GridDividerDecoration.java

public class GridDividerDecoration extends RecyclerView.ItemDecoration {

    private static final int[] ATTRS = {android.R.attr.listDivider};

    private Drawable mDivider;
    private int mInsets;

    public GridDividerDecoration(Context context) {
        TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();

        mInsets = context.getResources().getDimensionPixelSize(R.dimen.card_insets);
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        drawVertical(c, parent);
        drawHorizontal(c, parent);
    }

    /**
     * Draw dividers at each expected grid interval
     */
    public void drawVertical(Canvas c, RecyclerView parent) {
        if (parent.getChildCount() == 0) return;

        final int childCount = parent.getChildCount();

        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            final RecyclerView.LayoutParams params =
                    (RecyclerView.LayoutParams) child.getLayoutParams();

            final int left = child.getLeft() - params.leftMargin - mInsets;
            final int right = child.getRight() + params.rightMargin + mInsets;
            final int top = child.getBottom() + params.bottomMargin + mInsets;
            final int bottom = top + mDivider.getIntrinsicHeight();
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(c);
        }
    }

    /**
     * Draw dividers to the right of each child view
     */
    public void drawHorizontal(Canvas c, RecyclerView parent) {
        final int childCount = parent.getChildCount();

        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            final RecyclerView.LayoutParams params =
                    (RecyclerView.LayoutParams) child.getLayoutParams();

            final int left = child.getRight() + params.rightMargin + mInsets;
            final int right = left + mDivider.getIntrinsicWidth();
            final int top = child.getTop() - params.topMargin - mInsets;
            final int bottom = child.getBottom() + params.bottomMargin + mInsets;
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(c);
        }
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        //We can supply forced insets for each item view here in the Rect
        outRect.set(mInsets, mInsets, mInsets, mInsets);
    }
}