Why `PagerAdapter::notifyDataSetChanged` is not updating the View?
android, android-adapter, android-viewpager, android-viewpager2
Solution
There are several ways to achieve this.
The first option is easier, but bit more inefficient.
Override `getItemPosition` in your `PagerAdapter` like this:
public int getItemPosition(Object object) {
return POSITION_NONE;
}
This way, when you call `notifyDataSetChanged()`, the view pager will remove all views and reload them all. As so the reload effect is obtained.
The second option, suggested by Alvaro Luis Bustamante (previously alvarolb), is to `setTag()` method in `instantiateItem()` when instantiating a new view. Then instead of using `notifyDataSetChanged()`, you can use `findViewWithTag()` to find the view you want to update.
Conclusion
If you have a lot of views, or want to support modifying any specific item and/or view (fastly at any time), then the second approach (tagging) is very flexible and high performant, as it prevents recreating all the not modified views. (Kudos to alvarolb for the original research.)
But if your App has only a "refresh" feature (without single item changes being even allowed), or has just few items, use the first approach, as it saves development time.
Problem
I'm using the ViewPager from the compatibility library. I have succussfully got it displaying several views which I can page through. However, I'm having a hard time figuring out how to update the ViewPager with a new set of Views. I've tried all sorts of things like calling `mAdapter.notifyDataSetChanged()`, `mViewPager.invalidate()` even creating a brand new adapter each time I want to use a new List of data. Nothing has helped, the textviews remain unchanged from the original data. Update: I made a little test project and I've almost been able to update the views. I'll paste the class below. What doesn't appear to update however is the 2nd view, the 'B' remains, it should display 'Y' after pressing the update button. ``` public class ViewPagerBugActivity extends Activity { private ViewPager myViewPager; private List<String> data; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); data = new ArrayList<String>(); data.add("A"); data.add("B"); data.add("C"); myViewPager = (ViewPager) findViewById(R.id.my_view_pager); myViewPager.setAdapter(new MyViewPagerAdapter(this, data)); Button updateButton = (Button) findViewById(R.id.update_button); updateButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { updateViewPager(); } }); } private void updateViewPager() { data.clear(); data.add("X"); data.add("Y"); data.add("Z"); myViewPager.getAdapter().notifyDataSetChanged(); } private class MyViewPagerAdapter extends PagerAdapter { private List<String> data; private Context ctx; public MyViewPagerAdapter(Context ctx, List<String> data) { this.ctx = ctx; this.data = data; } @Override public int getCount() { return data.size(); } @Override public Object instantiateItem(View collection, int position) { TextView view = new TextView(ctx); view.setText(data.get(position)); ((ViewPager)collection).addView(view); return view; } @Override public void destroyItem(View collection, int position, Object view) { ((ViewPager) collection).removeView((View) view); } @Override public boolean isViewFromObject(View view, Object object) { return view == object; } @Override public Parcelable saveState() { return null; } @Override public void restoreState(Parcelable arg0, ClassLoader arg1) { } @Override public void startUpdate(View arg0) { } @Override public void finishUpdate(View arg0) { } } } ```