Question

I have a semi-complicated problem and hoping that someone here will be able to help me.

On a click event I create a thread and start a long-running operation based on this method. After the long-running task is completed, it does a callback to another method, which does a post to the handler:

@Override
public void contentSearchModelChanged(Model_ContentSearch csm, ArrayList<Class_Reminder> newRemindersList) {
    remindersList = newRemindersList;
    mHandler.post(mUpdateDisplayRunnable);
}

Which calls a Runnable:

// post this to the Handler when the background thread completes
private final Runnable mUpdateDisplayRunnable = new Runnable() {
  public void run() {
    updateDisplay();
  }
};

Finally, here is what my updateDisplay() method is doing:

private void updateDisplay() {
    if (csModel.getState() != Model_ContentSearch.State.RUNNING) {
        if(remindersList != null && remindersList.size() > 0){
                r_adapter = new ReminderAdapater(Activity_ContentSearch.this, remindersList, thisListView);
                thisListView.setAdapter(r_adapter);
                r_adapter.notifyDataSetChanged();
        }
    }
}

This works beautifully when I do this normally. However, if I change the orientation while the long-running operation is running, it doesn’t work. It does make the callback properly, and the remindersList does have items in it. But when it gets to this line:

r_adapter.notifyDataSetChanged();

Nothing happens. The odd thing is, if I do another submit and have it run the whole process again (without changing orientation), it actually updates the view twice, once for the previous submit and again for the next. So the view updates once with the results of the first submit, then again with the results of the second submit a second later. So the adapater DID get the data, it just isn’t refreshing the view.

I know this has something to do with the orientation change, but I can’t for the life of me figure out why. Can anyone help? Or, can anyone suggest an alternative method of handling threads with orientation changes?

Bara

Answer 1

The problem is that when you change orientations a new activity is spun up from the beginning (onCreate). Your long running process has a handle to the old (no longer visible) activity. You are properly updating the old activity but since it isn’t on screen anymore, you don’t see it.

This is not an easy problem to fix. There is a library out there that may help you though. It is called DroidFu. Here is a blog post that (much more accurately than I) describes the root cause of what you are seeing and how the DroidFu library combats it: http://brainflush.wordpress.com/2009/11/16/introducing-droid-fu-for-android-betteractivity-betterservice-and-betterasynctask/

Edit: (Adding code for tracking active activity)

In your application class add this:

private Activity _activeActivity;
public void setActiveActivity(Activity activity) {
    _activeActivity = activity;
}
public Activity getActiveActivity() {
    return _activeActivity;
}

In your Activities, add this:

@Override
public void onResume() {
    super.onResume();
    ((MyApplicationClassName)getApplication()).setActiveActivity(this);
}

Now you can get the active activity by calling MyApplicationClassName.getActiveActivity();

This is not how DroidFu does it. DroidFu sets the active activity in onCreate but I don’t feel that is very robust.