Dart Completer sample

main.dart

import 'dart:async';
import 'dart:math';

class API {
  void startMethod() {
    Timer(Duration(seconds: 2), onMethodDone);
  }

  bool onMethodDone() {
    final randomBool = Random().nextBool();
    print('after 2 seconds, we have decided to ${randomBool ? "succeed" : "fail"}');
    return randomBool;
  }
}

class APIAsync extends API {
  final Completer completer;

  APIAsync(this.completer) : super() {
    super.startMethod();
  }

  @override
  bool onMethodDone() {
    final result = super.onMethodDone();
    result ? completer.complete(result) : completer.completeError('the API call failed');
    return result;
  }
}

void main() {
  final _completer = Completer();
  APIAsync(_completer);
  _completer.future
      //.then is called on `completer.complete()`
      .then((value) => print('API completed: $value'))
      //.onError is called on `completer.completeError()`
      .onError((error, _) => print('API failed with error: $error'));
}