Iterable.forEach
, Map.forEach
, and Stream.forEach
are meant to execute some code on each element of a collection for side effects. They take callbacks that have a void
return type. Consequently, those .forEach
methods cannot use any values returned by the callbacks, including returned Future
s. If you supply a function that returns a Future
, that Future
will be lost, and you will not be able to be notified when it completes. You therefore cannot wait for each iteration to complete, nor can you wait for all iterations to complete.
Do NOT use .forEach
with asynchronous callbacks.
Instead, if you want to wait for each asynchronous callback sequentially, just use a normal for
loop:
for (var mapEntry in gg.entries) {
await Future.delayed(const Duration(seconds: 5));
}
(In general, I recommend using normal for
loops over .forEach
in all but special circumstances. Effective Dart has a mostly similar recommendation.)
If you really prefer using .forEach
syntax and want to wait for each Future
in succession, you could use Future.forEach
(which does expect callbacks that return Future
s):
await Future.forEach(
gg.entries,
(entry) => Future.delayed(const Duration(seconds: 5)),
);
If you want to allow your asynchronous callbacks to possibly run in parallel, you can use Future.wait
:
await Future.wait([
for (var mapEntry in gg.entries)
Future.delayed(const Duration(seconds: 5)),
]);
See https://github.com/dart-lang/linter/issues/891 for a request for an analyzer warning if attempting to use an asynchronous function as a Map.forEach
or Iterable.forEach
callback (and for a list of many similar StackOverflow questions).
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