PagerAdapter not called inside fragment when I move to the fragment for the second time in Android

Question

I'm working with viewpager inside fragments. I'm using pagerAdapter to switch between two pages via XML. Here is the code:

public class DemoPagerAdapter extends FragmentPagerAdapter {

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

    @Override
    public Fragment getItem(int index) {
        Fragment f;
        switch (index) {
        case 0:
            f = new FirstFragment(); 
            break;
        case 1:
            f =  new SecondFragment();
            break;
        }
        return f;
    }

    @Override
    public int getCount() {
        return 2;
    }
}

And I'm calling it like this:

public class DemoFragment1 extends Fragment {
private ViewPager viewPager;
private DemoPagerAdapter mAdapter;
public DemoFragment1() {

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    final View rootView = inflater.inflate(R.layout.security, container, false);
    mAdapter = new DemoPagerAdapter(getActivity().getSupportFragmentManager());
    viewPager.setAdapter(mAdapter);
    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
                        showPages(position);
        }
        @Override
        public void onPageScrolled(int arg0, float arg1, int arg2) {
        }
        @Override
        public void onPageScrollStateChanged(int arg0) {
        }
    });
    return rootView;
}

Everything works fine. But if I move to DemoFragment2 and then come back to DemoFragment1, PagerAdapter is not called.

Answer 1

Well, I encontered this issue last week.

When I put the ViewPager in the fragment with a FragmentPagerAdapter, after I move forward to anoter Fragment and back, the pager' s view not exists.

I supposed is the FragmentPagerAdapter won' t retain its view in the ViewPager ' s host Fragment, Their life cycle is not the same. Though you put the VewPager' s host Fragment in the back stack, but the FragmentPagerAdapter' s fragment not. And Then, I gave up FragmentPagerAdapter and tried android.support.v4.view.PagerAdapter, (note, it wont' t use fragment to inject view!), everything is fine!

below is the sample:

private final PagerAdapter coverPager = new PagerAdapter() {

    private LayoutInflater mLayoutInflater;

    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public boolean isViewFromObject(View view, Object object) {
        return view == object;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View) object);
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        if (mLayoutInflater == null) {
            mLayoutInflater = LayoutInflater.from(getActivity());
        }
        switch (position) {
            case 0:
                View frontPage = mLayoutInflater.inflate(R.layout.profile_cover, container, false);
                ImageView avatar = (ImageView) frontPage.findViewById(R.id.avatar);
                Picasso.with(getActivity())
                        .load(mAvatarUrl)
                        .placeholder(R.drawable.error)
                        .error(R.drawable.error)
                        .into(avatar);
                TextView screenName = (TextView) frontPage.findViewById(R.id.screen_name);
                screenName.setText("@" + mScreenName);
                TextView remark = (TextView) frontPage.findViewById(R.id.remark);
                remark.setText(TextUtils.isEmpty(mRemark) ? mScreenName : mRemark);
                if (mVerified) {
                    frontPage.findViewById(R.id.verified).setVisibility(View.VISIBLE);
                }
                container.addView(frontPage);
                return frontPage;
            case 1:
                View introPage = mLayoutInflater.inflate(R.layout.profile_intro, container, false);
                CatnutUtils.setText(introPage, R.id.description, TextUtils.isEmpty(mDescription)
                        ? getString(R.string.no_description) : mDescription);
                CatnutUtils.setText(introPage, R.id.location, mLocation);
                CatnutUtils.setText(introPage, R.id.profile_url, Constants.WEIBO_DOMAIN + mProfileUrl);
                container.addView(introPage);
                return introPage;
            default:
                return null;
        }
    }
};

and then in your onViewCreate or OnCreateView, put mViewPager.setAdapter(coverPager);

the instantiateItem is the place to put the page' s view, similar like the FragmentPagerAdapter!

It' s just like put the FragmentAdapter' s view int the ViewPager' s host Fragment, not the other fragment!

Hope it' s helpful!

Answer 2

Your ViewPager is nested in a Fragment, you should use the ChildFragmentManager, modify:

mAdapter = new DemoPagerAdapter(getChildFragmentManager());

I also assume the ViewPager will keep the off screen fragment in memory, thus it might not call getItem(int index) again. Add

viewPager.setOffscreenPageLimit(0);
return rootView;