How do I dynamically add fragments to start of ViewPager as user reaches the end of the ViewPager?

Question

I currently have a basic ViewPager that utilizes a FragmentStatePagerAdapter to initialize fragments.

This ViewPager starts at the end and scrolls backwards (i.e. scrolls to the left).

As the user nears the start of the ViewPager's list, I want there to be more items added to the start of the adapter's list programmatically, essentially creating an infinitely scrollable ViewPager (going left) without disrupting the user's experience (i.e. not reloading their current fragment as the ViewPager has fragments added on to it). However, I am unable to accomplish this.

Currently, I am detecting when my adapter's getItem() function passes a position of 0, populating the start of the list the adapter uses with more items, and then calling notifyDataSetChanged(). However, what happens is that the fragments at position 1 (which triggers the getItem() call as soon as it comes into focus) and 0 remain at the start of the ViewPager and the new fragments are loaded all out of order.

Here is my FragmentStatePagerAdapter:

class WeekyMoodFragmentAdapter extends FragmentStatePagerAdapter {

    private final int NUM_WEEKS_TO_INITIALIZE = 4;
    private final List<Long> timestampList = new ArrayList<>();
    private final ViewPager mViewPager;

    WeekyMoodFragmentAdapter(FragmentManager fragmentManager, ViewPager viewPager) {
        super(fragmentManager);

        mViewPager = viewPager;
        initializeTimestampList();
    }

    private void initializeTimestampList() {

        DateTime mondayOfThisWeek = new DateTime(System.currentTimeMillis());
        while (mondayOfThisWeek.getDayOfWeek() != DateTimeConstants.MONDAY) {
            mondayOfThisWeek = new DateTime(mondayOfThisWeek.getMillis()
                    - TimeUnit.HOURS.toMillis(DateTimeConstants.HOURS_PER_DAY));
        }

        long weekStartMS = mondayOfThisWeek.withTimeAtStartOfDay().getMillis()
                - TimeUnit.DAYS.toMillis(DateTimeConstants.DAYS_PER_WEEK * NUM_WEEKS_TO_INITIALIZE);

        addTimestampsToList(weekStartMS);
    }

    private void addTimestampsToList(long weekStartMS) {
        for (int i = 0; i < NUM_WEEKS_TO_INITIALIZE; ++i) {
            weekStartMS += TimeUnit.DAYS.toMillis(DateTimeConstants.DAYS_PER_WEEK);

            timestampList.add(i, weekStartMS);
        }
        notifyDataSetChanged();
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = WeeklyMoodFragment.newInstance(timestampList.get(position));
        if (position == 0) {
            addTimestampsToList(timestampList.get(position)
                    - TimeUnit.DAYS.toMillis
                    (DateTimeConstants.DAYS_PER_WEEK * (NUM_WEEKS_TO_INITIALIZE+1)));
        }
        return fragment;
    }

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

And how I initialize my ViewPager:

ViewPager moodViewPager = (ViewPager) view.findViewById(R.id.weeklyMoodViewPager);

moodViewPager.setAdapter(new WeekyMoodFragmentAdapter(getChildFragmentManager());
moodViewPager.setCurrentItem(moodViewPager.getAdapter().getCount());

snachmsm · Answer 1 · 2017-03-15T20:22:41.260

have you considered a bit hacky way with getCount() returning Integer.MAX_VALUE and newInstance created from timestampList.get(position%timestampList.size())? and set initial page setCurrentItem as Integer.MAX_VALUE/2 or nearest appriopiate

@Override
public Fragment getItem(int position) {
    return WeeklyMoodFragment.newInstance(
        timestampList.get(position % timestampList.size()));
        //                         ^  here important
}

@Override
public int getCount() {
    return Integer.MAX_VALUE;
}

    ...

moodViewPager.setCurrentItem(Integer.MAX_VALUE/2);
//or 
WeekyMoodFragmentAdapteradapter = new WeekyMoodFragmentAdapter(getChildFragmentManager());
moodViewPager.setAdapter(adapter);
//dividing integers will round
int roundedCount = (Integer.MAX_VALUE/2) / adapter.getCount();
moodViewPager.setCurrentItem(roundedCount * adapter.getCount())

How do I dynamically add fragments to start of ViewPager as user reaches the end of the ViewPager?

1 Answers1