// Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been cancelled, or could not be cancelled for some other reason.
// If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
             // Escape early if cancel() is called
             // 注意下面这行，如果检测到cancel，则及时退出
             if (isCancelled()) break;
         }
         return totalSize;
     }
 
     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }
 
     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

// AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent pacakge such as Executor, ThreadPoolExecutor and FutureTask.

// java.lang.IllegalArgumentException: Receiver not registered ...

// just sample, 可以写入工具类
// 第一眼我看到这段代码，靠，太粗暴了，但是回头一想，要的就是这么简单粗暴，不要把一些简单的东西搞的那么复杂。
private void unregisterReceiverSafe(BroadcastReceiver receiver) {
    try {
        getContext().unregisterReceiver(receiver);
    } catch (IllegalArgumentException e) {
        // ignore
    }
}

private void startTimer(){ 
    if (mTimer == null) { 
        mTimer = new Timer(); 
    } 
 
    if (mTimerTask == null) { 
        mTimerTask = new TimerTask() { 
            @Override 
            public void run() { 
                // todo
            } 
        }; 
    } 
 
    if(mTimer != null && mTimerTask != null ) 
        mTimer.schedule(mTimerTask, 1000, 1000); 
 
} 

private void cancelTimer(){ 
        if (mTimer != null) { 
            mTimer.cancel(); 
            mTimer = null; 
        } 
        if (mTimerTask != null) { 
            mTimerTask.cancel(); 
            mTimerTask = null; 
        }
    }

// 其实也非常简单，只不过ContentObserver是系统的例子，有必要单独拿出来提示一下大家，不可掉以轻心
private final ContentObserver mSettingsObserver = new ContentObserver(new Handler()) {
    @Override
    public void onChange(boolean selfChange, Uri uri) {
        // todo
    }
};
 
@Override
public void onStart() {
    super.onStart();
 
    // register the observer
    getContentResolver().registerContentObserver(Settings.Global.getUriFor(
            xxx), false, mSettingsObserver);
}
 
@Override
public void onStop() {
    super.onStop();
 
    // unregister it when stoping
    getContentResolver().unregisterContentObserver(mSettingsObserver);
 
}

private final class SettingsObserver implements Observer {
    public void update(Observable o, Object arg) {
        // todo ...
    }  
}
 
 mContentQueryMap = new ContentQueryMap(mCursor, Settings.System.XXX, true, null);
 mContentQueryMap.addObserver(new SettingsObserver());

private Observer mSettingsObserver;
@Override
public void onResume() {
    super.onResume();
    if (mSettingsObserver == null) {
        mSettingsObserver = new SettingsObserver();
    }  
    mContentQueryMap.addObserver(mSettingsObserver);
}
 
@Override
public void onStop() {
    super.onStop();
    if (mSettingsObserver != null) {
        mContentQueryMap.deleteObserver(mSettingsObserver);
    }  
    mContentQueryMap.close();
}

// 只是参考，不必死板
/*
addCallback             <==>     removeCallback
registerReceiver        <==>     unregisterReceiver
addObserver             <==>     deleteObserver
registerContentObserver <==>     unregisterContentObserver
... ...
*/

Handler handler = new Handler() {
    public void handleMessage(Message msg) {
        switch (msg.what) {
        case MESSAGE_1:
            // isFinishing == true, 则不处理，尽快结束
            if (!isFinishing()) {
                // 不退出
                // removeDialog()
                // showDialog()
            }  
            break;
        default:
            break;
        }  
        super.handleMessage(msg);
    }  
};