[移动开发]flutter DropDown Button之 There should be exactly one item with [DropdownButton]‘s value: One...错误

解决方案: flutter要求DropdownButton中的value值与List<String> items中的值有重合

换句话说,让你的value有一个固定的初始值,这里用图举例

附上官方代码(可以直接copy):

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  String dropdownValue = 'One';

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      value: dropdownValue,
      icon: const Icon(Icons.arrow_downward),
      elevation: 16,
      style: const TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      onChanged: (String? newValue) {
        setState(() {
          dropdownValue = newValue!;
        });
      },
      items: <String>['One', 'Two', 'Free', 'Four']
          .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}

附上我的垃圾代码(仅供参考,与本人记录):

  List<String> bookitem = ["Brand New", "Almost New", "Good","Acceptable"];
  String dropdownValue = "Brand New";



 Positioned(
              top: HYSizeFit.setPx(300),
              left: HYSizeFit.setPx(110),
              child: DropdownButton(
                icon: const Icon(Icons.arrow_downward),
                elevation: 16,
                items: bookitem
                    .map((e) => DropdownMenuItem(
                          child: Text(e),
                          value: e,
                        ))
                    .toList(),
                value: dropdownValue,
                onChanged: (value) {
                  setState(() {
                    dropdownValue= value.toString();
                  });
                },
              ),
            )

参考链接:

1.https://api.flutter.dev/flutter/material/DropdownButton-class.html

2.https://www.youtube.com/watch?v=w2EwuiKsUqU