CODEWITHSUNDEEP
androidandroid-fragmentsandroid-viewpageroncreatefragmentpageradapter
Jasper
Jasper

Reputation: 2409

Fragment in ViewPager returns empty object onResume

I use a FragmentPagerAdapter to switch from fragments. I need some functions to be called when a fragmentswitch is made and had some troubles with OnPause and OnResume, so as suggested by THIS question I have implemented an interface OnPageSelectListener :

public interface OnPageSelectListener {
    void onPageSelected();
    void onPageNotVisible();
}

It calls the function OnPageSelected whenever this page comes to the foreground. Works nice, except that I want to call a function on my adapter. I thought that would work, except that my adapter returns NULL all the times (even though it is initialized and data is loaded in my listview as prefered).

    public class AfterCheckFragment extends Fragment implements OnPageSelectListener{


        private   ListView listView;
        private List<Check> checkList;
        private CheckListAdapter adapter;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view =  inflater.inflate(R.layout.fragment_check, container, false);

        System.out.println("VIEW create  called");

//(.. some other stuff, not relevant for question..)

        //initializing the adapter
          listView = (ListView) view.findViewById(R.id.listView);
        adapter = new CheckListAdapter(checkList,getActivity(),trainPosition);

        listView.setAdapter(adapter);
        adapter.handleButtonVisibility();
        return view;

    }
@Override
    public void onPageSelected() {
        if(this.adapter != null) {
            System.out.println("adapter not null");
            this.adapter.checkForActive();
        }else{
            System.out.println("Adapter is NULL");
        }

    }

        @Override
        public void onPageNotVisible() { //page is moved to backgroung
            System.out.println("AFTER not active any more ");
        }
    }

Now is my question: Why does adapter (or any other object in the fragment) return null when I return to my fragment? When the fragmentPager is initialized the onActivityCreate function of the fragment is called one time, but after that not any more, and the adapter return null....

Upvotes: 6

Views: 2538

Answers (6)

j2emanue
j2emanue

Reputation: 62519

for me i had to call this on my viewpager:

myViewPager.setSaveFromParentEnabled(false);

Upvotes: 1

Andrei Tudor Diaconu
Andrei Tudor Diaconu

Reputation: 2257

Here is why I think your CheckListAdapter (i'll call it listAdapter) is null:

  1. You give the pagerAdapter to the ViewPager
  2. The ViewPager asks the pagerAdapter for a new Fragment
  3. The ViewPager tells the FragmentManager to use it
  4. onPageSelected gets called
  5. You try and use listAdapter. It hasn't been initialized yet at this point. (NPE)
  6. The FragmentManager drags the Fragment through all its stages.
  7. onCreateView gets called. Your listAdapter is created.

Don't try and use internal data of a fragment outside of it. It is meant to work as a standalone unit, it won't be very good if you use it differently. Since the fragment is initialized at a later stage, you can't use it like you intend.
You can try and do what you want to do in the fragment, rather than the pagerAdapter, or write a method in the hosting Activity and call it from the fragment when ready, or even launch an event.

Upvotes: 2

Geraldo Neto
Geraldo Neto

Reputation: 4030

Why don't you use a viewpager.addOnPageChangeListener, in you pager , after setting its adapter and the setOffscreenPageLimit() instead of implements it on your fragment?

Heres a sample code:

viewpager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

    }

    @Override
    public void onPageSelected(int position) {
       if(position == 1){  // if you want the second page, for example
           //Your code here
       }
    }

    @Override
    public void onPageScrollStateChanged(int state) {

    }
});

Make it in your Activity, where you setup your ViewPager, of course.

Upvotes: 1

manjusg
manjusg

Reputation: 2263

This is because Viewpager calls OnpageSelected way before Fragments in oncreateView()/onActivityCreated() is called .

The best way for you is to inflate your views in the constructor of the Fragment and set the Adapters.

Or

Use a member variable to store whether the Fragment is active or not. And use the variable in oncreateview() to call function on your adapter.

Upvotes: 1

Shivani Gupta
Shivani Gupta

Reputation: 271

you have to call the onPageSelected() after initialization of the adapter and setAdapter() otherwise adapter will return null always

Upvotes: 3

kris larson
kris larson

Reputation: 30985

ViewPager will create and destroy fragments as the user changes pages (see ViewPager.setOffscreenPageLimit()). So onActivityCreated() is only called on the fragment when it is being restored or set up for the first time. Hence, fragments can be created without ever having onActivityCreated() called.

Instead of onActivityCreated(), I would recommend overriding onViewCreated() and setting up your adapter there. No fragment can be displayed without having a view created, so this is a good place to do that kind of stuff.

If you have your OnPageSelectListener logic working, that's good. I found the best way to know when your fragment is actually in front of the user is by overriding setPrimaryItem() in the FragmentPagerAdapter. Getting the page out of view event is a little trickier, since you have to keep a reference to the fragment from the previous setPrimaryItem() call.

Upvotes: 2

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