async* blocking function body from executing - dart

Question

When adding async* to listen method it isn't executing the function body
import 'dart:async';
main(List<String> args) {
print('====');
tranStream();
}
Stream<int> intStreamer() async* {
int c = 0;
while (c <= 30) {
await Future.delayed(Duration(seconds: 1));
yield c++;
}
}
tranStream() {
intStreamer().listen((event) async* { // If i remove async* from here it will execute print statement
print(event);
});
}
If i remove async* from intStreamer().listen it will execute print statement. What is happening here?

Answer 1

When you are using async*, the method will only start being executed when the returned Stream gets a subscriber. Your code does not really make any sense since listen takes a method which returns void. So nobody are going to listen on the returned Stream which the given method will automatically return (based on the async* keyword).
I would also properly rewrite your code so you instead of listen uses await for which I think makes it more clear what happens`:
import 'dart:async';
Future<void> main(List<String> args) async {
print('====');
await tranStream();
}
Stream<int> intStreamer() async* {
int c = 0;
while (c <= 30) {
await Future<void>.delayed(const Duration(seconds: 1));
yield c++;
}
}
Future<void> tranStream() async {
await for (final event in intStreamer()) {
print(event);
}
}
Update with example of tranStream returns a Stream:
import 'dart:async';
Future<void> main(List<String> args) async {
print('====');
await for (final event in tranStream()) {
print('main got: $event');
}
}
Stream<int> intStreamer() async* {
int c = 0;
while (c <= 30) {
await Future<void>.delayed(const Duration(seconds: 1));
yield c++;
}
}
Stream<int> tranStream() async* {
await for (final event in intStreamer()) {
print('tranStream got: $event');
yield event;
}
}