Customize GridLayoutManager sample

MainActivity.java

public class MainActivity extends AppCompatActivity {

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

        final RecyclerView recyclerView = new RecyclerView(this);
        setContentView(recyclerView);

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new CustomGridLayoutManager(this, 6));

        recyclerView.setAdapter(new MyAdapter());
    }

    public static class ViewHolder extends RecyclerView.ViewHolder {

        @NonNull
        static ViewHolder create(ViewGroup parent) {
            final int height = (int) (100 * parent.getResources().getDisplayMetrics().density);
            final TextView textView = new TextView(parent.getContext());
            textView.setGravity(Gravity.CENTER);
            textView.setBackgroundResource(R.drawable.border);
            textView.setLayoutParams(new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, height));
            return new ViewHolder(textView);
        }

        final TextView textView;

        private ViewHolder(TextView itemView) {
            super(itemView);
            textView = itemView;
        }
    }

    public static class MyAdapter extends RecyclerView.Adapter<ViewHolder> {

        @Override
        public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            return ViewHolder.create(parent);
        }

        @Override
        public void onBindViewHolder(ViewHolder holder, int position) {
            holder.textView.setText(String.valueOf(position));
        }

        @Override
        public int getItemCount() {
            return 50;
        }
    }

    public static class CustomGridLayoutManager extends GridLayoutManager {

        private OrientationHelper orientationHelperForHorizontal;
        private int offset;

        public CustomGridLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
            super(context, attrs, defStyleAttr, defStyleRes);
        }

        public CustomGridLayoutManager(Context context, int spanCount) {
            super(context, spanCount);
        }

        public CustomGridLayoutManager(Context context, int spanCount, int orientation, boolean reverseLayout) {
            super(context, spanCount, orientation, reverseLayout);
        }

        private int getExtra() {
            return super.getWidth();
        }

        @Override
        public int getWidth() {
            // 魔改造 その1
            return super.getWidth() + getExtra();
        }

        @Override
        public boolean canScrollHorizontally() {
            return true;
        }

        @Override
        public int scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler, RecyclerView.State state) {
            if (getChildCount() == 0 || dx == 0) {
                return 0;
            }
            ensureLayoutState();
            final int layoutDirection = dx > 0 ? 1 : -1;
            final int absDy = Math.abs(dx);
            final int consumed = calculateHorizontalOffsetForScroll(layoutDirection);
            if (consumed < 0) {
                return 0;
            }
            final int scrolled = absDy > consumed ? layoutDirection * consumed : dx;
            orientationHelperForHorizontal.offsetChildren(-scrolled);
            return scrolled;
        }

        private int calculateHorizontalOffsetForScroll(int layoutDirection) {
            int scrollingOffset;
            if (layoutDirection == 1) {
                scrollingOffset = getExtra() + offset;
            } else {
                scrollingOffset = -offset;
            }
            return scrollingOffset;
        }


        void ensureLayoutState() {
            if (orientationHelperForHorizontal == null) {
                orientationHelperForHorizontal = OrientationHelper.createOrientationHelper(this, OrientationHelper.HORIZONTAL);
            }
        }

        @Override
        public void layoutDecoratedWithMargins(View child, int left, int top, int right, int bottom) {
            // 魔改造 その2
            super.layoutDecoratedWithMargins(child, left + offset, top, right + offset, bottom);
        }

        @Override
        public void offsetChildrenHorizontal(int dx) {
            super.offsetChildrenHorizontal(dx);
            this.offset += dx;
        }
    }
}