为什么我的片段在viewpager中重新初始化？

Question

I have a problem with my viewpager which contains 3 Fragments : when I go to the left one, the informations of the right one disapear, and when I go to the right one, the informations of the left one disapear. By "informations" I mean textViews labels and datas from my project.

This problem causes NullPointerExceptions.

My code :

public class Principal extends FragmentActivity {

private PagerAdapter mPagerAdapter;
private List<Fragment> fragments;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    super.setContentView(R.layout.principal);
    DataModel.getInstance().setFragmentActivity(this);

    // Création de la liste de Fragments que fera défiler le PagerAdapter
    fragments = new Vector<Fragment>();

    // Ajout des Fragments dans la liste
    fragments.add(Fragment.instantiate(this,PageGaucheFragment.class.getName()));
    fragments.add(Fragment.instantiate(this,PageMilieuFragment.class.getName()));
    fragments.add(Fragment.instantiate(this,PageDroiteFragment.class.getName()));

    // Création de l'adapter qui s'occupera de l'affichage de la liste de Fragments
    this.mPagerAdapter = new MyPagerAdapter(super.getSupportFragmentManager(), fragments);

    ViewPager pager = (ViewPager) super.findViewById(R.id.viewpager);
    // Affectation de l'adapter au ViewPager
    pager.setAdapter(this.mPagerAdapter);
    pager.setCurrentItem(1);

}
...



public class MyPagerAdapter extends FragmentPagerAdapter {

private final List<Fragment> fragments;

//On fournit à l'adapter la liste des fragments à afficher
public MyPagerAdapter(FragmentManager fm, List<Fragment> fragments) {
    super(fm);
    this.fragments = fragments;
}

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

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

}

And my fragments begin like this :

public class PageGaucheFragment extends Fragment {

private TextView textViewIlluminePrecedent = null;

private View V;

private List<TextView> listeTextViewCombinaisons = new ArrayList<>();

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

I looked for a solution for this problem, maybe it's a "nested" fragments problem but I don't know what it is.

Thank for your help !

Sorry for my english, I'm french ;)

And tell me if I didn't write enough details

Answer 1

By default a ViewPager only keeps 1 offscreen page on either side of the current page in memory at a time. This means that you have a maximum of three pages in memory, and if you are on the first or last item you only have two.

You can adjust this by calling setOffscreenPageLimit(int) on your ViewPager.

However, you should do so cautiously, as this can have a serious impact on memory, especially when dealing with Fragments.

It sounds like the real problem is that your Fragments aren't handling being recreated properly. If you are encountering NullPointerExceptions when the ViewPager recreates your Fragments, you will likely encounter them in other situations as well.

I would spend some time looking into what is causing those NullPointerExceptions, and try to address those bugs. Pay special attention to when the Fragment's lifecycle methods are called when you change ViewPager pages and that should help shed light on the source of the issue.