Android开发笔记之:消息循环与Looper的详解

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!");
      for (int i = 1; i < 100; i++) {
   sb.append(".");
   Message newMsg = Message.obtain();
   newMsg.obj = sb.toString();
   mMainHandler.sendMessage(newMsg);
   Log.e(TAG, "workthread, working" + sb.toString());
   SystemClock.sleep(100);
      }
      Log.e(TAG, "workerthread, your work is done.");
      sb.append("\nyour work is done");
      Message newMsg = Message.obtain();
      newMsg.obj = sb.toString();
      mMainHandler.sendMessage(newMsg);
  }
     };
     Looper.loop();
 }

 public void exit() {
     if (mLooper != null) {
  mLooper.quit();
  mLooper = null;
     }
 }

 // This method returns immediately, it just push an Message into Thread's MessageQueue.
 // You can also call this method continuously, the task will be executed one by one in the
 // order of which they are pushed into MessageQueue(they are called).
 public void executeTask(String text) {
     if (mLooper == null || mHandler == null) {
  Message msg = Message.obtain();
  msg.obj = "Sorry man, it is out of service";
  mMainHandler.sendMessage(msg);
  return;
     }
     Message msg = Message.obtain();
     msg.obj = text;
     mHandler.sendMessage(msg);
 }
    }
}