Reload data on tab selected in a fragment using view pager

Question

I have my fragments inside view page but I would like to reload the data on the tab selected.I tried returning PositionNone in the GetItemPosition method in my FragmentPagerAdapter but it does not work.

I tried adding notifyDataSetChanged(); on tab selected but it throws nullpointer exception. I even tried setting the viewPager.setOffscreenPageLimit I managed to find the link : Replace Fragment inside a ViewPager but it is on java. I have managed to convert most of the code to c# but gets stuck on the commented code. I am not sure how to call the listener and instantiate the class at the same time? There is a lot of similar question but unfortunately not one in Xamarin Android.

I need to do this in xamarin android. Any help will be greatly appreciated.

This is my FragmentAdapter

public class TabsFragmentPagerAdapter : FragmentPagerAdapter
{
    private readonly Android.Support.V4.App.Fragment[] fragments;
    static readonly int NUM_ITEMS = 2;
    private readonly Android.Support.V4.App.FragmentManager FragmentManager;
    private Android.Support.V4.App.Fragment mFragmentAtPos0;
    private readonly ICharSequence[] titles;

    public TabsFragmentPagerAdapter(Android.Support.V4.App.FragmentManager fm, Android.Support.V4.App.Fragment[] fragments, ICharSequence[] titles) : base(fm)
    {
        this.fragments = fragments;
        this.titles = titles;

    }
    public override int Count
    {
        get
        {
            return fragments.Length;
        }
    }

    public override int GetItemPosition(Java.Lang.Object objectValue)
    {
        if (objectValue.GetType() == typeof(startJobFrag) && mFragmentAtPos0.GetType() == typeof(jobCaptureFrag))
        {
            return PositionNone;
        }
        return PositionUnchanged;
    }


    public override ICharSequence GetPageTitleFormatted(int position)
    {
        return titles[position];
    }


    public override Android.Support.V4.App.Fragment GetItem(int position)
    {
        if (position == 0)
        {
            if (mFragmentAtPos0 == null)
            {
                //not working
            }
            return mFragmentAtPos0;
        }
        else
        {
            return new jobCaptureFrag();
        }
    }

This is my Actvity

 void FnInitTabLayout()
    {
        var fragments = new Android.Support.V4.App.Fragment[]
        {
new frag 1(),
new frag 2(),
new frag 3(),
        };

        //Tab title array
        var titles = CharSequence.ArrayFromStringArray(new[] {
    "Page 1",
    "Page 2",
     "Page 3",
   });

        var viewPager = FindViewById<ViewPager>(Resource.Id.viewpager);
        //viewpager holding fragment array and tab title text
        //viewPager.
        viewPager.Adapter = new TabsFragmentPagerAdapter(SupportFragmentManager, fragments, titles);

        // Give the TabLayout the ViewPager 
        tabLayout.SetupWithViewPager(viewPager);

        viewPager.PageSelected += (object sender, ViewPager.PageSelectedEventArgs e) =>
        {
            //FnInitTabLayout();
            if(e.Position == 0)
            {
              //handle tab click/selected

            }
        };
}

public interface FirstPageFragmentListener
{
    void onSwitchToNextFragment();
}

Fragment

public class Fragment1: Android.Support.V4.App.Fragment
{        
    public override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Create your fragment here
    }

    public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        // Use this to return your custom view for this Fragment
        // return inflater.Inflate(Resource.Layout.YourFragment, container, false);

        View view = inflater.Inflate(Resource.Layout.myview, container, false);

   }
}

Answer 1

I managed to solve it on my own. In My activity I called NotifyDataSetChanged() on Pageselected event

Activity

  ViewPager.PageSelected += (object sender,                 ViewPager.PageSelectedEventArgs e) =>
        {
            if (e.Position == 1)
            {
                _pager.Adapter.NotifyDataSetChanged();
            }
        };

Then I changed my Adapter from FragmentPagerAdapter to FragmentStatePagerAdapter and override the GetItemPosition.

Adapter

    public class TabsFragmentPagerAdapter : FragmentStatePagerAdapter
    {
        public override int GetItemPosition(Java.Lang.Object objectValue)
        {
            return PositionNone;
        }
    }

Refresh a specific tab - inside the Adapter

 public override Android.Support.V4.App.Fragment GetItem(int position)
    {
        if(position == 1) //second tab selected
        {
            NotifyDataSetChanged();
        }            
        return fragments[position];
    }

I'm sure there's a better way to do it but this works.