Usage the Synchronized package and locking the async operations demo

main.dart

import 'dart:async';

import 'package:synchronized/synchronized.dart';

///
/// without synchronized logout should be the same:
/// emitting 1
/// listening the step 1
/// listening the step 1
/// emitting 1
/// listening the step 1
/// listening the step 1
/// emitting 2
/// emitting 2
/// emitting 3
/// emitting 3
/// emitting 4
/// emitting 4
/// With synchronized logout should be look like this
/// emitting 1
/// listening the step 1
/// listening the step 1
/// emitting 2
/// emitting 3
/// emitting 4
/// emitting 1
/// emitting 2
/// emitting 3
/// emitting 4
///

void main() {
  final sync = SyncService();
  final subscription1 = sync.startUpload()
    ..onDone(() {
      print('done');
    });

  final subscription2 = sync.startUpload()
    ..onDone(() {
      print('done');
    });

  Future.delayed(Duration(seconds: 4), () => subscription1.cancel());
  Future.delayed(Duration(seconds: 4), () => subscription2.cancel());
}

class SyncService {
  final lock = Lock();

  final StreamController<int> _streamController =
      StreamController<int>.broadcast();

  Stream<int> get onSync => _streamController.stream;

  StreamSubscription<int> startUpload() {
    _upload();

    return onSync.listen((event) {
      print('listening the step $event');
    });
  }

  Future<void> _upload() => lock.synchronized(() async {
        await Future.delayed(Duration(seconds: 2));
        print('emitting 1');
        _streamController.add(1);

        await Future.delayed(Duration(seconds: 2));
        print('emitting 2');
        _streamController.add(2);

        await Future.delayed(Duration(seconds: 2));
        print('emitting 3');
        _streamController.add(3);

        await Future.delayed(Duration(seconds: 2));
        print('emitting 4');
        _streamController.add(4);
      });
}