如何在 flutter 中将 dropdownbutton 实现为一个函数

Question

我正在处理一个公司项目。但是我不能简单地了解如何将基本的下拉按钮实现到一个函数中，但我似乎无法改变下拉函数中的值你认为我做错了什么这是我的代码：

Widget buildDropdownField({
  required String dropdownHeader,
  required String dropdownValue,
}) {
  return Column(
    children: <Widget>[
      Text(dropdownHeader),
      const SizedBox(
        height: 10,
      ),
      //dropdownField
      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>['-', 'Geçti', 'Kaldı', 'Belirsiz']
            .map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
      )
    ],
  );
}

Answer 1

用 StatefulBuilder 包装它会起作用。

Widget buildDropdownField(
      {required String dropdownHeader, required String dropdownValue}) {
    return Column(
      children: <Widget>[
        Text(dropdownHeader),
        const SizedBox(
          height: 10,
        ),
        StatefulBuilder(
          builder: (_, setDropState) {
            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) {
                setDropState(() {
                  dropdownValue = newValue!;
                });
              },
              items: <String>['-', 'Geçti', 'Kaldı', 'Belirsiz']
                  .map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value),
                );
              }).toList(),
            );
          },
        )
      ],
    );
  }

Answer 2

试试下面的代码希望对你有帮助。使用 StatefulBuilder 参考 here

你的下拉函数：

buildDropdownField({
  required String dropdownHeader,
  required String dropdownValue,
}) {
  return Column(
    children: <Widget>[
      Text(dropdownHeader),
      const SizedBox(
        height: 10,
      ),
      //dropdownField
      StatefulBuilder(builder: (context, StateSetter setState) {
        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>['-', 'Geçti', 'Kaldı', 'Belirsiz']
              .map<DropdownMenuItem<String>>((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(value),
            );
          }).toList(),
        );
      })
    ],
  );
}

您的小部件：

buildDropdownField(
      dropdownHeader: 'dropdownHeader',
      dropdownValue: '-',
    ),

结果->

选择后的结果->

Answer 3

首先，您不应该用新值更新参数。它确实更新了参数，但该函数仍会从它的调用中获取值。

我不知道 buildDropdownField 函数是否在 class 中，但没关系，我会为这两种情况提供解决方案。

内Class

您需要在函数外的 class 中创建一个变量。

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String _dropDownValue = '-';

  Widget buildDropdownField({required String dropdownHeader}) {
    return Column(
      children: <Widget>[
        Text(dropdownHeader),
        const SizedBox(
          height: 10,
        ),
        //dropdownField
        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>['-', 'Geçti', 'Kaldı', 
  'Belirsiz'].map<DropdownMenuItem<String>>((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(value),
            );
          }).toList(),
        )
      ],
    );
  }
}

外Class

您需要将其转换为状态小部件，以便下拉文本发生变化。一旦下拉列表是有状态的小部件，您可以使用上面的解决方案或回调来对父 class.

进行更改

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String _dropDownValue = '-';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: DropDownWidget(
          dropdownHeader: 'Name',
          dropdownValue: _dropDownValue,
          onChanged: (String? newValue) {
            setState(() {
              _dropDownValue = newValue!;
            });
          },
        ),
      ),
    );
  }
}

class DropDownWidget extends StatefulWidget {
  final String dropdownHeader;
  final String dropdownValue;
  final Function(String?)? onChanged;

  DropDownWidget({required this.dropdownHeader, required this.dropdownValue, this.onChanged, Key? key}) : super(key: key);

  @override
  _DropDownWidgetState createState() => _DropDownWidgetState();
}

class _DropDownWidgetState extends State<DropDownWidget> {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Text(widget.dropdownHeader),
        const SizedBox(
          height: 10,
        ),
        //dropdownField
        DropdownButton<String>(
          value: widget.dropdownValue,
          icon: const Icon(Icons.arrow_downward),
          elevation: 16,
          style: const TextStyle(color: Colors.deepPurple),
          underline: Container(
            height: 2,
            color: Colors.deepPurpleAccent,
          ),
          onChanged: widget.onChanged,
          items: <String>['-', 'Geçti', 'Kaldı', 'Belirsiz'].map<DropdownMenuItem<String>>((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(value),
            );
          }).toList(),
        )
      ],
    );
  }
}

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