颤动中的下拉按钮不会将值切换到所选值 [英] Drop down button in flutter not switching values to the selected value

问题描述

我最近开始使用 dart 和 Flutter 进行编程，我的应用程序一切顺利，尽管最近我想添加下拉菜单来为用户提供多种选择.一切都按计划进行，但是当我从列表中选择一个值时，它不会更改框中的值，它会返回到提示或空框.任何帮助将不胜感激！

I've recently started programming using dart and flutter and everything has been going smoothly for my app, although recently i wanted to add drop down menu to provide the user with multiple options to pick from. everything worked as planned however when i pick a value from the list it doesn't change the value in the box, it goes back to the hint or an empty box. any help would be appreciated!

这是我的下拉按钮代码:

here is my code for the dropdownbutton:

Widget buildDropdownButton() {
String newValue;

return new Padding(
  padding: const EdgeInsets.all(24.0),
  child: new Column(
    mainAxisAlignment: MainAxisAlignment.start,
    children: <Widget>[
      new ListTile(
        title: const Text('Frosting'),
        trailing: new DropdownButton<String>(
            hint: Text('Choose'),
            onChanged: (String changedValue) {
              newValue=changedValue;
              setState(() {
                newValue;
                print(newValue);
              });
            },
            value: newValue,
            items: <String>['None', 'Chocolate', 'Vanilla', 'ButterCream']
                .map((String value) {
              return new DropdownMenuItem<String>(
                value: value,
                child: new Text(value),
              );
            }).toList()),
      ),
    ],
  ),
);
}

推荐答案

错误是因为你声明了一个方法变量 newValue 你必须在你的 StatefulWidget 中将该变量声明为全局变量代码>.

The error is because you are declaring a method variable newValue you must declare that variable as global inside your StatefulWidget.

   String newValue;

  Widget buildDropdownButton() {
  return new Padding(
    padding: const EdgeInsets.all(24.0),
    child: new Column(
      mainAxisAlignment: MainAxisAlignment.start,
      children: <Widget>[
        new ListTile(
          title: const Text('Frosting'),
          trailing: new DropdownButton<String>(
              hint: Text('Choose'),
              onChanged: (String changedValue) {
                newValue=changedValue;
                setState(() {
                  newValue;
                  print(newValue);
                });
              },
              value: newValue,
              items: <String>['None', 'Chocolate', 'Vanilla', 'ButterCream']
                  .map((String value) {
                return new DropdownMenuItem<String>(
                  value: value,
                  child: new Text(value),
                );
              }).toList()),
        ),
      ],
    ),
  );
  }

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