How can I know when my code completes all the async work in zone, in Dart?

Question

Consider I have code like this:

import 'dart:async';

foo() {
  print('foo');
}

bar() {
  print('bar');
}

void main() {
  runZoned(() {
    new Timer(const Duration(seconds: 1), foo);
    new Timer(const Duration(seconds: 2), bar);
  });
}

How can I know when all the async work inside a Zone is completed? That is, is it possible to know when all the async methods (either via timer, future, etc) that were registered within a zone are complete?

Answer 1

Initially zones did have an "onDone" callback. (See a usage in an old test here). However we discovered that it is too hard to keep track of outstanding callbacks. For example: future.then(callback) registers a callback, but there is absolutely no guarantee that this callback will ever be invoked.

In fact, it is quite common that futures are never completed. Zones, themselves, contribute to this: when an error originates in a zone that has an error-handler the error will never leave the zone. That means that future-chains outside the zone won't be completed (neither with a value nor the error that got intercepted by the zone).

TL;DR: we had onDone on zones but removed it because it was impractical.