一个构建函数返回 null，有问题的 widget 是：BlocBuilder<NotificationBloc、NotificationState> 在flutter_bloc包中

Question

我是颤振和flutter_bloc的初学者，我试图在 blocbuilder 中获取小部件，但得到"A build 函数返回空。有问题的小部件是：BlocBuilder。生成函数绝不能返回 null。下面是bloc代码以及来自控制台的空错误。

Column(
children: <Widget>[
Container(
color: Colors.white,
child: BlocListener<NotificationBloc, NotificationState>(
listener: (context, state) {
},
child: BlocBuilder<NotificationBloc, NotificationState>(
builder: (context, state) {
if (state is NotificationLoadedState) {
return NotificationIconBuild();
}
},
),
),
)
],
)
Widget NotificationIconBuild() {
return Column(
children: <Widget>[
IconButton(
icon: Icon(Icons.notifications),
onPressed: () {},
),
IconButton(
icon: Icon(Icons.notifications),
onPressed: () {},
),])}

控制台日志中的错误

I/flutter (13632): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (13632): The following assertion was thrown building BlocBuilder<NotificationBloc, NotificationState>(dirty,
I/flutter (13632): state: _BlocBuilderBaseState<NotificationBloc, NotificationState>#7fa62):
I/flutter (13632): A build function returned null.
I/flutter (13632): The offending widget is:
I/flutter (13632):   BlocBuilder<NotificationBloc, NotificationState>
I/flutter (13632): Build functions must never return null.
I/flutter (13632): To return an empty space that causes the building widget to fill available room, return
I/flutter (13632): "Container()". To return an empty space that takes as little room as possible, return
I/flutter (13632): "Container(width: 0.0, height: 0.0)".
I/flutter (13632): 
I/flutter (13632): The relevant error-causing widget was:
I/flutter (13632):   BlocBuilder<NotificationBloc, NotificationState>
I/flutter (13632):   file:///C:/Users/Nabil/AndroidStudioProjects/flutter_save/lib/home_page.dart:77:24

Answer 1

发生这种情况是因为您只为您的状态指定了一个if条件，即NotificationLoadedState。您的集团必须尝试构建未指定的其他状态，因此BlocBuilder返回 null。

为了快速修复，您现在可以将BlocBuilder更改为类似的东西。

child: BlocBuilder<NotificationBloc, NotificationState>(
builder: (context, state) {
if (state is NotificationLoadedState) {
return NotificationIconBuild();
} else {
return Container();,
}
},
),

Answer 2

发生这种情况是因为 BlocBuilder 需要返回。但是，您仅在条件为真时定义了返回。但是当条件为假时，它将返回 null。这就是为什么它给出此错误的原因。所以你必须添加else块或返回语句。

在新版的 bloc 中，如果你想同时使用 BlocBuilder 和 BlocListener，你可以使用BlocConsumerwidget。

你可以检查一下：集团消费者示例

Answer 3

非常愚蠢的一件事是我用BlocListener而不是BlocBuilder，它抛出了上面的错误，希望它可以帮助某人。