Android：从PagerAdapter获取片段

Question

So I'm trying to implement a ViewPager with a custom FragmentStatePagerAdapter. In my MainActivity, I'm trying to access the fragment being displayed so I can modify the children inside of that fragment's view.

Here's the custom fragment:

public class CustomFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        ViewGroup rootView = (ViewGroup) inflater.inflate(R.layout.fragment_custom, container, false);
        return rootView;
    }
}

Here's the FragmentStatePagerAdapter:

public class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {

    private int NUM_PAGES;

    public ScreenSlidePagerAdapter(FragmentManager fm, int NUM_PAGES) {
       super(fm);
       this.NUM_PAGES = NUM_PAGES;
    }

    @Override
    public int getCount() {
        return NUM_PAGES;
    }

    @Override
    public Fragment getItem(int position) {
        return new CustomFragment();
    }

}

Here's the important part in the MainActivity. This is where I try and get the fragment currently being displayed by the pager adapter. mPagerAdapter.instantiateItem returns a fragment, but when I call primaryFragment.getView(), I get a null pointer.

    moveViewPagerToIndex(); // Move the view pager to NUM_PAGES / 2 to start
    primaryFragment = (CustomFragment) mPagerAdapter.instantiateItem(mViewPager, mViewPager.getCurrentItem());
    updateFragment();

The updateFragment():

    ArrayList<Button> arrayList = Utility.getCellsFromCalendarFragment(primaryFragment.getView());
    for (int i = 0; i < arrayList.size(); i++) {
         // do stuff with all the cells
    }

The Utility.getCellsFromCalendarFragment(View rootView)

public static ArrayList<Button> getCellsFromCalendarFragment(View rootView) {
    ArrayList<Button> cellList = new ArrayList<>();
    cellList.add((Button) rootView.findViewById(R.id.cell00));
    cellList.add((Button) rootView.findViewById(R.id.cell01));
    cellList.add((Button) rootView.findViewById(R.id.cell02));
    // There are a lot more than 3 cells... There's roughly 50 cells.
    return cellList
}

I ran through the debugger in Android Studio and watched the variable 'primaryFragment'. When I call mViewPager.instantiateItem(...), I get an actual CustomFragment back. The problem is that 'primaryFragment' now has a null view so when I try to access the view to modify the cells, I get a NullPointerException.

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference

How do I get the view from this fragment? Is this even the right way to get the fragment from the view pager? I've been ripping my hair out over this. I just want to get the fragment being displayed and modify its view and the children.

Thanks!

Answer 1

FragmentStatePagerAdapter.instantiateItem() creates new Fragment by calling getItem() . Your getItem() creates a new instance of CustomFragment every time it is called. So when you call isntantiateItem() you are getting a new instance of CustomFragment and not the one that is being used by the adapter.

You may need to store the instances of CustomFragment in you adapter and write a method to get them by index.

I am not sure about your full use case so I can not advice on how you could go on to modify your adapter. I mean I do not know which fragment you want to access, if it is the one that is currently shown or some distant one that is far gone out of screen.