How to Async/await in List.forEach() in Dart

Question

I'm writing some kind of bot (command line application) and I'm having trouble with async execution when I'm using the "forEach" method. Here is a simplified code of what I'm trying to do :

main() async {
  print("main start");
  await asyncOne();
  print("main end");
}

asyncOne() async {
  print("asyncOne start");
  [1, 2, 3].forEach(await (num) async {
    await asyncTwo(num);
  });
  print("asyncOne end");
}

asyncTwo(num) async
{
  print("asyncTwo #${num}");
}

And here is the output :

main start
asyncOne start
asyncOne end
main end
asyncTwo #1
asyncTwo #2
asyncTwo #3

What I'm trying to get is :

main start
asyncOne start
asyncTwo #1
asyncTwo #2
asyncTwo #3
asyncOne end
main end

If someone knows what I'm doing wrong I would appreciate it.

Solution 1

I don't think it's possible to achieve what you want with the forEach method. However it will work with a for loop. Example;

asyncOne() async {
  print("asyncOne start");
  for (num number in [1, 2, 3])
    await asyncTwo(number);
  print("asyncOne end");
}

Solution 2

You need to use Future.forEach.

main() async {
  print("main start");
  await asyncOne();
  print("main end");
}

asyncOne() async {
  print("asyncOne start");
  await Future.forEach([1, 2, 3], (num) async {
    await asyncTwo(num);
  });
  print("asyncOne end");
}

asyncTwo(num) async
{
  print("asyncTwo #${num}");
}

Solution 3

You can't use forEach for this because it doesn't actually look at the return values of its callbacks. If they are futures, they will just be lost and not awaited.

You can either do a loop like Steven Upton suggested, or you can use Future.wait if you want the operations to run simultaneously, not one after the other:

asyncOne() async {
  print("asyncOne start");
  await Future.wait([1, 2, 3].map(asyncTwo));
  print("asyncOne end");
}

Solution 4

I know this is an old question, but I'll leave here a new answer, hoping this help someone in the future.

You can use forEach for what you're trying to achieve by doing something like this:

  asyncOne() async {
  print("asyncOne start");
  await Future.forEach([1, 2, 3],(num) async {
    await asyncTwo(num);
  });
  print("asyncOne end");
}

View more solutions

Comments

[Ουιλιαμ Αρκευα]

Asynchronous calls don't work with <List>.forEach(), you need to use traditional loops. Why? This article explains it very well. I just discovered that Dart and Node.JS are almost the same.

[aelayeb]

That's what I was starting to think. Thank you for confirming it. I thought that "await" expression precedes whatever is surrounding it but oh well...

[Grahambo]

This should be the approved answer.

[skytect]

For anyone reading this, do note that this unfortunately only works for iterables.

[mike-gallego]

Yesss this is so perfect on what I needed because of internet connection issues with the app. It'll load the list and add it, when it is complete, notify.

[Rahul Sharma]

You just saved me hours of debugging and searching over internet.

[Mangesh Kadam]

this does not work with Future.delayed in asyncTwo method. output is main start asyncOne start asyncOne end main end asyncTwo #1 asyncTwo #2 asyncTwo #3

[Keith DC]

That is literally a subset of what @Stephane posted above.

[Jules Lee]

Perfect answer!

[David Chopin]

Note that while this is the best approach, the compiler may not know the type of num within the block. So you may have to call something like await Future.forEach<int>(someIntList, (item) async {