StreamProvider未更新状态 [英] StreamProvider not updating state
问题描述
我正在尝试使用 StreamProvider
(来自此很棒的软件包),但是我一直在努力使特定的流工作。
I am trying to use a StreamProvider
(from this awesome package), but I've been struggling to get a particular stream to work.
我创建了一个 StreamController
,用于将数据添加到其 Stream
通过其接收器
。所有这些似乎都工作正常。但是,当将此 Stream
与 StreamProvider
一起使用时,小部件树不会反映流
。但是,使用 StreamBuilder
可以正常工作。
I create a StreamController
which I use to add data to its Stream
via its Sink
. All of this seems to be working fine. But when using this Stream
with a StreamProvider
, the widget tree does not reflect the changes of the Stream
. It does however work fine using a StreamBuilder
.
使用 StreamProvider
的代码:
class TestPage extends StatelessWidget {
final Mockup exchange = ExchangeApi.mockup;
@override
Widget build(BuildContext context) {
return StreamProvider<List<Data>>(
builder: (BuildContext context) => Wrapper.mockup.stream,
initialData: null,
catchError: (BuildContext context, e) {
print("Error: $e");
return null;
},
child: TestPageBody(),
);
}
}
class TestPageBody extends StatelessWidget {
@override
Widget build(BuildContext context) {
List<Data> dataList = Provider.of<List<Data>>(context);
return ListView.builder(
itemCount: dataList?.length ?? 0,
itemBuilder: (BuildContext context, int index) {
return Text(dataList[index].name);
},
);
}
}
我一直在寻找为什么这行不通,但是尚未找到答案。但是我确实找到了一些东西:
I have been searching why this does not work, but haven't found an answer yet. But here are some things I did find:
- 使用Flutter Desktop Embedding,当窗口调整大小时,UI确实反映了流中的变化(从而迫使重建)。使用热刷新时,会看到相同的效果。
- 流中不断添加新数据,我通过调试并简单地打印出数据来测试这一点
任何帮助将不胜感激!
推荐答案
大多数人的默认行为提供程序(不包括 ChangeNotifierProvider
)将假定传递的值是不可变的。
The default behavior of most providers (excluding ChangeNotifierProvider
) is to assume that the values passed are immutable.
因此,如果您的流发出
有两种解决方案:
-
使流发出的值不可变,这样执行
previousValue == newValue
即可正常工作。
覆盖 updateShouldNotify
以在值不变时不过滤值。
override updateShouldNotify
to not filter values if they didn't change.
一个简单的 updateShouldNotify:(_,__)=>
A simple updateShouldNotify: (_, __) => true
will do.
在
这可以通过在将对象发送到 streamController.add(value)<之前复制对象来完成。 / code>:
This can be done by making a copy of your object before sending it to streamController.add(value)
:
List<T> value;
streamController.add(List.from(value));
第二个(可选)步骤是覆盖 updateShouldNotify
(或您对象上的 operator == )通知提供者该值没有更改。
A second (optional) step is to override updateShouldNotify
(or operator==
on your object) to notify the provider that the value didn't change.
使用列表
,可以使用 collection / collection.dart中的
: ListEquality
完成此操作
With List
, this can be done using ListEquality
from collection/collection.dart
:
import 'package:provider/provider.dart';
import 'package:collection/collection.dart';
StreamProvider<List<int>>(
builder: (_) => stream,
updateShouldNotify: const ListEquality<int>().equals,
);
这篇关于StreamProvider未更新状态的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!