ViewPager PagerAdapter not updating the View

Question

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) {
        }
    }
}

Answer 1

I found very interesting decision of this problem. Instead of using FragmentPagerAdapter, which keep in memory all fragments, we can use FragmentStatePagerAdapter (android.support.v4.app.FragmentStatePagerAdapter), that reload fragment each time, when we select it.

Realisations of both adapters are identical. So, we need just change "extend FragmentPagerAdapter" on "extend FragmentStatePagerAdapter"

Answer 2

what worked for me was going viewPager.getAdapter().notifyDataSetChanged();

and in the adapter putting your code for updating the view inside getItemPosition like so

@Override
public int getItemPosition(Object object) {

    if (object instanceof YourViewInViewPagerClass) { 
        YourViewInViewPagerClass view = (YourViewInViewPagerClass)object;
        view.setData(data);
    }

    return super.getItemPosition(object);
}

might not be the most correct way of going about it but it worked (the return POSITION_NONE trick caused a crash for me so wasnt an option)

Answer 3

Change the FragmentPagerAdapter to FragmentStatePagerAdapter.

Override getItemPosition() method and return POSITION_NONE.

Eventually, it will listen to the notifyDataSetChanged() on view pager.

Answer 4

After a lot of searching for this problem, I found a really good solution that I think is the right way to go about this. Essentially, instantiateItem only gets called when the view is instantiated and never again unless the view is destroyed (this is what happens when you override the getItemPosition function to return POSITION_NONE). Instead, what you want to do is save the created views and either update them in the adapter, generate a get function so someone else can update it, or a set function which updates the adapter (my favorite).

So, in your MyViewPagerAdapter add a variable like:

private View updatableView;

an in your instantiateItem:

 public Object instantiateItem(View collection, int position) {
        updatableView = new TextView(ctx); //My change is here
        view.setText(data.get(position));
        ((ViewPager)collection).addView(view);
        return view;
    }

so, this way, you can create a function that will update your view:

public updateText(String txt)
{
    ((TextView)updatableView).setText(txt);
}

Hope this helps!

Answer 5

The code below worked for me.

Create a class which extends the FragmentPagerAdapter class as below.

public class Adapter extends FragmentPagerAdapter {

private int tabCount;
private Activity mActivity;
private Map<Integer, String> mFragmentTags;
private FragmentManager mFragmentManager;
private int container_id;
private ViewGroup container;
private List<Object> object;

public Adapter(FragmentManager fm) {
    super(fm);
}

public Adapter(FragmentManager fm, int numberOfTabs , Activity mA) {
    super(fm);
    mActivity = mA;
    mFragmentManager = fm;
    object = new ArrayList<>();
    mFragmentTags = new HashMap<Integer, String>();
    this.tabCount = numberOfTabs;
}

@Override
public Fragment getItem(int position) {
    switch (position) {
        case 0:
            return Fragment0.newInstance(mActivity);
        case 1:
            return Fragment1.newInstance(mActivity);
        case 2:
            return Fragment2.newInstance(mActivity);
        default:
            return null;
    }}


@Override
public Object instantiateItem(ViewGroup container, int position) {
    Object object = super.instantiateItem(container, position);
    if (object instanceof Fragment) {
        Log.e("Already defined","Yes");
        Fragment fragment = (Fragment) object;
        String tag = fragment.getTag();
        Log.e("Fragment Tag","" + position + ", " + tag);
        mFragmentTags.put(position, tag);
    }else{
        Log.e("Already defined","No");
    }
    container_id = container.getId();
    this.container = container;
    if(position == 0){
        this.object.add(0,object);
    }else if(position == 1){
        this.object.add(1,object);
    }else if(position == 2){
        this.object.add(2,object);
    }
    return object;
}

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
    super.destroyItem(container, position, object);
    if (object instanceof Fragment) {
        Log.e("Removed" , String.valueOf(position));
    }
}

@Override
public int getItemPosition (Object object)
{   int index = 0;
    if(this.object.get(0) == object){
        index = 0;
    }else if(this.object.get(1) == object){
        index = 1;
    }else if(this.object.get(2) == object){
        index = 2;
    }else{
        index = -1;
    }
    Log.e("Index" , "..................." + String.valueOf(index));
    if (index == -1)
        return POSITION_NONE;
    else
        return index;
}

public String getFragmentTag(int pos){
    return "android:switcher:"+R.id.pager+":"+pos;
}

public void NotifyDataChange(){
    this.notifyDataSetChanged();
}

public int getcontainerId(){
    return container_id;
}

public ViewGroup getContainer(){
    return this.container;
}

public List<Object> getObject(){
    return this.object;
}

@Override
public int getCount() {
    return tabCount;
}}

Then inside each Fragment you created, create an updateFragment method. In this method you change the things you need to change in the fragment. For example in my case, Fragment0 contained a GLSurfaceView which displays a 3d object based on a path to a .ply file, so inside my updateFragment method I change the path to this ply file.

then create a ViewPager instance,

viewPager = (ViewPager) findViewById(R.id.pager);

and an Adpater instance,

adapter = new Adapter(getSupportFragmentManager(), 3, this);

then do this,

viewPager.setAdapter(adapter);
viewPager.setOffscreenPageLimit(1);

Then inside the class were you initialized the Adapter class above and created a viewPager, every time you want to update one of your fragments (in our case Fragment0) use the following:

adapter.NotifyDataChange();

adapter.destroyItem(adapter.getContainer(), 0, adapter.getObject().get(0)); // destroys page 0 in the viewPager.

fragment0 = (Fragment0) getSupportFragmentManager().findFragmentByTag(adapter.getFragmentTag(0)); // Gets fragment instance used on page 0.

fragment0.updateFragment() method which include the updates on this fragment

adapter.instantiateItem(adapter.getContainer(), 0); // re-initialize page 0.

This solution was based on the technique suggested by Alvaro Luis Bustamante.

Answer 6

I don't think there is any kind of bug in the PagerAdapter. The problem is that understanding how it works is a little complex. Looking at the solutions explained here, there is a misunderstanding and therefore a poor usage of instantiated views from my point of view.

The last few days I have been working with PagerAdapter and ViewPager, and I found the following:

The notifyDataSetChanged() method on the PagerAdapter will only notify the ViewPager that the underlying pages have changed. For example, if you have created/deleted pages dynamically (adding or removing items from your list) the ViewPager should take care of that. In this case I think that the ViewPager determines if a new view should be deleted or instantiated using the getItemPosition() and getCount() methods.

I think that ViewPager, after a notifyDataSetChanged() call takes it's child views and checks their position with the getItemPosition(). If for a child view this method returns POSITION_NONE, the ViewPager understands that the view has been deleted, calling the destroyItem(), and removing this view.

In this way, overriding getItemPosition() to always return POSITION_NONE is completely wrong if you only want to update the content of the pages, because the previously created views will be destroyed and new ones will be created every time you call notifyDatasetChanged(). It may seem to be not so wrong just for a few TextViews, but when you have complex views, like ListViews populated from a database, this can be a real problem and a waste of resources.

So there are several approaches to efficiently change the content of a view without having to remove and instantiate the view again. It depends on the problem you want to solve. My approach is to use the setTag() method for any instantiated view in the instantiateItem() method. So when you want to change the data or invalidate the view that you need, you can call the findViewWithTag() method on the ViewPager to retrieve the previously instantiated view and modify/use it as you want without having to delete/create a new view each time you want to update some value.

Imagine for example that you have 100 pages with 100 TextViews and you only want to update one value periodically. With the approaches explained before, this means you are removing and instantiating 100 TextViews on each update. It does not make sense...