Performing long running operation in onDestroy

Question

I have a "long-running" cleanup operation that I need to perform in onDestroy() of my Activity. What is the best way to do this?

If I use a Thread to do this, my onDestroy() will return immediately; but what happens to the Thread reference? I am looking for advice on any implications/gotchas/trip-wires that I need to be aware of here since I assume that the process will still be alive even after the Activity is destroyed.

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Background:

I am using JmDNS in my app. When the user is done with my app, I want to clean up the JmDNS instance. I do this using the close() method of JmDNS class. However, this method takes more than 5 seconds to complete. As a result, the user sees my Activity on screen for a long time after touching the Back key.

I am yet to figure out why the close() takes that long, but in the meanwhile I also realized that I really don't need to wait for the close to complete successfully. All I need is a way to "trigger" the close and be done with it.

Answer 1

I ended up doing what I had asked in the question - I start a Thread to perform the long-running operation in onDestroy().

One case I had to consider was when the user re-opens my app even before the long-running has completed. In my app, this means a new instance of JmDNS gets created. So, I clean up each instance separately in my onDestroy.

Your use case might differ - you might want launch the cleanup thread only if it is not already running (using Thread's isAlive() method or some such technique).

Here's some sample code. To appreciate the "clean up each instance separately" part, perform the following sequence of steps:

1. Launch the app
2. Press the back button. You will see the clean up operation in LogCat
3. Re-launch the app.

4. Again, exit the app. Now, you will see two sets of cleanup logs - the first one representing cleanup for the first instance; and the second set corresponding to the second instance.

   public class DelayedExitActivity extends Activity {
   
       private static final String LOG_TAG = "DelayedExit";
       private final  Runnable longOperation = new Runnable(){
           @Override
           public void run() {
               for (int i=0 ; i < 50; i++){
                   Log.d(LOG_TAG, "Iteration "+i);
                   try {
                       Thread.sleep(2 * 1000);
                   } catch (InterruptedException e) {
                       // TODO Auto-generated catch block
                       e.printStackTrace();
                   }
               }
           }
       };
       private Thread longThread ;
   
       /** Called when the activity is first created. */
       @Override
       public void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.main);
       }
   
       @Override
       protected void onDestroy() {
           if(longThread == null){
               longThread = new Thread(longOperation);
           }
           longThread.start();
           super.onDestroy();
       }
   }
   

Answer 2

Try to start your thread in onBackPressed() and call destroy() when your thread will be finished.