An ItemDecoration that assigns even spacing to a grid of items, with the spacing between the items the same size as the outer spacing of the grid.

GridSpacingDecoration.java

import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

public class GridSpacingDecoration extends RecyclerView.ItemDecoration {

    private int cols;
    private int spacing;

    public GridSpacingDecoration(int cols, int spacing) {
        this.cols = cols;
        this.spacing = spacing;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        int position = parent.getChildLayoutPosition(view);

        outRect.bottom = spacing;

        if (position <= cols-1) {
            outRect.top = spacing;
        } else {
            outRect.top = 0;
        }
        outRect.left = spacing;
        outRect.right = spacing;
        if (position % cols == 0) {
            outRect.right /= 2;
        } else if(position % cols == cols - 1) {
            outRect.left /= 2;
        }
    }
}