问在GridLayoutManager中找到第一个可见位置

EN

public abstract class EndlessRecyclerOnScrollListener extends RecyclerView.OnScrollListener 
{
    public static String TAG = "EndlessScrollListener";

    private int previousTotal = 0; // The total number of items in the dataset after the last load
    private boolean loading = true; // True if we are still waiting for the last set of data to load.
    private int visibleThreshold = 5; // The minimum amount of items to have below your current scroll position before loading more.
    int firstVisibleItem, visibleItemCount, totalItemCount;

    private int currentPage = 1;

    RecyclerViewPositionHelper mRecyclerViewHelper;

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        mRecyclerViewHelper = RecyclerViewPositionHelper.createHelper(recyclerView);
        visibleItemCount = recyclerView.getChildCount();
        totalItemCount = mRecyclerViewHelper.getItemCount();
        firstVisibleItem = mRecyclerViewHelper.findFirstVisibleItemPosition();


        if (loading) {
            if (totalItemCount > previousTotal) {
                loading = false;
                previousTotal = totalItemCount;
            }
        }
        if (!loading && (totalItemCount - visibleItemCount)
                <= (firstVisibleItem + visibleThreshold)) {
            // End has been reached
            // Do something
            currentPage++;

            onLoadMore(currentPage);

            loading = true;
        }
    }

    //Start loading
    public abstract void onLoadMore(int currentPage);
}

public class RecyclerViewPositionHelper {

    final RecyclerView recyclerView;
    final RecyclerView.LayoutManager layoutManager;

    RecyclerViewPositionHelper(RecyclerView recyclerView) {
        this.recyclerView = recyclerView;
        this.layoutManager = recyclerView.getLayoutManager();
    }

    public static RecyclerViewPositionHelper createHelper(RecyclerView recyclerView) {
        if (recyclerView == null) {
            throw new NullPointerException("Recycler View is null");
        }
        return new RecyclerViewPositionHelper(recyclerView);
    }

    /**
     * Returns the adapter item count.
     *
     * @return The total number on items in a layout manager
     */
    public int getItemCount() {
        return layoutManager == null ? 0 : layoutManager.getItemCount();
    }

    /**
     * Returns the adapter position of the first visible view. This position does not include
     * adapter changes that were dispatched after the last layout pass.
     *
     * @return The adapter position of the first visible item or {@link RecyclerView#NO_POSITION} if
     * there aren't any visible items.
     */
    public int findFirstVisibleItemPosition() {
        final View child = findOneVisibleChild(0, layoutManager.getChildCount(), false, true);
        return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
    }

    /**
     * Returns the adapter position of the first fully visible view. This position does not include
     * adapter changes that were dispatched after the last layout pass.
     *
     * @return The adapter position of the first fully visible item or
     * {@link RecyclerView#NO_POSITION} if there aren't any visible items.
     */
    public int findFirstCompletelyVisibleItemPosition() {
        final View child = findOneVisibleChild(0, layoutManager.getChildCount(), true, false);
        return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
    }

    /**
     * Returns the adapter position of the last visible view. This position does not include
     * adapter changes that were dispatched after the last layout pass.
     *
     * @return The adapter position of the last visible view or {@link RecyclerView#NO_POSITION} if
     * there aren't any visible items
     */
    public int findLastVisibleItemPosition() {
        final View child = findOneVisibleChild(layoutManager.getChildCount() - 1, -1, false, true);
        return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
    }

    /**
     * Returns the adapter position of the last fully visible view. This position does not include
     * adapter changes that were dispatched after the last layout pass.
     *
     * @return The adapter position of the last fully visible view or
     * {@link RecyclerView#NO_POSITION} if there aren't any visible items.
     */
    public int findLastCompletelyVisibleItemPosition() {
        final View child = findOneVisibleChild(layoutManager.getChildCount() - 1, -1, true, false);
        return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
    }

    View findOneVisibleChild(int fromIndex, int toIndex, boolean completelyVisible,
                             boolean acceptPartiallyVisible) {
        OrientationHelper helper;
        if (layoutManager.canScrollVertically()) {
            helper = OrientationHelper.createVerticalHelper(layoutManager);
        } else {
            helper = OrientationHelper.createHorizontalHelper(layoutManager);
        }

        final int start = helper.getStartAfterPadding();
        final int end = helper.getEndAfterPadding();
        final int next = toIndex > fromIndex ? 1 : -1;
        View partiallyVisible = null;
        for (int i = fromIndex; i != toIndex; i += next) {
            final View child = layoutManager.getChildAt(i);
            final int childStart = helper.getDecoratedStart(child);
            final int childEnd = helper.getDecoratedEnd(child);
            if (childStart < end && childEnd > start) {
                if (completelyVisible) {
                    if (childStart >= start && childEnd <= end) {
                        return child;
                    } else if (acceptPartiallyVisible && partiallyVisible == null) {
                        partiallyVisible = child;
                    }
                } else {
                    return child;
                }
            }
        }
        return partiallyVisible;
    }
}

                recyclerView?.clearOnScrollListeners()
                recyclerView?.addOnScrollListener(object : RecyclerView.OnScrollListener() {
                    override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
                        val focusedChild = recyclerView.layoutManager?.focusedChild
                        if (focusedChild != null) {
                            mScrolledPosition = recyclerView.getChildAdapterPosition(focusedChild)
                        }
                    }
                })