onViewCreated is only called when the Fragments are first created. A ViewPager does not change the state of the Fragments. You can think of the hidden Fragments being stored, visible, to the sides of the currently visible Fragment.
I accomplished by setting up a callback when the TabLayout changes. The TabLayout is where the listener for this is. You can then have it call a callback method in your FragmentPagerAdapter. In each Fragment, you should implement a method that returns a callback. In the FragmentPagerAdapter, use the getItem(int position) to store the callback from the Fragment in a Map. The callback method in the adapter takes the position from the TabLayout
First, in your Fragments, give them a method that returns some kind of callback (I used a Runnable) that will be called from the adapter.
public class PlaceHolderFragment extends Fragment {
...
public Runnable getOnTabChangedListener() {
return new Runnable() {
@Override
public void run() {
scrollView.smoothScrollTo(0, 0);
}
};
}
...
}
Then, in your FragmentPagerAdapter (I called mine SectionsPagerAdapter), store the callbacks in a map in the getItem() method, and then add a custom callback.
public class SectionsPagerAdapter extends FragmentPagerAdapter {
...
HashMap<Integer, Runnable> tabChangedCallbacks = new HashMap<>();
...
@Override
public Fragment getItem(int position) {
// getItem is called to instantiate the fragment for the given page.
// Return a PlaceholderFragment (defined as a static inner class below).
PlaceholderFragment fragment = PlaceholderFragment.newInstance(position + 1);
tabChangedCallbacks.put(position, fragment.getOnTabChangedListener());
return fragment;
}
public void onTabChanged(int position) {
tabChangedCallbacks.get(position).run();
}
...
}
In your activity that contains the TabLayout, you will want to call addOnTabSelectedListener() on the TabLayout in the onCreate() method.
protected void onCreate(Bundle savedInstanceState) {
...
final SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());
final ViewPager viewPager = findViewById(R.id.view_pager);
viewPager.setAdapter(sectionsPagerAdapter);
TabLayout tabs = findViewById(R.id.tabs);
tabs.setupWithViewPager(viewPager);
tabs.addOnTabSelectedListener(
new TabLayout.ViewPagerOnTabSelectedListener(viewPager) {
@Override
public void onTabSelected(TabLayout.Tab tab) {
super.onTabSelected(tab);
sectionsPagerAdapter.onTabChanged(tab.getPosition());
}
});
...
}
EDIT
The principle is the same, but for this to work using ViewPager2 and FragmentStateAdapter, you will need the follow changes:
The Fragment stays the same.
The adapter is the same, functionally, with some updated methods:
public class SectionsStateAdapter extends FragmentStateAdapter {
HashMap<Integer, Runnable> tabChangedCallbacks = new HashMap<>();
...
public void onTabChanged(int position) {
tabChangedCallbacks.get(position).run();
}
@NonNull
@Override
public Fragment createFragment(int position) {
PlaceholderFragment fragment = PlaceholderFragment.newInstance(position + 1);
tabChangedCallbacks.put(position, fragment.getOnTabChangedListener());
return fragment;
}
@Override
public int getItemCount() {
return 2;
}
...
}
The activity changes the most:
public class MainActivity extends AppCompatActivity {
...
@Override
protected void onCreate(Bundle savedInstanceState) {
...
final SectionsStateAdapter sectionsStateAdapter = new SectionsStateAdapter(this);
final ViewPager2 viewPager = findViewById(R.id.view_pager);
viewPager.setAdapter(sectionsStateAdapter);
TabLayout tabs = findViewById(R.id.tabs);
// connect the TabLayout to the ViewPager2
new TabLayoutMediator(tabs, viewPager, new TabLayoutMediator.TabConfigurationStrategy() {
@Override
public void onConfigureTab(@NonNull TabLayout.Tab tab, int position) {
// set tab text, etc
}
}).attach();
// set the change listener on the ViewPager2
viewPager.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
@Override
public void onPageSelected(int position) {
super.onPageSelected(position);
sectionsStateAdapter.onTabChanged(position);
}
});
...
}
}
Note that the only real change is that the ViewPager2 doesn't need the TabLayout to handle the page change. You could still use TabLayout.addOnTabSelectedListener() if you wanted, but you would have to implement your own TabLayout.OnTabSelecterListener since TabLayout.ViewPagerOnTabSelectedListener doesn't work with ViewPager2.
For example:
tabs.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
@Override
public void onTabSelected(TabLayout.Tab tab) {
sectionsStateAdapter.onTabChanged(tab.getPosition());
}
@Override
public void onTabUnselected(TabLayout.Tab tab) {
}
@Override
public void onTabReselected(TabLayout.Tab tab) {
}
});
