future_zip_sample.dart

class Post {
  final String? content;
  const Post([this.content]);
}

class UserProfile {
  final List<Post> posts;
  final int followersCount;

  const UserProfile(this.posts, this.followersCount);
}

class User {
  final int followersCount;

  const User(this.followersCount);
}

class Api {
  Future<List<Post>> getPosts() async {
    await Future.delayed(Duration(seconds: 2));
    return [Post()];
  }

  Future<User> getUser() async {
    await Future.delayed(Duration(seconds: 2));
    return User(123);
  }
}

extension FutureZipX<T> on Future<T> {
  Future<(T, T2)> zipWith<T2>(Future<T2> future2) async {
    late T result1;
    late T2 result2;
    await Future.wait(
        [then((it) => result1 = it), future2.then((it) => result2 = it)]);
    return (result1, result2);
  }
}


void main() async {
  final api = Api();
  Stopwatch stopwatch = Stopwatch()..start();
  UserProfile(await api.getPosts(), (await api.getUser()).followersCount); // sequential 2s + 2s
  print(stopwatch.elapsed); // 4s~

  stopwatch = Stopwatch()..start();
  final responses = await Future.wait([api.getPosts(), api.getUser()]); // parallel w/o types // max(2s, 2s)
  UserProfile(responses[0] as List<Post>, (responses[1] as User).followersCount); // error-prone
  print(stopwatch.elapsed); // 2s~

  stopwatch = Stopwatch()..start();
  final (posts, user) = await api.getPosts().zipWith(api.getUser()); // parallel w/ types  // max(2s, 2s)
  final userProfile = UserProfile(posts, user.followersCount);
  print(stopwatch.elapsed); // 2s~
  print(userProfile.posts.length);
  print(userProfile.followersCount);
}