instantiateItem nor getItem are called on FragmentPagerAdapter

问题

I've one activity with a viewpager with a custom FragmentAdapter.

Here is my adapter code:

this.viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            SparseArray<WeakReference<Fragment>> registeredFragments = new SparseArray<>(4);

            @NonNull
            @Override
            public Object instantiateItem(ViewGroup container, int position) {
                Fragment fragment = (Fragment) super.instantiateItem(container, position);
                this.registeredFragments.put(position, new WeakReference<>(fragment));
                return fragment;
            }


            @Override
            public int getCount() {
                return 4;
            }

            @Override
            public android.support.v4.app.Fragment getItem(int position) {
                WeakReference<Fragment> ret = this.registeredFragments.get(position);

                if (ret == null || ret.get() == null) {
                    if (position == 0)
                        ret = new WeakReference<>(new NewGiveawayStepFragment1());
                    else if (position == 1)
                        ret = new WeakReference<>(NewGiveawayStepFragment2.newInstance(getUser().getApiFacade().getProfilePicUrl()));
                    else if (position == 2)
                        ret = new WeakReference<>(new NewGiveawayStepFragment3());
                    else if (position == 3)
                        ret = new WeakReference<>(NewGiveawayStepFragment4.newInstance(getUser().getApiFacade().getProfilePicUrl()));
                }
                return ret.get();
            }
        });

The idea is after the fragments were created i store them into a SparseArray and avoid recreating them (i know i could have same behavior viewPager.setOffscreenPageLimit(); but it makes the activity take longer to load at first time

This idea was based on this answer:
https://stackoverflow.com/a/15261142/5679560

In a normal happy world everything works fine...

the problem is when user minimize the app -> android decides to saveState and close the app -> them user restores the app...

In this scenario when the user restores the app the pages setOffscreenPageLimit are never created

So let be more specific:
1-user loads activity
2-user loads viewpage 0
3-user minimizes the app and android close it
4-user restores the app OK
5-user goes to viewpage 1 OK
6-user goes to viewpage 2 instantiateItem is never called getItem is never called and a blank page is shown
7-user goes to viewpage 3 instantiateItem is never called getItem is never called and a blank page is shown

No exception is thrown. If the user leaves the app at page 1, when it restores page 0,1 and 2 are OK, if he leaves at page 3 then pages 2 and 3 are OK

This is my activity onRestoreInstanceState

  @Override
    public void onRestoreInstanceState(@Nullable Bundle savedInstanceState) {
        this.currentPage = savedInstanceState.getInt("currentPage");
        this.ignoreSpam = savedInstanceState.getBoolean("ignoreSpam", false);
        super.onRestoreInstanceState(savedInstanceState);
    }

Absolutelly nothing special

Does anyone understand why this happens?

来源：https://stackoverflow.com/questions/55446077/instantiateitem-nor-getitem-are-called-on-fragmentpageradapter