Update ViewPager dynamically?

When using FragmentPagerAdapter or FragmentStatePagerAdapter, it is best to deal solely with getItem() and not touch instantiateItem() at all. The instantiateItem()–destroyItem()–isViewFromObject() interface on PagerAdapter is a lower-level interface that FragmentPagerAdapter uses to implement the much simpler getItem() interface.

Before getting into this, I should clarify that

if you want to switch out the actual fragments that are being displayed, you need to avoid FragmentPagerAdapter and use
FragmentStatePagerAdapter.

An earlier version of this answer made the mistake of using FragmentPagerAdapter for its example – that won’t work because FragmentPagerAdapter never destroys a fragment after it’s been displayed the first time.

I don’t recommend the setTag() and findViewWithTag() workaround provided in the post you linked. As you’ve discovered, using setTag() and findViewWithTag() doesn’t work with fragments, so it’s not a good match.

The right solution is to override getItemPosition(). When notifyDataSetChanged() is called, ViewPager calls getItemPosition() on all the items in its adapter to see whether they need to be moved to a different position or removed.

By default, getItemPosition() returns POSITION_UNCHANGED, which means, “This object is fine where it is, don’t destroy or remove it.” Returning POSITION_NONE fixes the problem by instead saying, “This object is no longer an item I’m displaying, remove it.” So it has the effect of removing and recreating every single item in your adapter.

This is a completely legitimate fix! This fix makes notifyDataSetChanged behave like a regular Adapter without view recycling. If you implement this fix and performance is satisfactory, you’re off to the races. Job done.

If you need better performance, you can use a fancier getItemPosition() implementation. Here’s an example for a pager creating fragments off of a list of strings:

ViewPager pager = /* get my ViewPager */;
// assume this actually has stuff in it
final ArrayList<String> titles = new ArrayList<String>();

FragmentManager fm = getSupportFragmentManager();
pager.setAdapter(new FragmentStatePagerAdapter(fm) {
    public int getCount() {
        return titles.size();
    }

    public Fragment getItem(int position) {
        MyFragment fragment = new MyFragment();
        fragment.setTitle(titles.get(position));
        return fragment;
    }

    public int getItemPosition(Object item) {
        MyFragment fragment = (MyFragment)item;
        String title = fragment.getTitle();
        int position = titles.indexOf(title);

        if (position >= 0) {
            return position;
        } else {
            return POSITION_NONE;
        }
    }
});

With this implementation, only fragments displaying new titles will get displayed. Any fragments displaying titles that are still in the list will instead be moved around to their new position in the list, and fragments with titles that are no longer in the list at all will be destroyed.

What if the fragment has not been recreated, but needs to be updated anyway? Updates to a living fragment are best handled by the fragment itself. That’s the advantage of having a fragment, after all – it is its own controller. A fragment can add a listener or an observer to another object in onCreate(), and then remove it in onDestroy(), thus managing the updates itself. You don’t have to put all the update code inside getItem() like you do in an adapter for a ListView or other AdapterView types.

One last thing – just because FragmentPagerAdapter doesn’t destroy a fragment doesn’t mean that getItemPosition is completely useless in a FragmentPagerAdapter. You can still use this callback to reorder your fragments in the ViewPager. It will never remove them completely from the FragmentManager, though.