getSupportFragmentManager（）。findFragmentById使用FragmentPagerAdapter和ViewPager返回null

Question

I checked all the similar tickets with "getSupportFragmentManager().findFragmentById returns null" but none helped.

In Android Studio 3.0.1 I just created a new project based on the "Tabbed Activity" template project and navigation style set to "Action Bar Tabs (with ViewPager)".

Then I added 3 xml files

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/fragment1">
<!-- @+id/fragment2 in fragment2.xml-->
<!-- @+id/fragment3 in fragment3.xml-->
...some controls
</LinearLayout>

And the 3 corresponding .java class files

public class Fragment1 extends Fragment {
   //Fragment2, Fragment3

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment1, container, false);
        //R.layout.fragment2, R.layout.fragment3 
    }

}

In the SectionsPagerAdapter class of MainActivity class, I changed the overriden method getItem to the following.

@Override
public Fragment getItem(int position) {
     // getItem is called to instantiate the fragment for the given page.
     // Return a PlaceholderFragment (defined as a static inner class below).
     switch(position) {
         case 0: return new Fragment1();
            case 1: return new Fragment2();
            case 2: return new Fragment3();
            default:
                return new Fragment1();

     }
     //return PlaceholderFragment.newInstance(position + 1);
}

And finally, I want to trigger something in my first fragment.

 public void onClick(View view) {
       Fragment1 frag1 = (Fragment1) getSupportFragmentManager().findFragmentById(R.id.fragment1);
 }

My problem is that when I click a button in the toolbar and I reach its onClick event (in MainActivity), my frag1 is always null!

Please help! :)

Answer 1

you shouldn't override getItem like this way

I suggest to be like:

public class SectionsPagerAdapter extends FragmentPagerAdapter {
    private List<Fragment> fragments;

    public SectionsPagerAdapter(FragmentManager fm, List<Fragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    public void setFragments(List<Fragment> fragments) {
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int position) {
        return this.fragments.get(position);
    }

    @Override
    public int getCount() {
        return this.fragments.size();
    }


}

and if you want to get any fragment you want just call

((FragmentCallBack)sectionsPagerAdapter.get(thePositionOfYourFragment))
.trigger();