我有一个Gridview构建器，它生成以下内容，我有一个带有onLongPress的GestureDetector，它显示了这样的菜单：

我可以将菜单与gridview的相应元素对齐，例如，如果我长按绿色框，它应该显示框下方的菜单：

但是，我无法设置水平x位置。
如果我设置L或R的值，它会将菜单移到最左边或最右边，这不是我想要的。我想将菜单居中到绿色框的x轴的中间。

onLongPress: () {
        RenderBox box = key.currentContext.findRenderObject();
        double h = double.parse(box.size.height.toString());
        Offset position =
            box.localToGlobal(Offset.zero); //this is global position
        double y = position.dy;
        double x = position.dx;
        double w = double.parse(box.size.width.toString());

        print(x);
        showMenu(
            context: context,
            position: new RelativeRect.fromLTRB(x, 0, y + h, 0),
            items: <PopupMenuEntry>[
              PopupMenuItem(
                value: 1,
                child: Row(
                  children: <Widget>[
                    Icon(Icons.delete),
                    Text("Delete"),
                  ],
                ),
              )
            ]);
      },

简而言之，我想做一些类似的事情：

body: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          
          Expanded(
            child: GridView.builder(
              shrinkWrap: false,
              itemCount: data.length,
              gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                childAspectRatio: 10.0 / 10.0,
                crossAxisCount: 2,
              ),
              itemBuilder: (BuildContext context, int index) {
                GlobalKey _key = GlobalKey();

                return Padding(
                  padding: EdgeInsets.all(10),
                  child: CustomCard(data[index], _key),
                );
              },
            ),
          ),
        ],
      ),
    );

海关卡

return GestureDetector(
      onTapDown: _storePosition,
      onLongPress: () {
      
        showMenu(
            context: context,
            position:null,
            items: <PopupMenuEntry>[
              PopupMenuItem(
                value: 1,
                child: Row(
                  children: <Widget>[
                    Icon(Icons.delete),
                    Text("Delete"),
                  ],
                ),
              )
            ]);
      },
      child: Card(
        color: d.color,
        elevation: 5,
        semanticContainer: true,
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(10.0),
        ),
        clipBehavior: Clip.antiAlias,
        child: Padding(
          padding: EdgeInsets.symmetric(vertical: 20),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: <Widget>[
              Text(
                d.title,
                style: TextStyle(fontSize: 20, fontWeight: FontWeight.w500),
              ),
            
             
            ],
          ),
        ),
      ),
    );