DropdownButton 未更新 flutter 中的值

Question

当我尝试从物品中挑选任何价值时。 文字没有改变，它总是向我显示“Tone 1”。

  @override
  Widget build(BuildContext context)
  {
    // Set landscape orientation
     int _value=1;



    SystemChrome.setPreferredOrientations([
      DeviceOrientation.landscapeLeft,
      DeviceOrientation.landscapeRight,
    ]);

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Tones'),
          backgroundColor: Colors.black,
          actions: <Widget>[
         DropdownButton<int>(
        value: _value,
            dropdownColor:Colors.black26,
            items: [
              DropdownMenuItem(
                child: Text("Tone 1",
                  style: TextStyle(
                    color: Colors.white ,

                    fontSize: 20,
                    letterSpacing: 0.5,
                  ),
                ),
                value: 1,
              ),
              DropdownMenuItem(
                child: Text("Tone 2"
                  ,
                  style: TextStyle(
                    color: Colors.white ,
                    fontSize: 20,
                    letterSpacing: 0.5,
                  ),),
                value: 2,
              ),
              DropdownMenuItem(
                  child: Text("Tone 3",
                    style: TextStyle(
                      color: Colors.white ,
                      fontSize: 20,
                      letterSpacing: 0.5,
                    ),),
                  value: 3
              ),
              DropdownMenuItem(
                  child: Text("Tone 4",
                    style: TextStyle(
                      color: Colors.white ,
                      fontSize: 20,
                      letterSpacing: 0.5,
                    ),),
                  value: 4
              )
              ,
              DropdownMenuItem(
                  child: Text("Tone 5",
                    style: TextStyle(
                      color: Colors.white ,
                      fontSize: 20,
                      letterSpacing: 0.5,
                    ),),
                  value: 5
              )

            ],
            onChanged: (value) {
              setState(() {
                _value = value!;
              });
            }),
            IconButton(
              icon: Icon(
                Icons.send,
                color: Colors.white,
              ),
              onPressed: () async {
                // do something
                List<int> pressedToneList = [];
                final FirebaseFirestore _firebase  = FirebaseFirestore.instance;
                switch (_value) {
                  case 1:
                    pressedToneList=song1;
                    break;

                  case 2:
                    pressedToneList=song2;
                    break;

                  case 3:
                    pressedToneList=song3;
                    break;

                  case 4:
                    pressedToneList=song4;
                    break;

                  case 5:
                    pressedToneList=song5;
                    break;

                  case 6:
                    pressedToneList=song6;
                    break;

                  case 7:
                    pressedToneList=song7;
                    break;
                  case 8:
                    pressedToneList=song8;
                    break;

                  case 9:
                    pressedToneList=song9;
                    break;

                  case 10:
                    pressedToneList=song10;
                }
                await _firebase.collection('Tones').doc('tone').set({"Keys": pressedToneList});
                pressedToneList.clear();
              },
            ),

            IconButton(
              icon: Icon(
                Icons.home,
                color: Colors.white,
              ),
              onPressed: () async {
                // do something
                Navigator.pop(context);
              },
            ),
          ],
        ),
        body: SafeArea(
          child: Row(
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: <Widget>[
              myTone(1),
              myToneOpposite(3),
              myTone(5),
                myToneOpposite(7),
                myTone(9),

            ],
          ),
        ),
      ),
    );
  }
}

Answer 1

您似乎已经定义了int _value=1; 在您的构建 function 内部。

因此，它本身始终为1 ，因为每次构建小部件时，该值都设置为 1。

您必须在State class 中将其定义为成员变量。

像这样，

class ExampleWidgetState extends State<ExampleWidget> {
    
    int _value = 1;

    @override
    Widget build(BuildContext context) => Scaffold(

然后，你可以用同样的方式使用它，像这样

DropdownButton<int>(
    value: _value,

最后，您可以 setState 像这样更改它的值

onChanged: (value) {
    setState(() {
        _value = value!;
    });
}),