Update ViewPager dynamically?
I can't update the content in ViewPager.
A question: What is the relationship and 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 (but as said I can't change the content).
So I found this: ViewPager PagerAdapter not updating the View
Particulary one in which it's talked about method instantiateItem():
"My approach is to use the setTag() method for any instantiated view in the instantiateItem() method"
So now I want to implement instantiateItem() in order to do that. But I don't know what I have to return there (return is Object) and what is the relation with getItem(int position)?
Currently I'm using getItem to instantiate the Fragment, is that wrong? But then do I have to put the fragments in instance variable, or not implement getItem() at all...? I just don't understand it.
I tried reading 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 understand how are they related and what I have to do.
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
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.
Instead of returning POSITION_NONE from getItemPosition() and causing full view recreation, do this:
//call this method to update fragments in ViewPager dynamically
public void update(UpdateData xyzData) {
this.updateData = xyzData;
notifyDataSetChanged();
}
@Override
public int getItemPosition(Object object) {
if (object instanceof UpdateableFragment) {
((UpdateableFragment) object).update(updateData);
}
//don't return POSITION_NONE, avoid fragment recreation.
return super.getItemPosition(object);
}
Your fragments should implement UpdateableFragment interface:
public class SomeFragment extends Fragment implements
UpdateableFragment{
@Override
public void update(UpdateData xyzData) {
// this method will be called for every fragment in viewpager
// so check if update is for this fragment
if(forMe(xyzData)) {
// do whatever you want to update your UI
}
}
}
and the interface:
public interface UpdateableFragment {
public void update(UpdateData xyzData);
}
Your data class:
public class UpdateData {
//whatever you want here
}
I have encountered this problem and finally solved it today, so I write down what I have learned and I hope it is helpful for someone who is new to Android's ViewPager and update as I do. I'm using FragmentStatePagerAdapter in API level 17 and currently have just 2 fragments. I think there must be something not correct, please correct me, thanks. 
Serialized data has to be loaded into memory. This can be done using a CursorLoader / AsyncTask / Thread . Whether it's automatically loaded depends on your code. If you are using a CursorLoader , it's auto-loaded since there is a registered data observer.
After you call viewpager.setAdapter(pageradapter) , the adapter's getCount() is constantly called to build fragments. So if data is being loaded, getCount() can return 0, thus you don't need to create dummy fragments for no data shown.
After the data is loaded, the adapter will not build fragments automatically since getCount() is still 0, so we can set the actually loaded data number to be returned by getCount() , then call the adapter's notifyDataSetChanged() . ViewPager begin to create fragments (just the first 2 fragments) by data in memory. It's done before notifyDataSetChanged() is returned. Then the ViewPager has the right fragments you need.
If the data in the database and memory are both updated (write through), or just data in memory is updated (write back), or only data in the database is updated. In the last two cases if data is not automatically loaded from the database to memory (as mentioned above). The ViewPager and pager adapter just deal with data in memory.
So when data in memory is updated, we just need to call the adapter's notifyDataSetChanged() . Since the fragment is already created, the adapter's onItemPosition() will be called before notifyDataSetChanged(） returns. Nothing needs to be done in getItemPosition() . Then the data is updated.
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