ViewPager Fragments getting destroyed while app is in background?

Question

I can't for the life of me figure out why sometimes my Fragments become uncontrollable after my app has been launched, paused, and then resumed.

I hold references to my Fragments like this:

public class MainActivity ... {

    public static AccountFragment accountFragment;
    ....

    @Override
    protected void onCreate( ... {

        accountFragment = new AccountFragment(this);
        ...
    }

I have a custom toolbar with a Spinner which theoretically should allow the user to cause an AsyncTask in the AccountFragment to run. This is implemented like so:

if (accountFragment.getView() != null) {

    accountFragment.load()
}

I then create a ViewPager, a FragmentPagerAdapter, etc.

This works completely fine most of the time. The user can select a new item in the Spinner and the AccountFragment will update accordingly. However, sometimes after the app has been stopped and then later resumed, the AccountFragment will not respond at all to anything. Even other views in the app which should affect the AccountFragment are useless. It makes no sense to me because the AccountFragment's view cannot be null, and the AsyncTask which is executed by accountFragment.load() makes some very simple changes to the UI in its onPreExecute which should be very apparent (e.g. Everything disappears and a ProgressBar appears) which is simply not happening.

Any ideas?

score 1 · Accepted Answer · answered Oct 22 '14 at 03:03

1

There's not enough code to know exactly what's going wrong here, but there are also a bunch of stuff you're doing wrong.

1) Don't store fragment reference in a public static field. Make it just private, or protected at most. Read basic Java manuals for explanation.

2) Don't overload Fragment's constructor, it's discouraged by design. If you need to pass some values to it, do it by using arguments.

3) I guess you're not performing a check during onCreate() method in your Activity if it's being recreated or freshly created. You could end up with two instances of AccountFragment where one of them is in some weird detached state.

...

You should probably spend more time researching basics about Fragment/Activity relationship and how to avoid situations like this.

answered Oct 22 '14 at 03:03

Tomislav Novoselec

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Thanks for the comprehensive answer! 1: Okay, I will do that. 2: My fragments need reference to my MainActivity somehow. How else should that be done? 3: How would I tell if onCreate is recreating or being freshly created? What would I do differently if I discovered that it was being recreated? – zholmes1 Oct 22 '14 at 17:03
Your fragment has getActivity() method. You can cast returned Activity to your MainActivity. Regarding the recreation: if savedInstanceState in onCreate() method is null, then it usually means that Activity is freshly created. If it's not null, it's re-creating. – Tomislav Novoselec Oct 22 '14 at 17:30
Do you think that maybe surrounding my instantiation of my fragments with the following if block might work? if (savedInstanceState == null) { // instantiate Fragments } – zholmes1 Oct 24 '14 at 07:40
It's definitely a better approach, but without the rest of the code I can't really say will it fix everything or not – Tomislav Novoselec Oct 24 '14 at 17:02

score 1 · Answer 2 · answered Oct 22 '14 at 04:05

What you need is override these methods:

onSaveInstanceState(){}
onCreate (Bundle savedInstanceState){}
or onCreateView()

See http://developer.android.com/reference/android/app/Fragment.html#onSaveInstanceState(android.os.Bundle) . You put all the necessary parameters to bundle in onSaveInstanceState() and restore them in onCreate() or onCreateView().

ViewPager Fragments getting destroyed while app is in background?

2 Answers2