await inside foreach possible?

[Benjamin Strauß]

Hi, i have an async function that looks like this:

Future<bool> clear() async
{
    int result = 1;

    _caches.forEach((Cache cache)
                    {
                        bool succeeded = await cache.clear();

                        result &= succeeded ? 1 : 0;
                    });

    return result == 1;
}

This throws an error because the closure itself is not marked as async. If i change it to this everything works inside dartium.

Future<bool> clear() async
{
    int result = 1;

    _caches.forEach((Cache cache) async
                    {
                        bool succeeded = await cache.clear();

                        result &= succeeded ? 1 : 0;
                    });

    return result == 1;
}

But is this safe to compile to javascript? Am i allowed to mark callbacks as async, even if the resulting future is not used in the underlying system? Is there a better way to do this?

[Günter Zöchbauer]

This works fine. If it doesn't create a bug report.

[Justin Fagnani]

You need to use Future.forEach: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:async.Future#id_forEach

And you'll also need to await the forEach:

Future<bool> clear() async {

  var succeeded = true;

  await Future.forEach(_caches, (Cache cache) async {

    succeeded = succeeded && await cache.clear();

  });

  return succeeded;

}

By the way, your operation is an async fold on _caches. You can use Quiver's async.reduceAsync to make this shorter:

import 'package:quiver/async.dart' show reduceAsync;

Future<bool> clear() => reduceAsync(_caches, true, (s, cache) async => s && await cache.clear());

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[Benjamin Strauß]

Thank you, i already use quiver in my project, so i will just use the function. :)

[Kevin Moore]

Future<bool> clear() async {
  int result = 1;

  for (var cache in _caches) {

    if (!await cache.clear()) {

      return false;

    }
  }

  return true;
}

[Alex Tatumizer]

attempt of a parallel version (never tested):

Future<bool> clear() async {
  var results = await Future.wait(_caches.map((c)=>c.clear()));
  return results.fold(true, (acc, val)=> acc && val);
}