How to work with ViewPager.OnPageChangeListener

Question

As per this accepted answer, I was able to get my tabLyout as below:

What do I need now, is to be able to change the buttons text based on the fragment position, for example if the position is zero, the Back should not appear, and if the position is the last, the Next should be replaced by Done

I have used the ViewPager.OnPageChangeListener but did not work for me, below is the full code:

public class Slider extends AppCompatActivity {

    private TabLayout tabLayout;
    private ViewPager viewPager;
    private Button leftBtn, rightBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.start_up);

        leftBtn = (Button) findViewById(R.id.left_btn);
        rightBtn= (Button) findViewById(R.id.right_btn);

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

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

   // This is not working, even the `Toast` not appearing
    private ViewPager.OnPageChangeListener listener = new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int position) {
            Toast.makeText(Slider.this, "Current position: "+position, Toast.LENGTH_SHORT).show();
            if (position == 0) {
                leftBtn.setText("");
            }

            if (position == 3) {
                rightBtn.setText("DONE");
            }
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }
    };

    private void setupViewPager(ViewPager viewPager) {
        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
        adapter.addFrag(new Tab1(),"");
        adapter.addFrag(new Tab2(),"");
        adapter.addFrag(new Tab3(),"");
        adapter.addFrag(new Tab4(),"");
        viewPager.setAdapter(adapter);
    }

    static class ViewPagerAdapter extends FragmentPagerAdapter {
        private final List<Fragment> mFragmentList = new ArrayList<>();
        private final List<String> mFragmentTitleList = new ArrayList<>();

        public ViewPagerAdapter(FragmentManager manager) {
            super(manager);
        }

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

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

        public void addFrag(Fragment fragment, String title) {

            mFragmentList.add(fragment);
            mFragmentTitleList.add(title);
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return mFragmentTitleList.get(position);
        }
    }

    public void next_fragment(View view) {
        viewPager.setCurrentItem(viewPager.getCurrentItem()+1);
    }

    public void previous_fragment(View view) {
        viewPager.setCurrentItem(viewPager.getCurrentItem()-1);
    }

    public void start_app(View view) {
        startActivity(new Intent(Slider.this, TrustedNumber.class));
        finish();
    }
}

Answer 1

It's because you haven't added the listener to the viewpager

viewPager.addOnPageChangeListener(listener);