当用户长按ListTile时，必须将选定属性更改为true，反之亦然，并且必须将卡颜色更改为Grey[300]之类的内容

class cardy extends StatefulWidget {
  @override
  _cardyState createState() => new _cardyState();
}

class _cardyState extends State<cardy> {
  var isSelected = false;
  var mycolor=Colors.white;

  @override
  Widget build(BuildContext context) {
    return new Card(
      color: mycolor,
      child: new Column(mainAxisSize: MainAxisSize.min, children: <Widget>[
        new ListTile(
            selected: isSelected,
            leading: const Icon(Icons.info),
            title: new Text("Test"),
            subtitle: new Text("Test Desc"),
            trailing: new Text("3"),
            onLongPress: toggleSelection // what should I put here,
            )
      ]),
    );
  }

  void toggleSelection() {
    setState(() {
      if (isSelected) {
        mycolor=Colors.white;
        isSelected = false;
      } else {
        mycolor=Colors.grey[300];
        isSelected = true;
      }
    });
  }
}

EDIT：如果要获得Border着色效果，请将列设置为容器内部，并将装饰属性设置为变量，并将其命名为border，然后编辑select方法

void toggleSelection() {
    setState(() {
      if (isSelected) {
        border=new BoxDecoration(border: new Border.all(color: Colors.white));
        mycolor = Colors.white;
        isSelected = false;
      } else {
        border=new BoxDecoration(border: new Border.all(color: Colors.grey));
        mycolor = Colors.grey[300];
        isSelected = true;
      }
    });
  }