Persistant fragments and viewpager

Question

I want to use 3 fragments within an Android App, I red: Creating-and-Using-Fragments .But I want to use the viewpager for swiping and displaying the fragments like explained in: ViewPager-with-FragmentPagerAdapter .But this code or the default code of Android Studio Example, use newInstance to create the instances, each time it is needed and destroy when not needed.

// Returns the fragment to display for that page
    @Override
    public Fragment getItem(int position) {
        switch (position) {
        case 0: // Fragment # 0 - This will show FirstFragment
            return FirstFragment.newInstance(0, "Page # 1");
        case 1: // Fragment # 0 - This will show FirstFragment different title
            return FirstFragment.newInstance(1, "Page # 2");
        case 2: // Fragment # 1 - This will show SecondFragment
            return SecondFragment.newInstance(2, "Page # 3");
        default:
            return null;
        }
    }

but I want to create once for all :

        // Within an activity

private FragmentA fragmentA;
private FragmentB fragmentB;
private FragmentC fragmentC;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        fragmentA = FragmentA.newInstance("foo");
        fragmentB = FragmentB.newInstance("bar");
        fragmentC = FragmentC.newInstance("baz");
    }
}

and only hide/show them, as in the example:

    // ...onCreate stays the same

// Replace the switch method
protected void displayFragmentA() {
    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    if (fragmentA.isAdded()) { // if the fragment is already in container
        ft.show(fragmentA);
    } else { // fragment needs to be added to frame container
        ft.add(R.id.flContainer, fragmentA, "A");
    }
    // Hide fragment B
    if (fragmentB.isAdded()) { ft.hide(fragmentB); }
    // Hide fragment C
    if (fragmentC.isAdded()) { ft.hide(fragmentC); }
    // Commit changes
    ft.commit();
}

But how to do that with a FragmentPagerAdapter public Fragment getItem(int position) no longer has to be like that

Also, how to access a data

 public double [][] tableaux;

that is in the MainActivity from one Fragment. Data will be persistant if I assign a pointer of the Fragment I just created in the MainActivity onCreate to point on the MainActivity.tableaux

Answer 1

You can return you pre-initialized fragments in getItem method.

@Override
public Fragment getItem(int position) {
    switch (position) {
    case 0: return fragmentA;
    case 1: return fragmentB;
    case 2: return fragmentC;
    default: return null;
}

UPDATE: @Alok is right. You should not have fragment references in Activity. And I suggest instead of increasing offscreen page limit using setOffscreenPageLimit, you should consider saving & restoring fragment states using public void onSaveInstanceState(Bundle) and savedInstanceState argument of public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState).

Answer 2

@Lotfi you don't need to save your fragment inside activity, it is prone to leaks . Also you can assign id to each of your fragment and later when you need to reuse that fragment you can ask fragment manager to return the fragment by passing ie. by calling fragment manager method findFragmentByID() inside your getItem().