# Why need to use Completer class

**URL:** <https://forums.kodeco.com/t/why-need-to-use-completer-class/184951>\
**Category:** Dart Apprentice: Beyond the Basics\
**Created:** [March 10, 2023, 2:53pm UTC](https://forums.kodeco.com/t/why-need-to-use-completer-class/184951 "2023-03-10T14:53:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ellery](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/ellery/32/110937_2.png) [@ellery](https://forums.kodeco.com/u/ellery)\
**Post date:** [March 10, 2023, 2:53pm UTC](https://forums.kodeco.com/t/why-need-to-use-completer-class/184951/1 "2023-03-10T14:53:41Z")

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About the section Creating a Future From Scratch in Chapter 12 Futures, I just don’t understand why I need to use `Completer` class to build a `Future` instance.

The code using `async` keyword just work and very readable:

```dart
  Future<double> fetchTemperature(String city) async {
    if (city == 'Portland') {
      return 42.0;
    } else {
      return throw ArgumentError("$city doesn't exist.");
    }
  }

```

Compare the above for the code of `Completer` class:

```dart
@override
Future<double> fetchTemperature(String city) {
  final completer = Completer<double>();
  if (city == 'Portland') {
    completer.complete(42.0);
  } else {
    completer.completeError(ArgumentError("City doesn't exist."));
  }
  return completer.future;
}

```

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<div class="post-metadata">

**Author:** ![suragch](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/suragch/32/109020_2.png) [@suragch](https://forums.kodeco.com/u/suragch)\
**Post date:** [March 11, 2023, 6:36am UTC](https://forums.kodeco.com/t/why-need-to-use-completer-class/184951/2 "2023-03-11T06:36:08Z")

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That’s a good question that the chapter in it’s current form doesn’t adequately answer. This deserves more explanation in the next edition or perhaps in an advanced Dart book.

For the contrived examples here, I agree that the `async` version is more readable. I would use `async` if I had to choose between the two.

A `Completer` is more useful for more complex requirements. One major use case is where you want to wrap a callback-based API in order to convert it to a `Future`-based API. Callbacks can be a little messy because you have to pass anonymous functions in as parameters. Converting such an API to a `Future` makes it a little easier to work with.

Let me refer you to a couple of articles as you continue your research:

- [Futures and Completers in Dart and Flutter](https://dart.academy/futures-and-completers-in-dart-and-flutter/)
- [Completer in Dart/Flutter](https://medium.com/@jessicajimantoro/completer-in-dart-flutter-e15f5739e96d)

Feel free to ask any followup questions after reading those. I may or may not be able to answer since this topic bumps up against the edge of my Dart knowledge (the only time I recall using a completer was when I tried to make an API for low-level database calls), but your questions will serve as markers for further research in the future. (no pun intended)

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<div class="post-metadata">

**Author:** ![ellery](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/ellery/32/110937_2.png) [@ellery](https://forums.kodeco.com/u/ellery)\
**Post date:** [March 11, 2023, 9:46am UTC](https://forums.kodeco.com/t/why-need-to-use-completer-class/184951/3 "2023-03-11T09:46:58Z")

</div>

@suragch thank you. I will take a look after finishing the book may be.