颤动中下拉菜单中的状态是否存在问题

Question

我在管理下拉菜单的状态时遇到问题，

我有两个下拉菜单，第二个的项目是根据第一个中选择的项目构建的。

当我再次更改第一个选项时，我的问题是清理第二个。

如果我更新第一个的值，我当时尝试将第二个的值设置为 null，但即使它仍然存在问题。

import 'package:flutter/material.dart';
import 'package:flutter_mobx/flutter_mobx.dart';
import 'package:mobicar/app/stores/brand_store.dart';
import 'package:mobicar/app/stores/vehicle_store.dart';

class HomePage extends StatefulWidget {
  HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  var brandStore = BrandStore();
  var vehicleStore = VehicleStore();
  var selectedBrand;
  var selectedVehicle;

  @override
  void initState() {
    brandStore.getBrands().then((value) {});
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: _body(),
    );
  }

  Column _body() {
    return Column(
      children: [
        Row(
          mainAxisAlignment: MainAxisAlignment.end,
          children: [
            GestureDetector(
              onTap: () => _newItemDialog(),
              child: Container(
                margin: EdgeInsets.only(top: 5, right: 5),
                padding: EdgeInsets.all(8),
                decoration: BoxDecoration(
                  color: Colors.black,
                  borderRadius: BorderRadius.all(Radius.circular(8)),
                ),
                child: Text(
                  "new",
                  style: TextStyle(color: Colors.white),
                ),
              ),
            ),
          ],
        ),
        Expanded(
            child: ListView(
          children: [Text("Supervisor, selecione a viatura")],
        ))
      ],
    );
  }

  Future _newItemDialog() {
    return showDialog(
        barrierDismissible: false,
        context: context,
        builder: (context) => Container(
              child: AlertDialog(
                title: Row(
                  mainAxisAlignment: MainAxisAlignment.spaceBetween,
                  children: [
                    Text("Novo Veiculo "),
                    IconButton(
                        icon: Icon(Icons.close),
                        onPressed: () => Navigator.pop(context))
                  ],
                ),
                content: Observer(builder: (_) {
                  return Column(
                    mainAxisSize: MainAxisSize.min,
                    children: [
                      Image.asset("assets/images/car.png"),
                      _dropdownButton(
                          hint: "Marca",
                          itemlist: _dropDownItemBrands,
                          type: 'brand'),
                      _dropdownButton(
                          hint: "Modelo",
                          itemlist: _dropDownItemVehicles,
                          type: 'vehicle'),
                      _dropdownButton(
                          hint: "Ano", itemlist: _dropDownItemBrands, type: ''),
                    ],
                  );
                }),
                backgroundColor: Colors.white,
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(26),
                ),
              ),
            ));
  }

  _dropdownButton({required hint, required itemlist, required type}) {
    return Container(
      margin: EdgeInsets.only(top: 10),
      child: DropdownButtonFormField(
        hint: Text(hint),
        items: itemlist() ?? [],
        onChanged: (value) {
          switch (type) {
            case 'brand':
              vehicleStore.getVehicles(value);
              break;
            case 'vehicle':
              selectedVehicle = value;
              break;
            default:
          }
        },
        decoration: InputDecoration(
          contentPadding: EdgeInsets.only(left: 8, right: 0, top: 0, bottom: 0),
          border: OutlineInputBorder(borderRadius: BorderRadius.circular(4)),
        ),
      ),
    );
  }

  List<DropdownMenuItem<int>> _dropDownItemBrands() {
    List<DropdownMenuItem<int>> list = [];
    if (brandStore.brandList.isNotEmpty) {
      brandStore.brandList.forEach((element) {
        list.add(
          DropdownMenuItem<int>(
            child: Text(
              element.name,
              style: TextStyle(color: Colors.black),
            ),
            value: element.id,
          ),
        );
      });
      return list;
    } else {
      return list;
    }
  }

  List<DropdownMenuItem<int>> _dropDownItemVehicles() {
    List<DropdownMenuItem<int>> list = [];
    if (brandStore.brandList.isNotEmpty) {
      vehicleStore.vehiclesList.forEach((element) {
        list.add(
          DropdownMenuItem<int>(
            child: Text(
              element.name,
              style: TextStyle(color: Colors.black),
            ),
            value: element.id,
          ),
        );
      });
      return list;
    } else {
      return list;
    }
  }
}

Answer 1

当您想更改clean 用户界面上第二个下拉菜单的值时，您必须使用setState。

所以，

如果我更新，我当时尝试将秒的值设置为 null 第一个的值，但即使它仍然存在问题。

将其设置为null 或空列表[]，但在setState 内部：

//instead of yourSecondValue == null, use:
setState(() {
yourSecondValue == null;
});

这是假设将其设置为 null 会真正解决问题。