Access ViewPager Fragment Method from Activity

Many mobile applications utilize fragments, self-contained components representing a modular screen section. Managing multiple fragments using a view pager grants smooth navigation and page animations. Occasionally, developers face the need to perform specific actions within a fragment in response to external events, such as a user swipe on the view pager. However, implementing this functionality can encounter certain challenges.

Initial Approaches and Limitations

Initially, an attempt was made to detect fragment visibility by overriding the setUserVisibleHint method. While this approach worked, it introduced a noticeable lag during fragment transitions.

An alternative approach involved utilizing OnPageChangeListener, a listener registered to the view pager, which provided more control over the timing of fragment actions. However, attempting to execute a method within a fragment from the OnPageChangeListener resulted in a NullPointerException.

Solution

The solution lies in maintaining a reference to each fragment instance within the pager adapter. The ViewPager provides a method called instantiateItem that can be overridden to store fragment references in an array or collection. During the onPageSelected event, you can access the desired fragment instance from the adapter and perform necessary actions, such as initiating a network request.

Example Code (Java)

public class PagerAdapter extends FragmentPagerAdapter {
    private Fragment[] fragments;

    public PagerAdapter(FragmentManager fm, Context context) {
        super(fm);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
        fragments[position] = createdFragment;
        return createdFragment;
    }
}

// Within your activity that hosts view pager:

onPageChangeListener = new ViewPager.OnPageChangeListener() {
    @Override
    public void onPageSelected(int position) {
        // Assuming you named your fragment FragmentTwo
        Fragment frag = adapter.fragments[position];
        if (frag != null && frag instanceof FragmentTwo) {
            ((FragmentTwo) frag).sendGetRequest();
        }
    }
};

Kotlin Equivalent (with Kotlin Extensions and ViewPager2)

viewPager.registerOnPageChangeCallback(
    object : ViewPager2.OnPageChangeCallback() {
        override fun onPageSelected(position: Int) {
            val frag = mAdapter.fragments[position]
            if (frag is FragmentTwo) {
                frag.sendGetRequest()
            }
        }
    }
)

Conclusion

By implementing this approach, you can effectively communicate with fragments from your hosting activity, enabling seamless integration and enhanced user experiences within your mobile applications.