无法将参数类型 'void Function(Hospital)' 分配给参数类型 'void Function(Object?)?'。颤振下拉

Question

当我在 Dropdown Flutter 中使用 Hospital 作为 onChangeEvent 的参数时出现此错误：

The argument type 'void Function(Hospital)' can't be assigned to the parameter type 'void Function(Object?)?'.

医院

class Hospital {
  final int Id;
  final String Name;
  final int BranchId;
  const Hospital(
      {required this.Id, required this.Name, required this.BranchId});

  factory Hospital.fromJson(Map<String, dynamic> json) {
    return Hospital(
        Id: json['id'], Name: json['name'], BranchId: json['branchId']);
  }
}

下拉代码

                                DropdownButton(
                                      style: const TextStyle(
                                          color: Colors.deepPurple),
                                      underline: Container(
                                        height: 2,
                                        color: Colors.deepPurpleAccent,
                                      ),
                                      onChanged: (Hospital newValue) {
                                        setState(() {
                                          hospitalDropdownValue = newValue.id;
                                         branchDropdownValue = newValue.BranchId;
                                        });
                                      },
                                      items: _hospitals.map<DropdownMenuItem<Hospital>>((Hospital item) {
                                        return DropdownMenuItem<Hospital>(
                                          child: Text(item.Name),
                                          value: item,
                                        );
                                      }).toList(),
                                      value: hospitalDropdownValue,
                                      hint: const Text("Hospital Name"),
                                    ),

Answer 1

您可以尝试将 DropdownButton 类型指定为 Hospital 以及

                           DropdownButton<Hospital>(
                                      style: const TextStyle(
                                          color: Colors.deepPurple),
                                      underline: Container(
                                        height: 2,
                                        color: Colors.deepPurpleAccent,
                                      ),
                                      onChanged: (Hospital? newValue){//add null safe 
                                        setState(() {
                                          hospitalDropdownValue = newValue.id;
                                         branchDropdownValue = newValue.BranchId;
                                        });
                                      },