AttachmentMediaCornersDecoration.java

/*
    Related SO question: https://stackoverflow.com/questions/47990179/round-corners-itemdecoration/48003946
*/
public class AttachmentMediaCornersDecoration extends RecyclerView.ItemDecoration {
    private final float radius;
    private final RectF defaultRectToClip;
    private final List<Integer> allowedViewTypes = Arrays.asList(
            AttachmentImageViewItem.VIEW_TYPE,
            AttachmentVideoViewItem.VIEW_TYPE,
            AttachmentBlurViewItem.VIEW_TYPE);

    public AttachmentMediaCornersDecoration(Context context, @DimenRes int radiusRes) {
        defaultRectToClip = new RectF(Float.MAX_VALUE, Float.MAX_VALUE, 0, 0);
        radius = radiusRes != 0
                ? context.getResources().getDimension(radiusRes)
                : 0f;
    }

    @Override
    public void onDraw(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
        float maxBottom = 0f;
        final RectF roundRect = new RectF(defaultRectToClip);

        final Rect childRect = new Rect();
        for (int i = 0; i < parent.getChildCount(); i++) {
            final View child = parent.getChildAt(i);
            parent.getDecoratedBoundsWithMargins(child, childRect);

            maxBottom = Math.max(maxBottom, childRect.bottom);

            if (isMedia(parent, i)) {
                roundRect.left = Math.min(roundRect.left, childRect.left);
                roundRect.top = Math.min(roundRect.top, childRect.top);
                roundRect.right = Math.max(roundRect.right, childRect.right);
                roundRect.bottom = Math.max(roundRect.bottom, childRect.bottom);
            }
        }

        // has no `allowedViewTypes`
        if (roundRect.equals(defaultRectToClip)) {
            return;
        }

        // used to draw other attachments on canvas after clipping (below `roundRect`)
        final RectF otherRect = new RectF(roundRect);
        otherRect.top = otherRect.bottom;
        otherRect.bottom = maxBottom;

        final Path path = new Path();
        path.addRoundRect(roundRect, radius, radius, Path.Direction.CW);
        path.addRect(otherRect, Path.Direction.CW);
        canvas.clipPath(path);
    }

    private boolean isMedia(RecyclerView parent, int viewPosition) {
        final RecyclerView.Adapter adapter = parent.getAdapter();
        final int viewType = adapter.getItemViewType(viewPosition);
        return allowedViewTypes.contains(viewType);
    }

}