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

Exception or error:

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.

scenario

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;
    }
}
How to solve:

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.

###

After some comments with DEADMC, I decided to implement it with extra memory, saving in a list the open fragments.

I explain my solution. I hve two types of special fragments, host and root. Host Fragments are those which are the init of a tab. Root Fragments are those which hold a ViewPager. In this scenario, tab_b has a Root Fragment with an inner ViewPager.

Hierarchy

For each tab, this is, for each HostFragment, I have added a list fragments to save the fragments opened in this tab. Also a method getCurrentFragment to get the last displayed in this tab.

public class HostFragment extends BackStackFragment {

    private ArrayList<Fragment> fragments = new ArrayList<>();

    public Fragment getCurrentFragment() {
        return fragments.get(fragments.size() - 1);
    }

Also a method to remove the last fragment displayed in this tab is needed, to maintain a true state. Back button needs to be redirect to this method.

    public void removeCurrentFragment() {
        getChildFragmentManager().popBackStack();
        fragments.remove(fragments.size() - 1);
    }

Also previous methods need to be changed, to insert the new fragments opened into the list.

    public void replaceFragment(Fragment fragment, boolean addToBackstack) {
        // NEW: Add new fragment to the list.
        fragments.add(fragment);
        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();
        // NEW: Add first fragment to the list.
        hostFragment.fragments.add(fragment);
        return hostFragment;
    }
} // HostFragment.

RootFragment is an interface implemented by those fragments which holds a ViewPager.

public interface RootFragment {

    /**
     * Opens a new Fragment in the current page of the ViewPager held by this Fragment.
     *
     * @param fragment - new Fragment to be opened.
     */
    void openNewFragment(Fragment fragment);

    /**
     * Returns the fragment displayed in the current tab of the ViewPager held by this Fragment.
     */
    Fragment getCurrentFragment();
}

And then, the implementation would be:

MainActivity is not a Fragment, but I implement RootFragment interface for meaning.

public class MainActivity extends AppCompatActivity implements RootFragment {

    private void setupViewPager() {
        adapter = new ViewPagerAdapter(getSupportFragmentManager());
        // Wrap with HostFragment to get separate tabbed nagivation.
        adapter.addFrag(HostFragment.newInstance(new Fragment1()), null);
        adapter.addFrag(new RootFragment2(), null); // This is a Root, not a Host.
        adapter.addFrag(HostFragment.newInstance(new Fragment4()), null);
        viewPager.setAdapter(adapter);
        // 2 because TabLayout has 3 tabs.
        viewPager.setOffscreenPageLimit(2);
    }

    @Override
    public void openNewFragment(Fragment fragment) {
        Fragment hosted = adapter.getItem(viewPager.getCurrentItem());
        // Replace the fragment of current tab to [fragment].
        if (hosted instanceof HostFragment) {
            ((HostFragment) hosted).replaceFragment(fragment, true);
        // Spread action to next ViewPager.
        } else {
            ((RootFragment) hosted).openNewFragment(fragment);
        }
    }

    @Override
    public Fragment getCurrentFragment() {
        Fragment hosted = adapter.getItem(viewPager.getCurrentItem());
        // Return current tab's fragment.
        if (hosted instanceof HostFragment) {
            return ((HostFragment) hosted).getCurrentFragment();
        // Spread action to next ViewPager.
        } else {
            return ((RootFragment) hosted).getCurrentFragment();
        }
    }
}

and RootFragment2

public class RootFragment2 extends Fragment implements RootFragment {

    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);
        // 1 because TabLayout has 2 tabs.
        viewPager.setOffscreenPageLimit(1);
    }

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

    @Override
    public Fragment getCurrentFragment() {
        return ((HostFragment) adapter.getItem(viewPager.getCurrentItem())).getCurrentFragment();
    }
}

And then, I just need to call:

((MainActivity) getActivity()).getCurrentFragment();

###

You can create something like CustomFragment class which contains getClassName method and extends from Fragment class. Then extend all your fragments such as RootFragment2 from CustomFragment class instead of just Fragment.

So you can get fragment like this:

public CustomFragment getNeededFragment(String fragmentName) {
    List<Fragment> fragments = getSupportFragmentManager().getFragments();
    CustomFragment result = null;
    if (fragments != null) {
        for (Fragment fragment : fragments) {
            if (fragment != null && fragment.isVisible()) {
                try {
                    CustomFragment customFragment = (CustomFragment) customFragment;
                    if (customFragment.getClassName().equals(fragmentName)))
                    result = customFragment;
                } catch (ClassCastException e) {
                }
            }
        }
    }
    return result ;
}

###

I would do something like that:

1. Create a interface like this.

public interface ReturnMyself {
    Fragment returnMyself();
}

2. All fragments inside ViewPagers should implements this one.

3. Add OnPageChangeListener to your main VP. So You will always know current position.

4. Add OnPageChangeListener your inner(s) VP so you will know which one is there on screen.

5. Add method to your adapter (both main and inner) which returns your fragment from list passed to it.

public ReturnMyself getReturnMyselfAtPosition()

6. All Fragments should return this in returnMyself()

7. Fragment that has inner fragments should return in returnMyself something like.

Fragment returnMyself() {
    return this.myInnerFragmentAdapter.getReturnMyselfAtPosition().returnMyself(); 
}

8. From main fragment/activity you just call.

this.adapter.getReturnMyselfAtPosition().returnMyself();

And you got your current fragment.

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