RecyclerView doesn't update after NotifyDataSetChanged() , content shows up after turning on and off Screen

Question

I'm fairly new to Android and I'm building and app with a group of friends from college . The problem that we have run into is that when filling the list of elements to display by getting data from a server the data loads and gets added to a list but the recyclerView never displays it (even after calling NotifyDataSetChanged from my custom Adapter) . Back when we used hardcoded data to test the app , this never happened.

The strange part is that the list of elements get displayed on the recyclerView after I turn off the display and then turn it back on (onBindViewHolder of my custom Adapter gets called when I turn the screen back on).Moreover , if I replace the fragment that contains the recyclerView with another and then return to it the data gets loaded to the list but not shown.

What I want to know is why is this happening and what can I do to solve it (and by solve it I mean showing the recyclerView populated with the correct items right as I launch the fragment and updating the recyclerView when I add more elements to the list)

We are using a custom adpater and Retrofit to get data from the server.

Here is the code

Fragment that contains the recyclerView

    public class EventViewListFragment extends Fragment implements EventListAdapter.ClickListener{

    private RestClient restClient;
    private Builder builder;
    private String token ;

    private boolean userScrolled = true;
    int pastVisiblesItems, visibleItemCount, totalItemCount;

    private LinearLayoutManager mLayoutManager;

    private RecyclerView recyclerView;
    private EventListAdapter eventListAdapter;

    private Button btnNewEvent;

    FloatingActionButton suggestFAB;

    private int currentPage=1;


    public  EventViewListFragment(){
        restClient = new RestClient();
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        System.out.println("Se llamo al onCreate de EventViewListFragment");
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        System.out.println("se llamo al onCreateView de EvENT List Fragment");
        // Inflate the layout for this fragment
        View layout =inflater.inflate(R.layout.fragment_events_list, container, false);
        setUpElements(layout);
        addListeners();
        return layout;
    }

    private void setUpElements(View layout)
    {
        recyclerView = (RecyclerView) layout.findViewById(R.id.eventList);
        eventListAdapter = new EventListAdapter(getActivity());
        eventListAdapter.setClickListener(this);
        eventListAdapter.setData(getInitialData());
        recyclerView.setAdapter(eventListAdapter);
        mLayoutManager=new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(mLayoutManager);
        suggestFAB = (FloatingActionButton)  layout.findViewById(R.id.suggestFAB);
        builder = new Builder();
    }

    private void addListeners()
    {
        addNewEventListener();
        addScrollBottomListener();
    }



    private void addNewEventListener()
    {
        /*btnNewEvent.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view)
            {
                EventsActivity.getInstance().toNewEventForm();
            }
        });*/

        suggestFAB.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                EventsActivity.getInstance().toNewEventForm();
                currentPage=1;
            }
        });
    }

    public List<Event> getInitialData()
    {
        List<Event> data=new ArrayList<>();
        data = getEvents(data);
        return data;
    }

    @Override
    public void itemClicked(View view, int position) {
        EventsActivity.getInstance().toEventPage(eventListAdapter.getItemAtPos(position));
        currentPage=1;

    }
    private void addScrollBottomListener() {
        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {

            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {

                super.onScrollStateChanged(recyclerView, newState);
                if (newState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL) {
                    userScrolled = true;

                }

            }

            @Override
            public void onScrolled(RecyclerView recyclerView, int dx,
                                   int dy) {

                super.onScrolled(recyclerView, dx, dy);
                // Here get the child count, item count and visibleitems
                // from layout manager

                visibleItemCount = mLayoutManager.getChildCount();
                totalItemCount = mLayoutManager.getItemCount();
                pastVisiblesItems = mLayoutManager.findFirstVisibleItemPosition();
                if (userScrolled && (visibleItemCount + pastVisiblesItems) == totalItemCount) {
                    userScrolled = false;

                    addNewElementsToList();
                }

            }

        });

    }

    private void addNewElementsToList()
    {
        Toast.makeText(getActivity(), "Cargando Mas Elementos", Toast.LENGTH_SHORT).show();
        eventListAdapter.setData(getEvents(eventListAdapter.getData()));
    }

