How to explicitly return `void`? - flutter

Question

One flutter package has this kind of data requesting workflow:
final cancelListening = await request(
query: query,
onResponse: (response) {
streamController.add(response);
cancelListening(); // I need to cancel it here;
},
);
But this way I obviously have the error: cancelListening can't be referenced before it is declared. As request() returns Future<void Function()> I can do this:
void Function() cancelListening = () {};
cancelListening = await request(
...
And I got Omit type annotations for local variables.dart(omit_local_variable_types).
So, I write this way:
var cancelListening = () {}
cancelListening = await request(
...
But now cancelListening is Null Function() and I'm getting A value of type 'void Function()' can't be assigned to a variable of type 'Null Function()'.
So my questions is:
Is there is a way to explicitly return void in dart? Something like () => Void();
Should I simply ignore this linter rule or there is better way to handle this situation?
Thanks.

Answer 1

You might want to setup an intermediary function to call the received callback. VoidCallback should be a type for functions that specifically return void, instead of null.
VoidCallback? cancelListening;
void stopListening() {
cancelListening?.call();
}
void listen() async {
cancelListening = await request(
query: query,
onResponse: (response) {
streamController.add(response);
stopListening();
},
);
}
Optionally, instead of making cancelListening nullable, you could use the late keyword.
late VoidCallback cancelListening;
void stopListening() {
cancelListening(); // No optional operator `?` needed.
}