在颤动中我无法从下拉按钮更改值

Question

在我定义下拉按钮的 StatefulWidget class Customdropdownbutton 下方


class Customdropdownbutton extends StatefulWidget {
  Customdropdownbutton({@required this.defaultValue, @required this.listValue, this.enable});
  String defaultValue;
  List<String> listValue;
  bool enable;
  
  @override
  _CustomdropdownbuttonState createState() => _CustomdropdownbuttonState();
}

class _CustomdropdownbuttonState extends State<Customdropdownbutton> {
  @override
  Widget build(BuildContext context) {
    
    String _valore;
    return DropdownButton<String>(
      value: widget.defaultValue.isEmpty ?  widget.listValue[0] : _valore, 
      icon: const Icon(Icons.arrow_downward),
      iconSize: 24,
      elevation: 16,
      style: const TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      
      onChanged: widget.enable ? (String newValue) {
        setState(() {
          _valore = newValue;
        });
      } : null,
      items: widget.listValue.map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}

下面我尝试实例化 class Customdropdownbutton 并在 UI 中我看到下拉按钮但我无法更改值（选择的文本始终是 widget.listValue[0 ])


class MyMain extends StatefulWidget {

@override
  Widget build(BuildContext context) {
    final auth = Provider.of<AuthBase>(context, listen: false);
    final String emailMy = auth.currentUser.email;
    return scaffold(
      .....
      body: _buildFormChildren(emailMy),
  }

   List<Widget> _buildFormChildren(String emailMy) {
    List<String> listValue = ['1a', '2b'];
    String defaultValue = '1a';
    bool enable = true;
    return [
     **Customdropdownbutton(defaultValue: '', listValue : listValue, enable: true, ),**
    ],
  
  ....
  }
}

Answer 1

在你的 _CustomdropdownbuttonState 的 build 方法中，你检查 widget.defaultValue 是否为空（这总是因为你没有改变它）并且它评估 widget.listValue[0].即使您更改 _valore，条件也会继续检查 widget.defaultValue 是否为空。您可以执行以下操作：

class _CustomdropdownbuttonState extends State<Customdropdownbutton> {
  // Use _valore as a property instead of a local variable
  String _valore;

  @override
  void initState() {
    super.initState();
    // Use _valore to store the default value
     _valore = widget.defaultValue;
  }

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      // Check _valore instead of widget.defaultValue
      value: _valore.isEmpty ?  widget.listValue[0] : _valore, 
      icon: const Icon(Icons.arrow_downward),
      iconSize: 24,
      elevation: 16,
      style: const TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      
      onChanged: widget.enable ? (String newValue) {
        setState(() {
          _valore = newValue;
        });
      } : null,
      items: widget.listValue.map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}

在颤动中我无法从下拉按钮更改值

in flutter I can not change value from dropdown button

flutter

dropdownbutton