    private List<Event> getEvents(final List<Event> eventsList)
    {
        System.out.println("Asignando CALL");
        Call<JsonElement> eventPage = restClient.getConsumerService().getEvents(token, "", currentPage, 10);
        System.out.println("Enquequeing");
        eventPage.enqueue(new Callback<JsonElement>() {
            @Override
            public void onResponse(Response<JsonElement> response) {
                JsonObject responseBody = response.body().getAsJsonObject();
                if (responseBody.has("events")) {
                    JsonArray jsonArray = responseBody.getAsJsonArray("events");
                    System.out.println(jsonArray.size());
                    for (int i = 0; i < jsonArray.size(); i++) {
                        JsonObject storedObject = jsonArray.get(i).getAsJsonObject();
                        Event current = new Event();
                        current.setEventId(storedObject.get("id").getAsInt());
                        current.setName(storedObject.get("title").getAsString());
                        Calendar startCal = new GregorianCalendar();
                        startCal.setTimeInMillis((storedObject.get("starts_at").getAsLong()) * 1000);
                        current.setStartDateTime(startCal);
                        Calendar endCal = new GregorianCalendar();
                        endCal.setTimeInMillis((storedObject.get("ends_at").getAsLong()) * 1000);
                        current.setFinishDateTime(endCal);
                        current.setImgUrl(storedObject.get("image").getAsString());
                        eventsList.add(current);
                    }
                } else {
                    if (responseBody.has("error")) {
                        System.out.println("ERROR");
                        wan.wanmarcos.models.Error error = builder.buildError(responseBody.get("error").getAsJsonObject());
                        Toast.makeText(getActivity(), "Error : " + error.toString(), Toast.LENGTH_SHORT).show();
                    } else {

                    }
                }
            }

            @Override
            public void onFailure(Throwable t) {
            }
        });
        eventListAdapter.notifyDataSetChanged();
        currentPage++;
        return eventsList;
    }
}

Custom Adapter for RecyclerView

    public class EventListAdapter extends  RecyclerView.Adapter<EventListAdapter.EventListViewHolder>{
    private LayoutInflater inflater;
    private List<Event> data = Collections.emptyList();
    private Context context;
    private ClickListener clickListener;

    public EventListAdapter(Context context){
        inflater = LayoutInflater.from(context);
        this.context=context;
    }

    @Override
    public EventListViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view =  inflater.inflate(R.layout.event_list_item, parent, false);
        EventListViewHolder holder = new EventListViewHolder(view);
        return holder;
    }

    @Override
    public void onBindViewHolder(EventListViewHolder holder, int position) {
        System.out.println("se llamo al onBindViewHolder de ELA");
        Event current = getData().get(position);
        holder.title.setText(current.getName());
        Picasso.with(context)
                .load(current.getImgUrl())
                .into(holder.img);
        String startDateAndTime = current.CalendarToString(current.getStartDateTime())+" - "+current.CalendarToString(current.getFinishDateTime());
        holder.dateAndTime.setText(startDateAndTime);
    }

    public void setClickListener(ClickListener clickListener){

        this.clickListener=clickListener;
    }

    @Override
    public int getItemCount() {

        return getData().size();
    }

    public List<Event> getData() {
        return data;
    }

    public void setData(List<Event> data) {
        this.data = data;
    }

    class EventListViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {

        TextView title;
        TextView dateAndTime;
        ImageView img;

        public EventListViewHolder(View itemView) {

            super(itemView);
            itemView.setOnClickListener(this);
            title = (TextView) itemView.findViewById(R.id.eventListTitle);
            img = (ImageView) itemView.findViewById(R.id.eventListImage);
            dateAndTime =(TextView) itemView.findViewById(R.id.eventListDateAndTime);
        }


        @Override
        public void onClick(View v) {

            if(clickListener!=null)
            {
                clickListener.itemClicked(v,getPosition());
            }
        }

    }

    public Event getItemAtPos(int pos)
    {
        return getData().get(pos);
    }


    public interface ClickListener{
        public void itemClicked(View view,int position);
    }



}

Fragment XML File

    <?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
    android:id="@+id/main_content"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="wan.wanmarcos.fragments.EventViewListFragment">

            <android.support.v7.widget.RecyclerView
                android:id="@+id/eventList"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:scrollbars="vertical"
                >
            </android.support.v7.widget.RecyclerView>
    </RelativeLayout>

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/suggestFAB"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|right"
        app:layout_anchorGravity="bottom|right|end"
        android:layout_margin="16dp"
        android:clickable="true"
        android:src="@mipmap/ic_add_white_48dp" />

</android.support.design.widget.CoordinatorLayout>

Answer 1

eventsList in your Activity and data in your Adapter are two different collections. You are filling up the former but not the latter. Intialize data with new ArrayList<> instead of Collections.emptyList(); and then add a method in your Adapter, call addAll, like that:

public void addAll(final List<Event> new events) {
      final int currentCount = data.size();
      synchronized(data) {
         data.addAll(events);
      }
      if (Looper.getMainLooper() == Looper.myLooper()) {
          notifyItemRangeInserted(currentCount, events.size());
      } else {
          new Handler(Looper.getMainLooper()).post(new Runnable() {
            @Override
            public void run() {
                notifyItemRangeInserted(currentCount, events.size());
            }
          });
      }
}

the if else checks if you are calling addAll from the ui thread or from a different thread and call notifyItemRangeInserted in a safe way. In the Activity when onResponse is invoked, after you filled up completely eventsList, just call eventListAdapter.addAll(eventsList)