Android实战技巧：消息循环与Looper
Understanding Looper
Looper是用于给一个线程添加一个消息队列(MessageQueue)，并且循环等待，当有消息时会唤起线程来处理消息的一个工具，直到线程结束为止。通常情况下不会用到Looper，因为对于Activity，Service等系统组件，Frameworks已经为我们初始化好了线程(俗称的UI线程或主线程)，在其内含有一个Looper，和由Looper创建的消息队列，所以主线程会一直运行，处理用户事件，直到某些事件(BACK）退出。
如果，我们需要新建一个线程，并且这个线程要能够循环处理其他线程发来的消息事件，或者需要长期与其他线程进行复杂的交互，这时就需要用到Looper来给线程建立消息队列。
使用Looper也非常的简单，它的方法比较少，最主要的有四个:
public static prepare();
public static myLooper();
public static loop();
public void quit();
使用方法如下：
1. 在每个线程的run()方法中的最开始调用Looper.prepare()，这是为线程初始化消息队列。
2. 之后调用Looper.myLooper()获取此Looper对象的引用。这不是必须的，但是如果你需要保存Looper对象的话，一定要在prepare()之后，否则调用在此对象上的方法不一定有效果，如looper.quit()就不会退出。
3. 在run()方法中添加Handler来处理消息
4. 添加Looper.loop()调用，这是让线程的消息队列开始运行，可以接收消息了。
5. 在想要退出消息循环时，调用Looper.quit()注意，这个方法是要在对象上面调用，很明显，用对象的意思就是要退出具体哪个Looper。如果run()中无其他操作，线程也将终止运行。
下面来看一个实例
实例
这个例子实现了一个执行任务的服务：
[java]
public class LooperDemoActivity extends Activity {
private WorkerThread mWorkerThread;
private TextView mStatusLine;
private Handler mMainHandler;
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.looper_demo_activity);
mMainHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
String text = (String) msg.obj;
if (TextUtils.isEmpty(text)) {
return;
}
mStatusLine.setText(text);
}
};
mWorkerThread = new WorkerThread();
final Button action = (Button) findViewById(R.id.looper_demo_action);
action.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mWorkerThread.executeTask("please do me a favor");
}
});
final Button end = (Button) findViewById(R.id.looper_demo_quit);
end.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mWorkerThread.exit();
}
});
mStatusLine = (TextView) findViewById(R.id.looper_demo_displayer);
mStatusLine.setText("Press 'do me a favor' to execute a task, press 'end of service' to stop looper thread");
}
@Override
public void onDestroy() {
super.onDestroy();
mWorkerThread.exit();
mWorkerThread = null;
}
private class WorkerThread extends Thread {
protected static final String TAG = "WorkerThread";
private Handler mHandler;
private Looper mLooper;
public WorkerThread() {
start();
}
public void run() {
// Attention: if you obtain looper before Looper#prepare(), you can still use the looper
// to process message even after you call Looper#quit(), which means the looper does not
//really quit.
Looper.prepare();
// So we should call Looper#myLooper() after Looper#prepare(). Anyway, we should put all stuff between Looper#prepare()
// and Looper#loop().
// In this case, you will receive "Handler{4051e4a0} sending message to a Handler on a dead thread
// 05-09 08:37:52.118: W/MessageQueue(436): java.lang.RuntimeException: Handler{4051e4a0} sending message
// to a Handler on a dead thread", when try to send a message to a looper which Looper#quit() had called,
// because the thread attaching the Looper and Handler dies once Looper#quit() gets called.
mLooper = Looper.myLooper();
// either new Handler() and new Handler(mLooper) will work
mHandler = new Handler(mLooper) {
@Override
public void handleMessage(Message msg) {
/*
* Attention: object Message is not reusable, you must obtain a new one for each time you want to use it.
* Otherwise you got "android.util.AndroidRuntimeException: { what=1000 when=-15ms obj=it is my please
* to serve you, please be patient to wait!........ } This message is already in use."
*/
// Message newMsg = Message.obtain();
StringBuilder sb = new StringBuilder();
sb.append("it is my please to serve you, please be patient to wait!\n");
Log.e(TAG, "workerthread, it is my please to serve you, please be patient to wait!");
&nbs
补充：移动开发 , Android ,
