Dart firstThatResolves

f.dart

import 'dart:async';

extension FutureX<T> on Future<T> {
  /// This function accepts two functions returning a future,
  /// and returns whichever function resolves successfully first.
  ///
  /// Can be used in case one of the functions may fail
  /// even under normal circumstances as long as the other succeeds
  /// (so at least one of them should succeed).
  ///
  /// If both functions return an error, this static method will
  /// use the second error.
  static Future<T> firstThatResolves<T>(
      Future<T> Function() a,
      Future<T> Function() b,
      ) {
    final completer = Completer<T>();
    var aCompleted = false;
    var bCompleted = false;

    a().then((t) {
      aCompleted = true;
      if (bCompleted) return;
      completer.complete(t);
    }).catchError((Object e, StackTrace st) {
      aCompleted = true;
      if (bCompleted && completer.isCompleted) return;
      completer.completeError(e, st);
    });

    b().then((t) {
      bCompleted = true;
      if (aCompleted) return;
      completer.complete(t);
    }).catchError((Object e, StackTrace st) {
      bCompleted = true;
      if (aCompleted) return;
      completer.completeError(e, st);
    });

    return completer.future;
  }
}