any<T> method Null safety

Future<T> any<T>(
  1. Iterable<Future<T>> futures
)

Returns the result of the first future in futures to complete.

The returned future is completed with the result of the first future in futures to report that it is complete, whether it's with a value or an error. The results of all the other futures are discarded.

If futures is empty, or if none of its futures complete, the returned future never completes.

Example:

void main() async {
  final result =
      await Future.any([slowInt(), delayedString(), fastInt()]);
  // The future of fastInt completes first, others are ignored.
  print(result); // 3
}
Future<int> slowInt() async {
  await Future.delayed(const Duration(seconds: 2));
  return 2;
}

Future<String> delayedString() async {
  await Future.delayed(const Duration(seconds: 2));
  throw TimeoutException('Time has passed');
}

Future<int> fastInt() async {
  await Future.delayed(const Duration(seconds: 1));
  return 3;
}

Implementation

static Future<T> any<T>(Iterable<Future<T>> futures) {
  var completer = new Completer<T>.sync();
  void onValue(T value) {
    if (!completer.isCompleted) completer.complete(value);
  }

  void onError(Object error, StackTrace stack) {
    if (!completer.isCompleted) completer.completeError(error, stack);
  }

  for (var future in futures) {
    future.then(onValue, onError: onError);
  }
  return completer.future;
}