Update ViewPager dynamically?
android, android-fragments, android-viewpager
Solution
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.
Problem
I can't update the content in ViewPager. What is the correct usage of methods instantiateItem() and getItem() in FragmentPagerAdapter class? I was using only getItem() to instantiate and return my fragments: ``` @Override public Fragment getItem(int position) { return new MyFragment(context, paramters); } ``` This worked well. Except I can't change the content. So I found this: ViewPager PagerAdapter not updating the View "My approach is to use the setTag() method for any instantiated view in the instantiateItem() method" Now I want to implement instantiateItem() to do that. But I don't know what I have to return (the type is Object) and what is the relation with getItem(int position)? I read the reference: public abstract Fragment getItem (int position) Return the Fragment associated with a specified position. public Object instantiateItem (ViewGroup container, int position) Create the page for the given position. The adapter is responsible for adding the view to the container given here, although it only must ensure this is done by the time it returns from finishUpdate(ViewGroup). Parameters container The containing View in which the page will be shown. position The page position to be instantiated. Returns Returns an Object representing the new page. This does not need to be a View, but can be some other container of the page. but I still don't get it. Here's my code. I'm using support package v4. ViewPagerTest ``` public class ViewPagerTest extends FragmentActivity { private ViewPager pager; private MyFragmentAdapter adapter; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.pager1); pager = (ViewPager)findViewById(R.id.slider); String[] data = {"page1", "page2", "page3", "page4", "page5", "page6"}; adapter = new MyFragmentAdapter(getSupportFragmentManager(), 6, this, data); pager.setAdapter(adapter); ((Button)findViewById(R.id.button)).setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { reload(); } }); } private void reload() { String[] data = {"changed1", "changed2", "changed3", "changed4", "changed5", "changed6"}; //adapter = new MyFragmentAdapter(getSupportFragmentManager(), 6, this, data); adapter.setData(data); adapter.notifyDataSetChanged(); pager.invalidate(); //pager.setCurrentItem(0); } } ``` MyFragmentAdapter ``` class MyFragmentAdapter extends FragmentPagerAdapter { private int slideCount; private Context context; private String[] data; public MyFragmentAdapter(FragmentManager fm, int slideCount, Context context, String[] data) { super(fm); this.slideCount = slideCount; this.context = context; this.data = data; } @Override public Fragment getItem(int position) { return new MyFragment(data[position], context); } @Override public int getCount() { return slideCount; } public void setData(String[] data) { this.data = data; } @Override public int getItemPosition(Object object) { return POSITION_NONE; } } ``` MyFragment ``` public final class MyFragment extends Fragment { private String text; public MyFragment(String text, Context context) { this.text = text; } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View view = inflater.inflate(R.layout.slide, null); ((TextView)view.findViewById(R.id.text)).setText(text); return view; } } ``` Here is also somebody with a similar problem, no answers http://www.mail-archive.com/android-developers@googlegroups.com/msg200477.html