android - How to get the current fragment displayed in a specific tab of a viewpager?

Question

I want to get the last fragment in the backstack, or the current displayed it's the same for me, in the tab b_1. As you can see in the following image, I have a ViewPager, and another one inner tab b. Thus there are four current fragments displayed.

Question: How can I get the Fragment 2 instance?

I have seen another solutions, but none works for this scenario.

Annotation: The fragment to return is not necessary the hosted in the ViewPager. I can have opened two more fragments in a tab.

With this method I get all the current visible fragments, but not the one specific I want.

public ArrayList<Fragment> getVisibleFragment() {
    List<Fragment> fragments = getSupportFragmentManager().getFragments();
    ArrayList<Fragment> visibleFragments = new ArrayList<>();
    if (fragments != null) {
        for (Fragment fragment : fragments) {
            if (fragment != null && fragment.isVisible())
                visibleFragments.add(fragment);
        }
    }
    return visibleFragments;
}

Some interesting code

activity_main.xml

<android.support.design.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

        <android.support.design.widget.TabLayout
            android:id="@+id/tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:tabMode="fixed"
            app:tabGravity="fill"/>
    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

</android.support.design.widget.CoordinatorLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static ViewPagerAdapter adapter;
    private static ViewPager viewPager;
    private TabLayout tabLayout;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        viewPager = (ViewPager) findViewById(R.id.viewpager);
        setupViewPager();

        tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(viewPager);
        setupTabIcons();
    }

    private void setupViewPager() {
        adapter = new ViewPagerAdapter(getSupportFragmentManager());
        // Wrap with HostFragment to get separate tabbed nagivation.
        adapter.addFrag(HostFragment.newInstance(new Fragment1()), null);
        adapter.addFrag(HostFragment.newInstance(new RootFragment2()), null);
        adapter.addFrag(HostFragment.newInstance(new Fragment4()), null);
        viewPager.setAdapter(adapter);
        viewPager.setOffscreenPageLimit(2);
    }

    public void openNewFragment(Fragment fragment) {
        HostFragment hostFragment = (HostFragment) adapter.getItem(viewPager.getCurrentItem());
        hostFragment.replaceFragment(fragment, true);
    }
}

fragment_host.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/hosted_fragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

HostFragment.java

/**
 * This class implements separate navigation for a tabbed viewpager.
 *
 * Based on https://medium.com/@nilan/separate-back-navigation-for-
 * a-tabbed-view-pager-in-android-459859f607e4#.u96of4m4x
 */
public class HostFragment extends BackStackFragment {

    private Fragment fragment;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        super.onCreateView(inflater, container, savedInstanceState);
        View view = inflater.inflate(R.layout.fragment_host, container, false);
        if (fragment != null) {
            replaceFragment(fragment, false);
        }
        return view;
    }

    public void replaceFragment(Fragment fragment, boolean addToBackstack) {
        if (addToBackstack) {
            getChildFragmentManager().beginTransaction().replace(R.id.hosted_fragment, fragment).addToBackStack(null).commit();
        } else {
            getChildFragmentManager().beginTransaction().replace(R.id.hosted_fragment, fragment).commit();
        }
    }

    public static HostFragment newInstance(Fragment fragment) {
        HostFragment hostFragment = new HostFragment();
        hostFragment.fragment = fragment;
        return hostFragment;
    }

    public Fragment getFragment() {
        return fragment;
    }
}

fragment2_root.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/fragment2_root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <android.support.design.widget.TabLayout
        android:id="@+id/tab2_tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabMode="fixed"
        app:tabGravity="fill"/>

    <android.support.v4.view.ViewPager
        android:id="@+id/tab2_viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

</LinearLayout>

RootFragment2.java

public class RootFragment2 extends Fragment {

    private ViewPagerAdapter adapter;
    private ViewPager viewPager;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Inflate the layout for this fragment.
        View root = inflater.inflate(R.layout.fragment2_root, container, false);

        viewPager = (ViewPager) root.findViewById(R.id.tab2_viewpager);
        setupViewPager(viewPager);

        TabLayout tabLayout = (TabLayout) root.findViewById(R.id.tab2_tabs);
        tabLayout.setupWithViewPager(viewPager);

        return root;
    }

    private void setupViewPager(ViewPager viewPager) {
        adapter = new ViewPagerAdapter(getActivity().getSupportFragmentManager());
        // Wrap with HostFragment to get separate tabbed nagivation.
        adapter.addFrag(HostFragment.newInstance(new Fragment2()), null);
        adapter.addFrag(HostFragment.newInstance(new Fragment3()), null);
        viewPager.setAdapter(adapter);
        viewPager.setOffscreenPageLimit(1);
    }

    public ViewPagerAdapter getAdapter() {
        return adapter;
    }

    public ViewPager getViewPager() {
        return viewPager;
    }
}

Answer 1

First define a SparseArray in your ViewPagers' adapters like below. In this array we'll hold the instance of fragments.

SparseArray<Fragment> registeredFragments = new SparseArray<>();

And Override your Adapters' instantiateItem method.

@Override
public Object instantiateItem(ViewGroup container, int position) {
    Fragment fragment = (Fragment) super.instantiateItem(container, position);
    registeredFragments.put(position, fragment);
    return fragment;
}

Also Override destroyItem method of your ViewPagers

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    registeredFragments.remove(position);
    super.destroyItem(container, position, object);
}

And define a new method to get your ViewPager Fragments instance.

public Fragment getRegisteredFragment(int position) {
    return registeredFragments.get(position);
}

And finally set add a PageChangeListener to your ViewPagers:

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

    }

    @Override
    public void onPageSelected(int position) {
        // Here's your instance
        YourFragment fragment =(YourFragment)yourPagerAdapter.getRegisteredFragment(position);

    }

    @Override
    public void onPageScrollStateChanged(int state) {

    }
});

I hope this'll help you. Good luck.

Edit: I'm sorry i cannot understand exactly what you're planning to do but if you need to keep sub fragment (b_1, b_2) instance you can define a method to your activity such as

public void setCurrentFragment(Fragment fragment){
      this.currentFragment = fragment;
}

and in your sub view pager's adapter you can call this method like below:

subViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

    }

    @Override
    public void onPageSelected(int position) {
        // Here's your instance
        YourFragment fragment =(YourFragment)yourSubPagerAdapter.getRegisteredFragment(position);
        ((MyActivity)getActivity).setCurrentFragment(fragment);
    }

    @Override
    public void onPageScrollStateChanged(int state) {

    }
});

With this way you can keep one instance and your top fragment.