Navigating to a new screen when stream value in BLOC changes - ios

Question

In Flutter how would I call Navigator.push when the value of a stream changes? I have tried the code below but get an error.
StreamBuilder(
stream: bloc.streamValue,
builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
if (snapshot.hasData && snapshot.data == 1) {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SomeNewScreen()),
);
}
return Text("");
});

Answer 1

You should not use StreamBuilder to handle navigation.
StreamBuilder is used to build the content of a screen and nothing else.
Instead, you will have to listen to the stream to trigger side-effects manually. This is done by using a StatefulWidget and overriding initState/dispose as such:
class Example extends StatefulWidget {
final Stream<int> stream;
const Example({Key key, this.stream}) : super(key: key);
#override
ExampleState createState() => ExampleState();
}
class ExampleState extends State<Example> {
StreamSubscription _streamSubscription;
#override
void initState() {
super.initState();
_listen();
}
#override
void didUpdateWidget(Example oldWidget) {
super.didUpdateWidget(oldWidget);
if (oldWidget.stream != widget.stream) {
_streamSubscription.cancel();
_listen();
}
}
void _listen() {
_streamSubscription = widget.stream.listen((value) {
Navigator.pushNamed(context, '/someRoute/$value');
});
}
#override
void dispose() {
_streamSubscription.cancel();
super.dispose();
}
#override
Widget build(BuildContext context) {
return Container();
}
}
Note that if you're using an InheritedWidget to obtain your stream (typically BLoC), you will want to use didChangeDependencies instead of initState/didUpdateWidget.
This leads to:
class Example extends StatefulWidget {
#override
ExampleState createState() => ExampleState();
}
class ExampleState extends State<Example> {
StreamSubscription _streamSubscription;
Stream _previousStream;
void _listen(Stream<int> stream) {
_streamSubscription = stream.listen((value) {
Navigator.pushNamed(context, '/someRoute/$value');
});
}
#override
void didChangeDependencies() {
super.didChangeDependencies();
final bloc = MyBloc.of(context);
if (bloc.stream != _previousStream) {
_streamSubscription?.cancel();
_previousStream = bloc.stream;
_listen(bloc.stream);
}
}
#override
void dispose() {
_streamSubscription.cancel();
super.dispose();
}
#override
Widget build(BuildContext context) {
return Container();
}
}

Answer 2

You can extend StreamBuilder with custom listener like this:
typedef StreamListener<T> = void Function(T value);
class StreamListenableBuilder<T> extends StreamBuilder<T> {
final StreamListener<T> listener;
const StreamListenableBuilder({
Key key,
T initialData,
Stream<T> stream,
#required this.listener,
#required AsyncWidgetBuilder<T> builder,
}) : super(key: key, initialData: initialData, stream: stream, builder: builder);
#override
AsyncSnapshot<T> afterData(AsyncSnapshot<T> current, T data) {
listener(data);
return super.afterData(current, data);
}
}
Then connect listener for navigation this way:
StreamListenableBuilder(
stream: bloc.streamValue,
listener: (value) {
if (value==1) {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SomeNewScreen()),
);
}
},
builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
return Container();
});