Cause analysis

Activity saves the status of all added Fragments after adding Fragments to Activity; Activity recreates the original fragments and attach es them when it is reclaimed due to problems such as system memory; however, if we use a reference to save and use Fragments in ViewPager, it will cause an exception.

Problem solving

Programme 1

Activity does not call the parent class's method to save Fragment status.

@Override
protected void onSaveInstanceState(Bundle outState) {
 //   super.onSaveInstanceState(outState);
}

Programme 2

In the onCreate method of Fragment, the following method is called to clear all the sub-Fragments to which the current Fragment belongs.

private void clearFragments(){
        FragmentManager fm = getChildFragmentManager();
        if(fm != null && fm.getFragments() != null && fm.getFragments().size() > 0){
            FragmentTransaction ft = fm.beginTransaction();
            for(Fragment fragment : fm.getFragments()){
                ft.remove(fragment);
            }
            ft.commit();
        }
}

Programme 3

Override Activity's onCreate method:

@Override
protected void onCreate(Bundle savedInstanceState) {
        if(savedInstanceState != null){
            savedInstanceState.remove("android:support:fragments");
        }

        super.onCreate(savedInstanceState);

        ...
}

Programme 4

When obtaining Fragment s in ViewPager, do not get them by reference, instead, get them by the following way:uuuuuuuuuuuu

 /**
     * ViewPager Find the Fragment of the specified location based on position
     * @param fragmentManager
     * @param viewPagerId
     * @param fragmentPagerAdapter
     * @param position
     * @return
     */
    public static Fragment getFragment(FragmentManager fragmentManager, int viewPagerId, FragmentPagerAdapter fragmentPagerAdapter, int position){
        if(fragmentManager != null){
            String tag;
            if(fragmentPagerAdapter != null){
                tag = "android:switcher:" + viewPagerId + ":" + fragmentPagerAdapter.getItemId(position);
            } else {
                tag = "android:switcher:" + viewPagerId + ":" + position;
            }

            return fragmentManager.findFragmentByTag(tag);
        } else {
            return null;
        }
    }

conclusion

The above four methods can solve some problems caused by Activity's own reconstruction of Fragment, but the maintainability and scalability of scheme 1, 2 and 3 are not good, recommend scheme 4.

Thoughts on Problems

1. In scenario 2, when Activity destruction is rebuilt, sub-fragments will be rebuilt automatically, but after onCreate is cleaned up with code, our business code will be rebuilt later.

2. Schemes 1, 2 and 3 are all trying to eliminate the original Fragment and its state. Schemes 4 only modifies the way of obtaining Fragment and better retains the original Fragment state logic.