dart - 在Flutter中获取选定的DropdownMenuItem的值

我正在尝试使用DropdownButton构建多个ListView.builder，就像用户单击 float 操作按钮一样多

new FloatingActionButton(
          onPressed: () {
            setState(() {
              counter++;
            });
          },
          child: new Icon(Icons.add),
      )

new ListView.builder(
              itemBuilder: (BuildContext context, int index) {
                return buildfields(index); },
              itemCount: counter,
              scrollDirection: Axis.vertical,
            )

new DropdownButton<String>(
            onChanged: (String value) { setState((){
              setUn();
              _unit = value;
            });
            },
            hint: new Text('Course Unit'),
            value: _unit,
            items: <String>["1", "2", "3", "4", "5"].map((String value) {
              return new DropdownMenuItem<String>(
                value: value,
                child: new Text(value),
              );
            }).toList(),
          )

问题是，当用户生成多个DropdownButton并为“一个”选择值时，其他所有DropdownButton都将其“值”更改为新选择的值。如何为每个生成的DropdownButton设置唯一的ID？

最佳答案

使用ListView.Builder和一个列表来尝试保存值。

class MultipleDropDownPage extends StatefulWidget {
  MultipleDropDownPage({Key key}) : super(key: key);

  @override
  _MultipleDropDownPageState createState() => new _MultipleDropDownPageState();
}

class _MultipleDropDownPageState extends State<MultipleDropDownPage> {
  List<String> selectedValues;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    selectedValues = [];
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: Text('Multi Drop'),
      ),
      body: new ListView.builder(
        itemCount: selectedValues.length,
        itemBuilder: (context, index) {
          return new DropdownButton<String>(
            onChanged: (String value) {
              setState(() {
                selectedValues[index] = value;
              });
            },
            hint: new Text('Course Unit'),
            value: selectedValues[index],
            items: <String>["1", "2", "3", "4", "5"].map((String value) {
              return new DropdownMenuItem<String>(
                value: value,
                child: new Text(value),
              );
            }).toList(),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            selectedValues.add(null);
          });
        },
      ),
    );
  }
}

关于dart - 在Flutter中获取选定的DropdownMenuItem的值，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/52116722/