颤振，如何给gridview提供行空间

Question

 Widget selectTime(double itemHeight, double itemWidth) {
    return Expanded(
        child: Padding(
      padding: const EdgeInsets.all(10.0),
      child: GridView.builder(
        primary: false,
        // scrollDirection: Axis.vertical,
        shrinkWrap: true,
        gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisSpacing: 10.0,
          mainAxisSpacing: 20.0,
          crossAxisCount: 3,
          childAspectRatio: (itemHeight / itemWidth),
        ),
        itemCount: timeslotsList.length,
        itemBuilder: (BuildContext context, int index) {
          return Container(
            decoration: new BoxDecoration(
                color: const Color(0XFF9CA3F4),
                borderRadius: new BorderRadius.only(
                    topLeft: const Radius.circular(10.0),
                    topRight: const Radius.circular(10.0),
                    bottomLeft: const Radius.circular(10.0),
                    bottomRight: const Radius.circular(10.0))),
          );
        },
      ),
    ));
  }

我正在flutter应用程序中实现gridview。我现在成功创建了网格，我想在第二行之后给偶数行（Ex：2,4,6）之后给出间距，我想为gridview提供行间距