list - Flutter:选定的值未显示在下拉列表中

Question

我正在从SQLite数据库填充城市名称，并尝试显示为下拉列表。我按照教程进行操作，但是有一个小问题。所选值未显示在下拉列表中，而是继续显示默认提示值。但是，我能够分配和检索正确的选定值。

这是我的代码:
cities.dart

class Cities {
  int id;
  String name;

  Cities(this.id, this.name);

  Cities.fromMap(Map<String, dynamic> json) {
    this.id = json["id"];
    this.name = json["name"];
  }

  Map<String, dynamic> toMap() => {
        'id': null,
        'name': name,
      };
}

从数据库检索并返回值的 Function:

Future<List<Cities>> getCitiesList() async {
Database db = await instance.database;

final citiesData = await db.query('cities');

if (citiesData.length == 0) return null;

List<Cities> citiesList = citiesData.map((item) {
  return Cities.fromMap(item);
}).toList();

return citiesList;
}

构建的代码在 Widget build内下拉:

//these are defined above in the code
Cities _city;
final databaseHelper = DatabaseHelper.instance;

FutureBuilder<List<Cities>>(

    future: databaseHelper.getCitiesList(),
    builder: (BuildContext context, AsyncSnapshot<List<Cities>> snapshot) {
      if (!snapshot.hasData) return CircularProgressIndicator();
      return DropdownButton<Cities>(

        items: snapshot.data
            .map((city) => DropdownMenuItem<Cities>(
                  child: Text(city.name),
                  value: city,
                ))
            .toList(),
        onChanged: (Cities value) {
          setState(() {
            _city = value;
          });
        },
        isExpanded: true,
        // value: _city, //uncommenting this line breaks the layout
        hint: Text('Select City'),
      );
    },
  ),

控制台中的错误:

'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not true.

取消注释此 value: _city,会在显示中添加相同的错误(显示8次错误，而不是下拉列表)。

问题:

如何解决此问题？

如何设置列表中的默认值？ (默认情况下会选择)

Answer 1

您可以通过简单的方式进行操作，只需创建一个简单的字符串列表并将该列表传递到下拉菜单即可。

就是这样:

更新您的getCitiesList()函数:

Future<List<String>> getCitiesList() async {
  Database db = await instance.database;

  final citiesData = await db.query(tblCities);

  if (citiesData.length == 0) return null;

  return citiesData.map((Map<String, dynamic> row) {
    return row["name"] as String;
  }).toList();
}

将此添加到表单页面中:

//initialize these at top
List<String> _citiesList = <String>[];
String _city;

void _getCitiesList() async {
  final List<String> _list = await databaseHelper.getCitiesList();
  setState(() {
    _citiesList = _list;
  });
}

在_getCitiesList();中调用initState()。

将此添加到build方法中:

DropdownButtonHideUnderline(
    child: DropdownButton<String>(
      value: _city,
      items: _citiesList.map((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
      onChanged: (String newValue) {
      setState(() {
          _city = newValue;
      });
      },
)),