Item decoration to add padding vertically and horizontally to RecyclerViews using Linear and Grid layout managers.

PaddingItemDecoration.java

//
// Created by Ben on 28/09/2016.
// Copyright (c) 2016 SHAPE A/S. All rights reserved.
//
public class PaddingItemDecoration extends RecyclerView.ItemDecoration {

    private final Context _context;
    private Resources _resources;

    private Paint _paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    private Rect rect = new Rect();

    private final int _verticalPaddingPixels;
    private final int _horizontalPaddingPixels;
    private final boolean _externalPadding;

    private int _colorResId = -1;
    private int _drawableResId = -1;
    private Drawable _drawable;

    public PaddingItemDecoration(Context context, @DimenRes int paddingDimenRes) {
        _context = context;
        _verticalPaddingPixels = _horizontalPaddingPixels = (int) _context.getResources().getDimension(paddingDimenRes);
        _externalPadding = false;
    }

    public PaddingItemDecoration(Context context, @DimenRes int paddingDimenRes, boolean externalPadding) {
        _context = context;
        _verticalPaddingPixels = _horizontalPaddingPixels = (int) _context.getResources().getDimension(paddingDimenRes);
        _externalPadding = externalPadding;
    }

    public PaddingItemDecoration(Context context, @DimenRes int verticalPaddingDimenRes, @DimenRes int horizontalPaddingDimenRes) {
        _context = context;
        _verticalPaddingPixels = (int) _context.getResources().getDimension(verticalPaddingDimenRes);
        _horizontalPaddingPixels = (int) _context.getResources().getDimension(horizontalPaddingDimenRes);
        _externalPadding = false;
    }

    public PaddingItemDecoration(Context context, @DimenRes int verticalPaddingDimenRes, @DimenRes int horizontalPaddingDimenRes, boolean externalPadding) {
        _context = context;
        _verticalPaddingPixels = (int) _context.getResources().getDimension(verticalPaddingDimenRes);
        _horizontalPaddingPixels = (int) _context.getResources().getDimension(horizontalPaddingDimenRes);
        _externalPadding = externalPadding;
    }

    public void setItemBackgroundColorRes(@ColorRes int colorResId) {
        _colorResId = colorResId;
    }

    public void setItemBackgroundDrawableRes(@DrawableRes int drawableResId) {
        _drawableResId = drawableResId;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);

        // GridLayoutManager is a subclass of LinearLayoutManager, so we need to check for it first
        if (parent.getLayoutManager() instanceof GridLayoutManager) {

            GridLayoutManager layoutManager = (GridLayoutManager) parent.getLayoutManager();

            if (isVerticalOrientation(layoutManager)) { // Vertical grid

                if (_externalPadding) {

                    outRect.top = _verticalPaddingPixels;
                    outRect.right = _horizontalPaddingPixels;

                    if (isFullSpanItem(layoutManager, view) || isStartSpanEdgeItem(layoutManager, view)) {
                        outRect.left = _horizontalPaddingPixels;
                    }

                } else {

                    if (!isFirstSpanItem(parent, parent.getLayoutManager(), view)) {
                        outRect.top = _verticalPaddingPixels;
                    }

                    if (!isFullSpanItem(layoutManager, view) && !isStartSpanEdgeItem(layoutManager, view)) {
                        outRect.left = _horizontalPaddingPixels;
                    }
                }


            } else { // Horizontal grid

                if (_externalPadding) {

                    outRect.left = _verticalPaddingPixels;
                    outRect.bottom = _horizontalPaddingPixels;

                    if (isFullSpanItem(layoutManager, view) || isStartSpanEdgeItem(layoutManager, view)) {
                        outRect.top = _horizontalPaddingPixels;
                    }

                } else {

                    outRect.left = _verticalPaddingPixels;
                    if (isLastItem(parent, view)) {
                        outRect.right = _verticalPaddingPixels;
                    }

                    if (!isFullSpanItem(layoutManager, view) && !isStartSpanEdgeItem(layoutManager, view)) {
                        outRect.bottom = _horizontalPaddingPixels;
                    }
                }
            }

        } else if (parent.getLayoutManager() instanceof LinearLayoutManager) { // (Vertical) list

            if (_externalPadding) {

                outRect.top = _verticalPaddingPixels;

                // We want bottom padding on the bottom list item
                if (isLastItem(parent, view)) {
                    outRect.bottom = _verticalPaddingPixels;
                }

            } else {

                if (!isFirstSpanItem(parent, parent.getLayoutManager(), view)) {
                    outRect.top = _verticalPaddingPixels;
                }
            }
        }
    }

    @Override
    public void onDraw(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(canvas, parent, state);

        if (_colorResId == -1 && _drawableResId == -1) return;

        if (_resources == null) _resources = _context.getResources();

        int childCount = parent.getChildCount();
        for (int index = 0; index < childCount; index++) {

            View child = parent.getChildAt(index);
            if (child == null) continue;

            getItemOffsets(rect, child, parent, state);

            if (_drawableResId != -1)  {

                if (_drawable == null) _drawable = _resources.getDrawable(_drawableResId);
                _drawable.setBounds(parent.getLeft() + parent.getPaddingLeft(),
                                    child.getTop() - rect.top,
                                    parent.getRight() + parent.getPaddingRight(),
                                    child.getBottom() + rect.bottom);
                _drawable.draw(canvas);

            } else if (_colorResId != -1) {

                _paint.setColor(parent.getContext().getResources().getColor(_colorResId));
                canvas.drawRect(parent.getLeft() + parent.getPaddingLeft(),
                        child.getTop() - rect.top,
                        parent.getRight() + parent.getPaddingRight(),
                        child.getBottom() + rect.bottom,
                        _paint);
            }
        }
    }


    private boolean isStartSpanEdgeItem(GridLayoutManager layoutManager, View view) {

        int itemPosition = layoutManager.getPosition(view);

        int currentSpanIndex = 0;
        int totalSpanCount = layoutManager.getSpanCount();
        for (int index = 0; index < itemPosition; index++) {

            currentSpanIndex += totalSpanCount - layoutManager.getSpanSizeLookup().getSpanSize(index);
            currentSpanIndex = currentSpanIndex % totalSpanCount;
        }

        return currentSpanIndex == 0;
    }

    private boolean isFirstSpanItem(RecyclerView recyclerView, RecyclerView.LayoutManager layoutManager, View itemView) {

        if (layoutManager instanceof GridLayoutManager) {
            GridLayoutManager gridLayoutManager = (GridLayoutManager) layoutManager;
            return gridLayoutManager.getSpanSizeLookup().getSpanGroupIndex(recyclerView.getChildAdapterPosition(itemView), gridLayoutManager.getSpanCount()) == 0;
        }

        return recyclerView.getChildAdapterPosition(itemView) == 0;
    }

    private boolean isLastItem(RecyclerView recyclerView, View itemView) {
        return recyclerView.getChildAdapterPosition(itemView) == recyclerView.getAdapter().getItemCount();
    }

    private boolean isVerticalOrientation(LinearLayoutManager layoutManager) {
        return layoutManager.getOrientation() == LinearLayoutManager.VERTICAL;
    }

    private boolean isFullSpanItem(GridLayoutManager gridLayoutManager, View itemView) {
        return gridLayoutManager.getSpanSizeLookup().getSpanSize(gridLayoutManager.getPosition(itemView)) == gridLayoutManager.getSpanCount();
    }
}