Simple RecyclerView customization that implements "OnLastItemVisible" feature, where the number of "items left to load more" can be set

ExtRecyclerView.java

import android.content.Context;
import android.content.res.TypedArray;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;

public class ExtRecyclerView extends RecyclerView {
    private static final int DEFAULT_ITEMS_LEFT_TO_LOAD_MORE = 5;

    Context context;
    int itemsLeftToLoadMore = DEFAULT_ITEMS_LEFT_TO_LOAD_MORE;
    OnLastItemVisibleListener onLastItemVisibleListener = new EmptyLastItemListener();
    private boolean loading = false;
    final Object lock = new Object();

    public ExtRecyclerView(Context context) {
        super(context);
        this.context = context;
        init(null);
    }

    public ExtRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.context = context;
        init(attrs);
    }

    public ExtRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        this.context = context;
        init(attrs);
    }

    protected void init(AttributeSet attrs) {
        if(attrs != null){
            TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ExtRecyclerView);

            itemsLeftToLoadMore = a.getInt(R.styleable.ExtRecyclerView_itemsLeftToLoadMore, DEFAULT_ITEMS_LEFT_TO_LOAD_MORE );
        }
        addOnScrollListener(new OnScrollListener() {
            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);
            }

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

                LinearLayoutManager layoutManager = (LinearLayoutManager) getLayoutManager();
                int last = layoutManager.findLastVisibleItemPosition();
                int total = layoutManager.getItemCount();

                if( lastItemsVisible(last, total) && !isLoading() ){
                    loading = true;
                    onLastItemVisibleListener.onLastItemVisible(last, total);
                }
            }
        });
    }

    private boolean lastItemsVisible(int last, int total) {
        return (last >= total - itemsLeftToLoadMore);
    }

    public boolean isLoading() {
        synchronized (lock) {
            return loading;
        }
    }

    public void finishLoadingMore(){
        synchronized (lock) {
            loading = false;
        }
    }

    //region Listeners
    public void setOnLastItemVisibleListener(OnLastItemVisibleListener onLastItemVisibleListener) {
        this.onLastItemVisibleListener = onLastItemVisibleListener;
    }

    public interface OnLastItemVisibleListener{
        public void onLastItemVisible(int visibleIndex, int total); // See parameters
    }

    protected class EmptyLastItemListener implements OnLastItemVisibleListener{
        public void onLastItemVisible(int visibleIndex, int total) {}
    }
    //endregion
}

Also, in res/values/attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="ExtRecyclerView">
        <attr name="itemsLeftToLoadMore" format="integer" />
    </declare-styleable>
</resources>