我是Bloc编程模式的新手，在将其与Dropdown一起使用时遇到问题 那是在我的bloc类中：
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;
在我的窗口小部件页面中，我添加了以下下拉窗口小部件，以便所有内容均由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null。我知道值必须是列表中的某个值或为null。但是，我没有提出任何逻辑可以使此错误消失。 如果将值设置为null，则可以使用，但是所选的值不会显示。 我做错了吗？有没有更好的方法可行？我该如何解决这个问题？ 谢谢！
答案 0 :(得分：0)
那是因为您使用的是StreamBuilder，所以在第一次快照为空时，您必须进行验证：
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int create_random(int num);
int main()
{
int i=0,num=0;
printf("How many random number ? ");//Request from user..
scanf("%d",&num);
for(i=0 ; i<num ; ++i)
{
printf("%d\n",create_random(i));
}
return 0;
}
int create_random(int a)
{
a=0;
srand(time(NULL));
a=rand()%100;
return a;
}
显示一个空的小部件it('Should add', () => {
const fb = new FormBuilder();
const myObject = fb.group('myArray': fb.array());
component.new(myObject);
const packagesLength = component.myFormGroup.get('data')['controls'][0] ['controls']['items']['controls'].length;
expect(packagesLength).toEqual(2);
});
或 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);
答案 1 :(得分：0)
我在集团中使用两个流来解决它，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，并且当您同时获得两个快照和数据时，就可以加载构建。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，则使用循环进度指示器返回。 希望我能对您有所帮助。