下拉选项不显示Flutter

3 回答

• 0

  因为您需要使用“setState（（）{}）更新”value：“参数;”方法和新值必须与传递给“onChanged：”的类型相同 . 我建议你这样做，因为你可以动态改变你的图标 .

  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）在窗口小部件的状态下添加：

  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);
          });
    }
  
  }
  

  现在调用“drop（）”你想要你的DropDown . 再见！

  回复于 2024-04-29T19:20:48+08:00
• 0

  在我最近的应用程序中使用此代码，它的工作原理 . 也许你可以将它与你的代码进行比较 .

  return DropdownButton<String>(
      style: Theme.of(context).textTheme.title,
      items: _persons.map((String val) {
        return new DropdownMenuItem<String>(
          value: val,
          child: new Container(
            child: new Card(
              child: new Row(children: <Widget>[
                new Icon(Icons.person),
                new Text(val),
              ]),
            ),
          ),
        );
      }).toList(),
      hint: new Card(
          child: new Row(children: <Widget>[
        new Icon(Icons.person),
        new Text("check"),
      ])),
      value: _selectedPerson,
      onChanged: (newVal) {
        _selectedPerson = newVal;
        setState(() {});
      });
  

  回复于 2024-04-29T19:20:48+08:00
• 0

  您将下拉列表中的第一个城市硬编码为 value .

  new DropdownButton(
              value: cities.elementAt(0), //this one
  ...
  

  您需要有一些 state 变量来保存所选城市 .

  class YourStatefulWidgetState extends State<YourStatefulWidget> {
    @override
    initState() {
     super.initState();
     selectedCity = cities.elementAt(0)
    }
  
   @override
   Widget build(BuildContext context) {
     ...
     new DropdownButton(
              value: selectedCity,
              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) {
                setState(() {
                  selectedCity = getTravelCity(value);
                });
              },),
       ...
  

  回复于 2024-04-29T19:20:48+08:00