如何在 DropdownButton 周围绘制边框，如 TextFormField 边框？

Question

StackOverflow 上有很多解释如何在小部件周围绘制边框的答案。但是，我正在寻找类似 TextFormField 的东西。

普通的 DropdownButton 仅具有 underline 属性，但我正在寻找类似以下设计的东西：

正如您在此处看到的，下拉列表有一个边框和一个标题。 我可以从 DropdownButton 小部件中删除 underline 属性，但是是否有任何自定义小部件可以用来包装 DropdownButton？

Answer 1

您可以使用PopupMenuButton 复制它或将其包裹在InputDecorator 下，然后使用DropdownButtonHideUnderline 隐藏下划线

/// Flutter code sample for DropdownButton

// This sample shows a `DropdownButton` with a large arrow icon,
// purple text style, and bold purple underline, whose value is one of "One",
// "Two", "Free", or "Four".
//
// ![](https://flutter.github.io/assets-for-api-docs/assets/material/dropdown_button.png)

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title)),
        body: const Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

/// This is the stateful widget that the main application instantiates.
class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

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

/// This is the private State class that goes with MyStatefulWidget.
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  String dropdownValue = 'One';

  @override
  Widget build(BuildContext context) {
    return InputDecorator(
          decoration: InputDecoration(
            contentPadding: EdgeInsets.symmetric(
                horizontal: 20.0, vertical: 15.0),
            labelText: 'Label',
            border:
                OutlineInputBorder(borderRadius: BorderRadius.circular(5.0)),
          ),
      
      child: DropdownButtonHideUnderline( child:DropdownButton<String>(
      value: dropdownValue,
      icon: const Icon(Icons.arrow_drop_down),
      iconSize: 24,
      elevation: 16,
      style: const TextStyle(color: Colors.deepPurple),
    
      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(),
      ),  ),
    );
  }
}

Answer 2

我不知道还有一个专门创建用于表单的小部件。 DropdownButtonFormField 是小部件，它不需要@flakerimi 提到的所有额外行。

这是我的示例代码，想看的人可以看看。

DropdownButtonFormField<String>(
      validator: (value) =>
          dropdownValue == null ? S.of(context).general_make_selection : null,
      autovalidateMode: AutovalidateMode.onUserInteraction,
      value: dropdownValue,
      decoration: InputDecoration(
        labelText: S.of(context).bill_obj_type,
        filled: true,
      ),
      icon: const Icon(Icons.arrow_drop_down),
      iconSize: 24,
      elevation: 16,
      isExpanded: true,
      style: Theme.of(context)
          .textTheme
          .subtitle1
          .copyWith(color: AppColors.neutral1),
      onChanged: (String newValue) {
        setState(() {
          dropdownValue = newValue;
          vm.objectionType = newValue;
        });
      },
      items: _getDropdownMenuItems(),
    );