CODEWITHSUNDEEP
dartfuture
bitnick
bitnick

Reputation: 2073

Dartlang wait more than one future

I want to do something after a lot of future functions are done, but I do not know how to write the code in dart? the code is like this:

for (var d in data) {
  d.loadData().then()
}
// when all loaded
// do something here

but I don't want to wait for them one by one:

for (var d in data) {
  await d.loadData(); // NOT NEED THIS
}

how to write those code in dart?

Upvotes: 125

Views: 63258

Answers (4)

Tobias Marschall
Tobias Marschall

Reputation: 2579

If you want to wait for multiple futures of different types and also support null-safety then you can add a helper function similar to the following when using Dart >= 3.0:

Future<(T1, T2)> waitConcurrently<T1, T2>(
  Future<T1> future1,
  Future<T2> future2,
) async {
  late T1 result1;
  late T2 result2;

  await Future.wait([
    future1.then((value) => result1 = value),
    future2.then((value) => result2 = value)
  ]);

  return (result1, result2);
}

Dart < 3.0 does not support tuples natively, but there is a package from Google which does: https://pub.dev/packages/tuple

import 'package:tuple/tuple.dart';

Future<Tuple2<T1, T2>> waitConcurrently<T1, T2>(
    Future<T1> future1, Future<T2> future2) async {
  late T1 result1;
  late T2 result2;

  await Future.wait([
    future1.then((value) => result1 = value),
    future2.then((value) => result2 = value)
  ]);

  return Tuple2(result1, result2);
}

Upvotes: 23

Praytic
Praytic

Reputation: 2041

In addition, I'd like to supplement Günter Zöchbauer's answer with FutureOr variant. You'll need to convert your FutureOr<T> variable to Future<T> first and then call wait:

Future.wait(list.map((x) async => x))

Upvotes: 4

Aleksandar
Aleksandar

Reputation: 4144

Existing answer gives enough information, but I want to add a note/warning. As stated in the docs:

The value of the returned future will be a list of all the values that were produced in the order that the futures are provided by iterating futures.

So, that means that the example below will return 4 as the first element (index 0), and 2 as the second element (index 1).

import 'dart:async';

Future main() async {
  print('start');

  List<int> li = await Future.wait<int>([
    fetchLong(),  // longer (which gives 4) is first
    fetchShort(), // shorter (which gives 2) is second
  ]);

  print('results: ${li[0]} ${li[1]}'); // results: 4 2
}

Future<int> fetchShort() {
  return Future.delayed(Duration(seconds: 3), () {
    print('Short!');
    return 2;
  });
}

Future<int> fetchLong() {
  return Future.delayed(Duration(seconds: 5), () {
    print('Long!');
    return 4;
  });
}

Upvotes: 25

G&#252;nter Z&#246;chbauer
G&#252;nter Z&#246;chbauer

Reputation: 657148

You can use Future.wait to wait for a list of futures:

import 'dart:async';

Future main() async {
  var data = [];
  var futures = <Future>[];
  for (var d in data) {
    futures.add(d.loadData());
  }
  await Future.wait(futures);
}

DartPad example

Upvotes: 231

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