Flutter tests throw "Bad state: Future already completed" when testing a call to `notifyListeners`

Question

I am trying to test a class with a function like that

class A {
  void doStuff() {
    // Do stuff...
    notifyListeners();
  }
}

I am currently using the flutter-test package to automate my tests. My test looks like this:

void main() {
  final myInstance = A();

  group("Class A", () {
    test("should correctly do stuff", () async {
      myInstance.doStuff();
      expect(...);
    });
  });
}

The test is working and expect yields the correct result. However, console shows an error message:

The following StateError was thrown while dispatching notifications for Class A:
Bad state: Future already completed

What causes this error and how can I prevent it from happening?

Answer 1

Turns out, notifyListeners() was the root-cause here. It threw an error, presumably because it was not yet initialized.

By waiting for its initialization, the error has stopped occuring. See TestWidgetsFlutterBinding.ensureInitialized for reference

setUpAll(() {
  TestWidgetsFlutterBinding.ensureInitialized();
});