How does TabLayout work?

Question

I have a Tablayout inside my MainActivity and a Viewpager that shows three Fragments.

When the app starts, this happens:

onCreate Fragment 1
onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1
onActivityCreated Fragment 2
onResume Fragment 2

This happens when tapping the second tab:

onCreate Fragment 1
onActivityCreated Fragment 3
onResume Fragment 3

Tapping the third tab:

onSaveInstanceState Fragment 1

Tapping on second tab from the third tab:

onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1

Code

public class ViewPagerAdapter extends FragmentStatePagerAdapter {

private final List<Fragment> mFragmentList = new ArrayList<>();
private final List<String> mFragmentTitleList = new ArrayList<>();

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

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

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

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

public void addFragment(Fragment fragment, String title) {
    mFragmentList.add(fragment);
    mFragmentTitleList.add(title);
}
}

In MainActivity

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

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

private void setupViewPager(ViewPager viewPager) {
    ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
    adapter.addFragment(DishFragment.newInstance(1), "Кулинария");
    adapter.addFragment(DishFragment.newInstance(2), "Кондитерская");
    adapter.addFragment(DishFragment.newInstance(3), "Хлебобулочная");
    viewPager.setAdapter(adapter);
}

---

onCreate Fragment 1
onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1
onActivityCreated Fragment 2
onResume Fragment 2

onCreate Fragment 1
onActivityCreated Fragment 3
onResume Fragment 3

onSaveInstanceState Fragment 1

onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1

Answer 1

ViewPager holds the adjacent Fragments in memory for quick transitions between pages. I'm thinking there's a fix for the recreated Fragments (since you're storing them in a list, add not creating them yourself more than once).

When app started this process happens:

I'm not sure about the doubled onCreate #1, but Fragment 1 and 2 are started

onCreate Fragment 1
onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1
onActivityCreated Fragment 2
onResume Fragment 2

This happens when tapping at second tap:

Fragment 1 & 2 were already created, but Fragment 3 is now created

onCreate Fragment 1
onActivityCreated Fragment 3
onResume Fragment 3

Tapping at third tap:

The OS doesn't need to hold onto Fragment 1 anymore

onSaveInstanceState Fragment 1

Tapping on second tap from third tap:

Fragment 1 is created / resumed

onCreate Fragment 1
onActivityCreated Fragment 1
onResume Fragment 1