 请忽略attachInfo判断。
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().post(action);
return true;
}
public boolean postDelayed(Runnable action, long delayMillis) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.postDelayed(action, delayMillis);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);
return true;
}
二、弄清消息机制之前，提一个问题：如何在子线程实例化一个Handler发送message呢？代码很简单，如下
new Thread(){
@Override
public void run() {
super.run();
Looper.prepare();
new Handler().sendEmptyMessage(1);
Looper.loop();
}
}.start();
接下来，问题又来了：
1.发消息的时候为什么要调用Looper.prepare()；
2.消息发送后为什么要调用Looper.loop()；
3.为什么在主线程里发送信息，我们没有调用 这两个方法呢？
在解答这三个问题之前，先看一张图，大致了解一下消息机制是如何运行的？如下图：
总结一下上面那个图片：在UI线程中有一个消息队列MessageQueue，其它线程do something之后，在UI线程中的消息队列MessageQueue插入Message，而Looper负责轮循消息队列MessageQueue。然后来回答上面第1个问题，看代码：
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));
}
//初始化Looper，并且初始化消息队列
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
通过上图可得知，接收一个消息的前提是，该线程得拥有自己的消息队列MessageQueue，而第1个问题的答案就是创建一个该线程接收消息的一个消息队列。然后第二个问题请看以下代码：
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.recycleUnchecked();
}
}
通过上图可知消息机制中除了消息队列MessageQueue，还得有轮循，第二个问题就是执行轮循的操作。第三个问题请往下看代码：
public final class ActivityThread {
......
public static final void main(String[] args) {
......
//将当前线程初始化为Looper线程。最终会调用Looper.prepare()
Looper.prepareMainLooper();
......
// 开始循环处理消息队列
Looper.loop();
......
}
}
第三个问题的答案：android应用程序启动过程中，会在进程中执行ActivityThread中main方法，来初始化该应用中UI线程的Looper。这也就是为什么主线程里可以直接调用Handler，而子线程不能直接调用Handler发送消息。
参考：http://3dobe.com/archives/74/
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