Adding endless scroll support to a RecyclerView.

EndlessScrollSupport.java

/**
 * A class that adds endless scroll support to a {@link RecyclerView}.
 */
public class EndlessScrollSupport {
    private final RecyclerView recyclerView;
    private EndlessScrollListener endlessScrollListener;

    private int startPage = 0;

    /** The current offset index of data you have loaded. */
    private int currentPage = startPage;

    /**
     * The minimum amount of rows to have below your current scroll position
     * before loading more.
     */
    private int rowThreshold = 3;

    /** The total number of items in the dataset after the last load. */
    private int prevTotalItemCount = 0;

    /** Whether we are still waiting for the last set of data to load. */
    private boolean loading = true;

    private RecyclerView.OnScrollListener scrollListener = new RecyclerView.OnScrollListener() {
        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            if (endlessScrollListener != null) {
                RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
                int totalItemCount = layoutManager.getItemCount();
                int lastVisibleItem = findLastVisibleItemPosition(layoutManager);
                int visibleThreshold = findVisibleThreshold(layoutManager);

                if (totalItemCount == 0 && prevTotalItemCount > totalItemCount) {
                    // Assume the list is invalidated, so reset back to initial state.
                    loading = true;
                    prevTotalItemCount = totalItemCount;
                    currentPage = startPage;
                } else if (loading && prevTotalItemCount < totalItemCount) {
                    // Loading has finished.
                    loading = false;
                    prevTotalItemCount = totalItemCount;
                    currentPage++;
                } else if (!loading && lastVisibleItem + visibleThreshold >= totalItemCount) {
                    // Fetch more data.
                    loading = true;
                    endlessScrollListener.onLoadMore(currentPage + 1, totalItemCount);
                }
            }
        }
    };

    private EndlessScrollSupport(RecyclerView recyclerView) {
        this.recyclerView = recyclerView;
    }

    private int findLastVisibleItemPosition(RecyclerView.LayoutManager layoutManager) {
        int lastVisibleItem = RecyclerView.NO_POSITION;
        if (layoutManager instanceof LinearLayoutManager) {
            LinearLayoutManager manager = (LinearLayoutManager) layoutManager;
            lastVisibleItem = manager.findLastVisibleItemPosition();
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            StaggeredGridLayoutManager manager = (StaggeredGridLayoutManager) layoutManager;
            lastVisibleItem = PrimUtils.max(manager.findLastVisibleItemPositions(null));
        }
        return lastVisibleItem;
    }

    private int findVisibleThreshold(RecyclerView.LayoutManager layoutManager) {
        int visibleThreshold = rowThreshold;
        if (layoutManager instanceof GridLayoutManager) {
            GridLayoutManager manager = (GridLayoutManager) layoutManager;
            visibleThreshold = rowThreshold * manager.getSpanCount();
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            StaggeredGridLayoutManager manager = (StaggeredGridLayoutManager) layoutManager;
            visibleThreshold = rowThreshold * manager.getSpanCount();
        }
        return visibleThreshold;
    }

    public static EndlessScrollSupport addTo(@NonNull RecyclerView view) {
        EndlessScrollSupport support = (EndlessScrollSupport)
                view.getTag(R.id.endless_scroll_support);
        if (support == null) {
            support = new EndlessScrollSupport(view);
            support.attach(view);
        } else {
            Log.w("RecyclerView already has EndlessScrollSupport.");
        }
        return support;
    }

    private void attach(RecyclerView view) {
        view.setTag(R.id.endless_scroll_support, this);
        view.addOnScrollListener(scrollListener);
    }

    public static EndlessScrollSupport removeFrom(@NonNull RecyclerView view) {
        EndlessScrollSupport support = (EndlessScrollSupport)
                view.getTag(R.id.endless_scroll_support);
        if (support != null) {
            support.detach(view);
        } else {
            Log.w("RecyclerView does not have EndlessScrollSupport.");
        }
        return support;
    }

    private void detach(RecyclerView view) {
        view.removeOnScrollListener(scrollListener);
        view.setTag(R.id.endless_scroll_support, null);
        endlessScrollListener = null;
    }

    public int getRowThreshold() {
        return rowThreshold;
    }

    public EndlessScrollSupport setRowThreshold(int rowThreshold) {
        this.rowThreshold = rowThreshold;
        return this;
    }

    public int getStartPage() {
        return startPage;
    }

    public EndlessScrollSupport setStartPage(int startPage) {
        this.startPage = startPage;
        return this;
    }

    public EndlessScrollListener getEndlessScrollListener() {
        return endlessScrollListener;
    }

    public EndlessScrollSupport setEndlessScrollListener(EndlessScrollListener listener) {
        endlessScrollListener = listener;
        return this;
    }

    public interface EndlessScrollListener {
        void onLoadMore(int page, int totalItemCount);
    }
}

ids.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item name="endless_scroll_support" type="id" />
</resources>

PrimUtils.java

/**
 * A utility class containing helper primitive methods.
 */
public final class PrimUtils {
    private PrimUtils() {
        throw new IllegalStateException("Non-instantiable utility class");
    }

    public static int max(int... ints) {
        int max = Integer.MIN_VALUE;
        for (int i : ints) {
            max = i > max ? i : max;
        }
        return max;
    }
}