Flutter 不显示下拉选择

Question

我正在从 JSON 获取项目并将它们显示在下拉列表中。当这个人从下拉列表中选择一个项目时，我得到了选择，但所选项目没有改变。

例如，我们在列表中有 (tokyo, paris, new york)。默认选择是东京。当这个人选择巴黎时，我明白了，但下拉列表中的选择没有改变。

这是我的代码：

new DropdownButton(
  value: cities.elementAt(0),
  hint: new Text("Ville"),
  items: cities.map((String value) {
    return new DropdownMenuItem(
      value: value,
      child: new Row(
        children: <Widget>[
          new Icon(
            Icons.location_city,
            color: Colors.deepOrange,
          ),
          new Text(value)
        ],
      ),
    );
  }).toList(),
  onChanged: (String value) {
    getTravelCity(value);
  },
),

当人选择一个项目时，它仍然显示默认值。

Answer 1

确保您没有在 Widget 中声明 selectedValue 下面的示例非常适合我。

这是dartpad上的工作代码来测试它

var currentSelectedValue;
const deviceTypes = ["Mac", "Windows", "Mobile"];

 Widget typeFieldWidget() {
return Container(
  padding: EdgeInsets.symmetric(horizontal: 20),
  child: FormField<String>(
    builder: (FormFieldState<String> state) {
      return InputDecorator(
        decoration: InputDecoration(
            border: OutlineInputBorder(
                borderRadius: BorderRadius.circular(5.0))),
        child: DropdownButtonHideUnderline(
          child: DropdownButton<String>(
            hint: Text("Select Device"),
            value: currentSelectedValue,
            isDense: true,
            onChanged: (newValue) {
              setState(() {
                currentSelectedValue = newValue;
              });
              print(currentSelectedValue);
            },
            items: deviceTypes.map((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
        ),
      );
    },
  ),
);
}

Answer 2

This is the solution: "StatefulBuilder()"

       Future<void> AgregarContacto() async {
             String dropdownValue = 'Exento';
             return showDialog<void>(
           context: context,
           barrierDismissible: true, // user must tap button!
           builder: (BuildContext context) {
                 return StatefulBuilder(
                   builder: (BuildContext context, StateSetter setState) {
                   return AlertDialog(
                   title: Text('Agregar cliente', textAlign: TextAlign.center,),
                   content:
                     SingleChildScrollView(
                       child: ListBody(
                         children: <Widget>[
                           Center(
                             child: Material(
                               type: MaterialType.transparency,
                               child: DropdownButton<String>(
                                 items: <String>[
                                   'Responsable inscripto',
                                   'Monotributista',
                                   'Exento',
                                   'Consumidor final',
                                   'No responsable'
                                 ]
                                     .map<DropdownMenuItem<String>>((
                                     String value) {
                                   return DropdownMenuItem<String>(
                                     value: value,
                                     child: Text(value),
                                   ); //DropMenuItem
                                 }).toList(),
                                 value: dropdownValue,
                                 onChanged: (String newValue) {
                                   setState(() {
                                     dropdownValue = newValue;
                                     print("new${newValue}");
                                   }); //setState
                                 },
                                 //OnChange
                                 isExpanded: false,
                                 hint: Text('Responsable inscripto',
                                   style: TextStyle(color: Colors.black),),
                               ),
                             ), //Material
                           ), //Center
                         ],
                       ),
                     ),
                   actions: <Widget>[
                         FlatButton(
                           child: Text('Regret'),
                           onPressed: () {
                             Navigator.of(context).pop();
                          },
                       ),
                    ],
                 );
            }
            );
           },
         );
       }

Answer 3

因为您需要用“setState(() {});”更新“value:”参数 方法和新值必须与传递给“onChanged:”的类型相同。我建议您这样做，因为您可以动态更改图标。

1）在文件顶部声明一个新类；

class city {
  final String name;
  final IconData icon;

  const city({
    this.name,
    this.icon,
  });
}

2）将其添加到您的小部件的状态中：

class yourState extend<myWidget>{

List<city> cities = [
    new city(name: "tokio", icon: Icons.location_city),
    new city(name: "paris", icon: Icons.location_city),
    new city(name: "new york", icon: Icons.location_city),
  ];

int index = 0;

Widget drop() {
    return DropdownButton(
        value: cities[index],
        hint: new Text("Ville"),
        items: cities.map((city value) {
          return new DropdownMenuItem(
            value: value,
            child: new Row(
              children: <Widget>[
                new Icon(
                  value.icon,
                  color: Colors.deepOrange,
                ),
                new Text(value.name)
              ],
            ),
          );
        }).toList(),
        onChanged: (city value) {
          setState(() {
            index = cities.indexOf(value);
            print(index);
          });
          //getTravelCity(value.name);
        });
  }

}

现在在您想要 DropDown 的位置调用“drop()”。再见！！