Why isn't onBindViewHolder getting called when I make a selection?

By following the example at https://github.com/Thumar/recyclerview-selection/tree/master/app I was able to get it working. See my working code below.

But I still don't know which of the changes made it work and why.

If anyone figures out what was causing my problem, please post a comment or answer.

Any and all other feedback is welcome as well.

THANKS

Here's my new fragment:

package com.example.smartflashcards;

import android.content.Context;
import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.selection.ItemDetailsLookup;
import androidx.recyclerview.selection.Selection;
import androidx.recyclerview.selection.SelectionPredicates;
import androidx.recyclerview.selection.SelectionTracker;
import androidx.recyclerview.selection.StableIdKeyProvider;
import androidx.recyclerview.selection.StorageStrategy;
import androidx.recyclerview.widget.GridLayoutManager;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.view.LayoutInflater;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import java.io.File;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class StackSelectionFragment extends Fragment {

    // TODO: Customize parameter argument names
    private static final String ARG_COLUMN_COUNT = "column-count";
    // TODO: Customize parameters
    private int mColumnCount = 1;

    SelectionTracker<Long> tracker; //TODO: add private?

    private StackSelectionListener listener;
    public interface StackSelectionListener {
        public void onSelectStack(String stack);
    }

    String selections = "";


    /**
     * Mandatory empty constructor for the fragment manager to instantiate the
     * fragment (e.g. upon screen orientation changes).
     */
    public StackSelectionFragment() {
    }

    // TODO: Customize parameter initialization
    @SuppressWarnings("unused")
    public static StackSelectionFragment newInstance(int columnCount) {
        StackSelectionFragment fragment = new StackSelectionFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_COLUMN_COUNT, columnCount);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (getArguments() != null) {
            mColumnCount = getArguments().getInt(ARG_COLUMN_COUNT);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_stack_selection_list, container, false);
        File directory = getActivity().getFilesDir();

        List<String> filesList = new ArrayList(Arrays.asList(directory.list()));

        // Set the adapter
        if (view instanceof RecyclerView) {
            Context context = view.getContext();
            RecyclerView recyclerView = (RecyclerView) view;
            if (mColumnCount <= 1) {
                recyclerView.setLayoutManager(new LinearLayoutManager(context));
            } else {
                recyclerView.setLayoutManager(new GridLayoutManager(context, mColumnCount));
            }
            MyItemRecyclerViewAdapter adapter = new MyItemRecyclerViewAdapter(filesList);
            recyclerView.setAdapter(adapter);
            adapter.hasStableIds();

            this.tracker = new SelectionTracker.Builder<Long>(
                    "stack_selector",
                    recyclerView,
                    new StableIdKeyProvider(recyclerView),
                    new MyDetailsLookup(recyclerView),
                    StorageStrategy.createLongStorage()
            ).withSelectionPredicate(SelectionPredicates.<Long>createSelectAnything()).build();
            adapter.injectTracker(this.tracker);

            SelectionTracker.SelectionObserver<Long> observer = new SelectionTracker.SelectionObserver<Long>() {
                @Override
                public void onSelectionChanged() {
                    super.onSelectionChanged();

                    //TODO: replace this placeholder action with something useful
                    selections = "";
                    tracker.getSelection().forEach(item -> selections += adapter.getContent(item.intValue()));//selections += item);
                    //listener.onSelectStack("selections"); //This causes an error???
                    Context context = getContext();
                    Toast toast = Toast.makeText(context, selections, Toast.LENGTH_LONG);
                    toast.show();
                }
            };
            this.tracker.addObserver(observer);
        }
        return view;
    }

    final class MyDetailsLookup extends ItemDetailsLookup {

        private final RecyclerView mRecyclerView;

        MyDetailsLookup(RecyclerView recyclerView) {
            mRecyclerView = recyclerView;
        }

        public @Nullable
        ItemDetails getItemDetails(@NonNull MotionEvent e) {
            View view = mRecyclerView.findChildViewUnder(e.getX(), e.getY());
            if (view != null) {
                RecyclerView.ViewHolder holder = mRecyclerView.getChildViewHolder(view);
                if (holder instanceof MyItemRecyclerViewAdapter.ViewHolder) {
                    return ((MyItemRecyclerViewAdapter.ViewHolder)holder).getItemDetails();
                }
            }
            return null;
        }
    }

Here's my new adapter:

package com.example.smartflashcards;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.recyclerview.selection.ItemDetailsLookup;
import androidx.recyclerview.selection.SelectionTracker;
import androidx.recyclerview.widget.RecyclerView;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.List;

public class MyItemRecyclerViewAdapter extends RecyclerView.Adapter<MyItemRecyclerViewAdapter.ViewHolder> {

    private final List<String> mValues;
    private SelectionTracker<Long> selectionTracker;

    public MyItemRecyclerViewAdapter(List<String> items) {
        setHasStableIds(true); //TODO: remove redundancy between this and "recyclerView.getAdapter().hasStableIds()" in the Fragment
        mValues = items;
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragment_stack_selection, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        String item = this.mValues.get(position);
        holder.bind(item, selectionTracker.isSelected((long) position));
    }

    @Override
    public int getItemCount() {
        return mValues.size();
    }

    @Override
    public long getItemId(int position) {
        return (long)position;
    }

    public String getContent(int position) {
        return mValues.get(position);
    }

    public void injectTracker(SelectionTracker<Long> tracker)
    {
        this.selectionTracker = tracker;
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        public final TextView mContentView;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            mContentView = itemView.findViewById(R.id.content);
        }

        @Override
        public String toString() {
            return super.toString() + " '" + mContentView.getText() + "'";
        }

        public void bind(String item, Boolean activate) {
            this.mContentView.setText(item);
            itemView.setActivated(activate); //TODO: understand how "itemView" exists outside of the constructor
        }

        public ItemDetailsLookup.ItemDetails<Long> getItemDetails() {
            return new ItemDetailsLookup.ItemDetails<Long>() {
                @Override
                public int getPosition() {
                    return getAbsoluteAdapterPosition();
                }

                @Nullable
                @Override
                public Long getSelectionKey() {
                    return (long) getAbsoluteAdapterPosition();
                }
            };
        }
    }
}