Flutter DropDownButton - 如何在下拉按钮被禁用时显示所选值？

Question

Flutter DropdownButton 表现出一些奇怪的行为：它在禁用时显示小部件 disabledHint 而不是所选值（必须通过将 onChanged 设置为 null 来完成）。

如何显示所选值？

这是我的示例代码：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'DropdownButton disable problem',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _enabled = true;
  int value;
  List<DropdownMenuItem<int>> items = [
    DropdownMenuItem(
      value: 11,
      child: Text('asdf'),
    ),
    DropdownMenuItem(
      value: 27,
      child: Text('qwert'),
    ),
    DropdownMenuItem(
      value: 31,
      child: Text('yxcv'),
    )
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('DropdownButton problem'),
      ),
      body: Center(
        child: Column(
          children: <Widget>[
            Text(
              'Disabling the DropdownButton looses its selection',
            ),
            Switch(
              onChanged: (v) => setState(() {
                _enabled = v;
              }),
              value: _enabled,
            ),
            DropdownButton(
              items: items,
              onChanged: _enabled
                  ? (v) => setState(() {
                        value = v;
                      })
                  : null,
              value: value,
            )
          ],
        ),
      ),
    );
  }
}

Answer 1

如果您将 List 项作为 Map 的 List 并从中生成 DropdownMenuItems，您将更容易识别选择和设置的内容它作为 disabledHint:

class DropdownProblem extends StatefulWidget {
  @override
  _DropdownProblemState createState() => _DropdownProblemState();
}

class _DropdownProblemState extends State<DropdownProblem> {
  bool _enabled = true;
  int value;

  List<Map> _items = [
    {
      "value": 11,
      "text": 'asdf'
    },
    {
      "value": 27,
      "text": 'qwert'
    },
    {
      "value": 31,
      "text": 'yxcv'
    },
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('DropdownButton problem'),
      ),
      body: Center(
        child: Column(
          children: <Widget>[
            Text(
              'Disabling the DropdownButton looses its selection',
            ),
            Switch(
              onChanged: (v) => setState(() {
                _enabled = v;
              }),
              value: _enabled,
            ),
            DropdownButton(
              disabledHint: value != null
                ? Text(_items.firstWhere((item) => item["value"] == value)["text"])
                : null,
              items: _items.map((item) => DropdownMenuItem(
                value: item["value"],
                child: Text(item["text"]),
              )).toList(),
              onChanged: _enabled
                ? (v) => setState(() {
                  value = v;
                })
                : null,
              value: value,
            )
          ],
        ),
      ),
    );
  }
}

Answer 2

来自 flutter dropDown api 文档：

If the onChanged callback is null or the list of items is null then the dropdown 
button will be disabled, i.e. its arrow will be displayed in grey and it will not 
respond to input. A disabled button will display the disabledHint widget if it is non-
null. However, if disabledHint is null and hint is non-null, 
the hint widget will instead be displayed.`

Flutter DropDownButton - 如何在下拉按钮被禁用时显示所选值？

Flutter DropDownButton - How to display the selected value when dropdown button gets disabled?

flutter

dropdownbutton