Go to next page in ViewPager

Question

I have a ViewPager in my MainActivity. Every page in it is a Fragment. So, everytime I swipe right or left, a new instance of the fragment is created and the fragment view is updated accordingly.

I also have two buttons: LEFT and RIGHT for navigation. These buttons are inside the Fragment, not in the Activity. A user can either swipe or alternatively press the relevant button to navigate between the pages.

Here's the problem: Since I'm changing the views through my MainActivity, how do I detect the onClick events of those buttons in the Activity in order to update the fragment?

Here's the PagerAdapter class (Removed all the irrelevant code from everywhere):

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {

        // logic part for the views.

        return PlaceholderFragment.newInstance(sectionNumber, questionStatus, questionOrder, showNext, showPrevious);
    }

And here's the PlaceHolderFragment class:

public class PlaceholderFragment extends Fragment{

    //some global variables

    public static PlaceholderFragment newInstance(int sectionNumber, int questionStatus, String questionOrder, boolean showNext, boolean showPrevious) {
        PlaceholderFragment fragment = new PlaceholderFragment();

        //setting up the arguments
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.explanation_fragment, container, false);

        //code for filling up all the views

        RightButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //can I do something here?
            }
        });

        return rootView;
    }
}

MORE INFO: I have to keep the navigation buttons in the fragment itself and not in the activity.

Answer 1

In your fragment write one interface like:

public class PlaceholderFragment extends Fragment{
        private OnButtonClickListener mOnButtonClickListener;

        interface OnButtonClickListener{
        void onButtonClicked(View view);
        }

        @Override
        public void onAttach(Context context) {
            super.onAttach(context);
            try {
                mOnButtonClickListener = (OnButtonClickListener) context;
            } catch (ClassCastException e) {
                throw new ClassCastException(((Activity) context).getLocalClassName()
                            + " must implement OnButtonClickListener");
            }
        }

        yourButtons.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mOnButtonClickListener.onButtonClicked(v);
            }
        });
     }   

And in your mainactivity:

class MainActivity extends AppCompatActivity implements   OnButtonClickListener{

    @Override
    void onButtonClicked(View view){
        int currPos=yourPager.getCurrentItem();

        switch(view.getId()){

            case R.id.leftNavigation:
            //handle currPos is zero
            yourPager.setCurrentItem(currPos-1);
            break;

            case R.id.rightNavigation:
            //handle currPos is reached last item
            yourPager.setCurrentItem(currPos+1);
            break;
        }
    }
}

Answer 2

For Kotlin and ViewPager2

previous page:

val currPos: Int = xyzViewPager.currentItem
if (currPos != 0) {
   xyzViewPager.currentItem = currPos - 1
}

next page:

val currPos: Int = xyzViewPager.currentItem
if ((currPos + 1) != xyzViewPager.adapter?.count) {
    xyzViewPager.currentItem = currPos + 1
}