Flutter：如何对多个流进行排队

Question

I have stream like the below-mentioned code, here want to merge streams and wanted to call it one by one whenever it required.

main() {
  Stream<String> stream = new Stream.fromFuture(getData());

  stream.listen((data) {
    print("DataReceived: "+data);
  }, onDone: () {
    print("Task Done");
  });
  
}

Future<String> getData() async {
  await Future.delayed(Duration(seconds: 5)); //Mock delay 
  print("Fetched Data");
  return "This a test data";
}

onDone will be called after 5 seconds.

Answer 1

For it, StreamQueue can be used to combine multiple streams

Add Dependency:

async: ^2.4.1

Use StreamQueue:

void main() async {
  Stream<String> stream1 = new Stream.fromFuture(getData(2));
  Stream<String> stream2 = new Stream.fromFuture(getData(4));
  Stream<String> stream3 = new Stream.fromFuture(getData(6));
  var streams = StreamGroup.merge([stream1, stream2, stream3]);

  var data = StreamQueue(streams);
  var first = await data.next;
  print(first);
  var second = await data.next;
  print(second);
  var third = await data.next;
  print(third);
  var third1 = await data.next;
  print(third1);
}

Future<String> getData(int duration) async {
  await Future.delayed(Duration(seconds: duration)); //Mock delay
  return "This a test data for duration $duration";
}

Output:

I/flutter ( 7950): This a test data for duration 2
I/flutter ( 7950): This a test data for duration 4
I/flutter ( 7950): This a test data for duration 6

data.next will provide you the first stream in Queue which is in our case is stream1, here we can take the future value and use. stream.next gives next stream in the queue if nothing exists in queue & stream.next is performed then it will throw an exception.