如何在 Flutter 中使用 Bloc 正确设置 DropdownButton 的值？

Posted 2023-02-23

【中文标题】如何在 Flutter 中使用 Bloc 正确设置 DropdownButton 的值？

【英文标题】：How to properly set value of DropdownButton using Bloc in Flutter?

【发布时间】：2019-05-20 01:14:36

【问题描述】：

我是 Bloc 编程模式的新手，在使用 Dropdown 时遇到问题 那是在我的集体课上：

final _dropDown = BehaviorSubject<String>();
Stream<String> get dropDownStream => _dropDown.stream;
Sink<String> get dropDownSink => _dropDown.sink;
final _dropdownValues = BehaviorSubject<List<String>>(seedValue: [
    'One',
    'Two',
    'Three',
    'Four',
  ].toList());
  Stream<List<String>> get dropdownValuesStream => _dropdownValues.stream;

在我的小部件页面中，我添加了以下下拉小部件，以便所有内容都由 Bloc 类处理：

StreamBuilder<List<String>>(
                      stream: _exampleBloc.dropdownValuesStream,
                      builder: (BuildContext contextValues, AsyncSnapshot snapshotValues) 
                        return StreamBuilder<String>(
                            stream: _exampleBloc.dropDownStream,
                            builder: (BuildContext context, AsyncSnapshot snapshot) 
                              return InputDecorator(
                                decoration: InputDecoration(
                                  icon: const Icon(Icons.color_lens),
                                  labelText: 'DropDown',
                                ),
                                child: DropdownButtonHideUnderline(
                                  child: DropdownButton<String>(
                                    value: snapshot.data,
                                    onChanged: (String newValue) => _exampleBloc.dropDownSink.add(newValue),
                                    items: snapshotValues.data != null ? snapshotValues.data.map<DropdownMenuItem<String>>((String value) 
                                      return DropdownMenuItem<String>(
                                        value: value,
                                        child: Text(value),
                                      );
                                    ).toList() : <String>[''].map<DropdownMenuItem<String>>((String value) 
                                      return DropdownMenuItem<String>(
                                        value: value,
                                        child: Text(value),
                                      );
                                    ).toList(),
                                  ),
                                ),
                              );
                            ,
                          );
                      ,
                    ),

但是这样做，在设置 DropdownButton 的值（值：snapshot.data）时出现此错误：

I/flutter ( 5565): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5565): The following assertion was thrown building StreamBuilder<String>(dirty, state:
I/flutter ( 5565): _StreamBuilderBaseState<String, AsyncSnapshot<String>>#70482):
I/flutter ( 5565): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 514 pos 15: 'items == null ||
I/flutter ( 5565): value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not
I/flutter ( 5565): true.
I/flutter ( 5565):
I/flutter ( 5565): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter ( 5565): more information in this error message to help you determine and fix the underlying cause.
I/flutter ( 5565): In either case, please report this assertion by filing a bug on GitHub:
I/flutter ( 5565):   https://github.com/flutter/flutter/issues/new?template=BUG.md
I/flutter ( 5565):
I/flutter ( 5565): When the exception was thrown, this was the stack:
I/flutter ( 5565): #2      new DropdownButton (package:flutter/src/material/dropdown.dart:514:15)
I/flutter ( 5565): #3      _ExamplePageState.build.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:financeiro_mobile/src/ui/exemple/example_page.dart:129:42)
I/flutter ( 5565): #4      StreamBuilder.build (package:flutter/src/widgets/async.dart:423:74)
I/flutter ( 5565): #5      _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:125:48)
I/flutter ( 5565): #6      StatefulElement.build (package:flutter/src/widgets/framework.dart:3809:27)
I/flutter ( 5565): #7      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15)
I/flutter ( 5565): #8      Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter ( 5565): #9      StatefulElement.update (package:flutter/src/widgets/framework.dart:3878:5)
I/flutter ( 5565): #10     Element.updateChild (package:flutter/src/widgets/framework.dart:2742:15)
I/flutter ( 5565): #11     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3732:16)
I/flutter ( 5565): #12     Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)
I/flutter ( 5565): #13     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2286:33)
I/flutter ( 5565): #14     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:676:20)
I/flutter ( 5565): #15     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:219:5)
I/flutter ( 5565): #16     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)
I/flutter ( 5565): #17     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)
I/flutter ( 5565): #18     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842:5)
I/flutter ( 5565): #19     _invoke (dart:ui/hooks.dart:154:13)
I/flutter ( 5565): #20     _drawFrame (dart:ui/hooks.dart:143:3)
I/flutter ( 5565): (elided 2 frames from class _AssertionError)

我尝试了很多想法，例如在设置时检查 snapshotValues.data 是否不为空。我知道该值必须是列表中的某个值或 null。但是我放在那里的任何逻辑都没有使这个错误消失。 如果我将值设置为 null，它可以工作，但所选值不会显示。 我做错了吗？有更好的方法吗？我该如何解决这个问题？ 谢谢！

【问题讨论】：

我也尝试使用 StreamBuilder 实现 Dropdown（即应用我认为很棒的 Bloc 模式），但我得到了完全相同的错误。在此期间你能以某种方式修复它吗？

【参考方案1】：

我使用 bloc 中的两个流解决了它，一个用于 elemtns 列表，另一个用于值。因此，在构建中，您需要两个链式 StreamBuilder，当您获得两个包含数据的快照时，您将加载构建。像这样：

Widget _holdingDropDown() 
return StreamBuilder(
    stream: bloc.holding,
    builder: (BuildContext context, AsyncSnapshot<Holding> snapshot) 
      return Container(
        child: Center(
          child: snapshot.hasData
              ? StreamBuilder(
                  stream: bloc.obsHoldingList,
                  builder: (BuildContext context,
                      AsyncSnapshot<List<Holding>> holdingListSnapshot) 
                    return holdingListSnapshot.hasData ?
                    DropdownButton<Holding>(
                      value: snapshot.data,
                      items: _listDropDownHoldings,
                      onChanged: (Holding h) 
                        _changeDropDownItemHolding(h);
                      ,
                    ): CircularProgressIndicator();
                  ,
                )
              : CircularProgressIndicator(),
        ),
      );
    );

如果我没有包含数据的快照，我会使用循环进度指示器返回。 希望对您有所帮助。

【参考方案2】：

那是因为你使用的是 StreamBuilder，所以当你的快照第一次为空时，你必须做一个验证：

        return snapshot.hasData ? 
             InputDecorator(
                                        decoration: InputDecoration(
                                          icon: const Icon(Icons.color_lens),
                                          labelText: 'DropDown',
                                        ),
                                        child: DropdownButtonHideUnderline(
                                          child: DropdownButton<String>(
                                            value: snapshot.data,
                                            onChanged: (String newValue) => _exampleBloc.dropDownSink.add(newValue),
                                            items: snapshotValues.data != null ? snapshotValues.data.map<DropdownMenuItem<String>>((String value) 
                                              return DropdownMenuItem<String>(
                                                value: value,
                                                child: Text(value),
                                              );
                                            ).toList() : SizedBox(height: 0.0)
                                        ),
                                      ) : SizedBox(height: 0.0);

显示一个空的小部件SizedBox(height: 0.0) 或CircleProgressIndicator

【讨论】：

这样做时我仍然会遇到同样的错误，即使我在 snapshotValues 上做同样的事情

我更新了我的代码，如果错误仍然存​​在，请在这里写下错误所在的行。