How do I prevent device rotation from generating duplicate output?

Question

After user inputs parameters in MainActivity for my app (shown below), he taps Search, which calls MatchesActivity, which generates output on a new screen (shown further below), which is exited via tapping back.

But with MatchesActivity active, every time the device is rotated, Search is again executed because the Activity restarts. In the screenshot below, I rotated device from vertical to horizontal to vertical to horizontal back to vertical.

It looks silly.

The output is generated in MatchesActivity that is invoked in onCreate in MainActivity like so:

Intent matchesIntent;
matchesIntent = new Intent(MainActivity.this, MatchesActivity.class);
startActivity(matchesIntent);

Here's the essence of onCreate for MatchesActivity:

@Override protected void onCreate(Bundle savedInstanceState)
{    
  MainActivity.dbc.setDbProcesslistener(this); // to know txaMatches has been defined

  MainActivity.dbc.findDBMatches(); // generate output
}

I did research. I found some complicated ways of preventing an activity from restarting when the device is rotated. For example .

I'm hoping for a simpler solution. Any ideas?

The results you are showing inside the MatchesActivity are repeated , check the list of items and reset it inside oncreate if it has size greater than 0 . — Krishna Satwaji Khandagale, Jan 16 '16 at 14:09

score 1 · Answer 1 · answered Jan 16 '16 at 14:14

As you have found, one option is to prevent the activity from being recreated on configuration changes all together. This is not always the best option, as this will prevent other things depending on the configuration from being recreated/reloaded too (e.g. resources overridden with the "-land" qualifier).

Another option is to cache the result of the DB search somehow. This could be done by adding a wrapper around your database that memorizes the term and results of the last search. Another way to cache the results would be to use a fragment, and reuse that fragment across activity recreations. Whether a fragment is recreated along with its activity is controlled by this method: http://developer.android.com/reference/android/app/Fragment.html#setRetainInstance(boolean).

DSlomer64 · Accepted Answer · 2016-01-18T12:37:57.403

My solution was simple.

Introduce boolean variable outputIsShowing, set it to true in onCreate as MatchesActivity terminates, set it for false when onCreate or onResume are executed in MainActivity (i.e., when MatchesActivity terminates), and return immediately in onCreate for MatchesActivity if outputIsShowing is true.

So if MatchesActivity is active when device is rotated, outputIsShowing will be true, so don't execute again.

It may not be best practice, but I've extensively tested it under normal conditions and am happy enough so far. Not sure if anything is lurking out there as a "gotcha".

I plan to go back and study the suggestions made so far since the more general situation is definitely important. And I will have to do so if someone finds fault with what I've done.

    @Override protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

  // usual details prior to asking for matches

        if(outputIsShowing)
            return;

        MainActivity.dbc.setDbProcesslistener(this); // to know matches was defined

        MainActivity.dbc.findDBMatches();

        outputIsShowing = true;
    }

* EDIT *

Strangely, after embedding TextView txaMatches in a ScrollView to accomplish smooth, accelerated scrolling, I had to remove the references to outputIsShowing in order to see output after two device orientation changes.

And now, maybe I'll submit another question to address the fact that, very infrequently after screensaver forces waking the device, the output does NOT show if that is where the focus was when screensaver became active. Tapping 'back' to get to user input screen and then immediately tapping Search restores all to normal until about 100 (give or take) screensaver instances later, the output is again missing.

Such a bug makes me think I ought to follow the advice above.

If I do, or when I figure out the problem, I'll edit this again.

How do I prevent device rotation from generating duplicate output?

2 Answers2