How do I write a unit test for a Flutter method that completes later with a future? - flutter-test

Question

I'm writing a unit test for a Flutter method that calls an async method and then returns, leaving the async to complete as and when. My test fails "after it had already completed".
Here's my test:
test('mark as viewed', () {
final a = Asset();
expect(a.viewed, false);
a.markAsViewed();
expect(a.viewed, true);
});
and here's the method it's testing:
void markAsViewed() {
viewed = true;
Repository.get().saveToStorage();
}
The saveToStorage() method is an async that I just leave to execute in the background.
How do I make this work? The test failure tells me Make sure to use [expectAsync] or the [completes] matcher when testing async code. but I can't see how to do that. Can anyone explain or else point me to the right documentation please? I can't find anything about how to handle these asyncs when it's not a Future that's being returned, but just being left to complete separately.
To be clear - this unit test isn't about testing whether it's saved to storage, just a basic test on setting viewed to be true.
Edited
The error is as follows:
package:flutter/src/services/platform_channel.dart 319:7 MethodChannel.invokeMethod
===== asynchronous gap ===========================
dart:async _asyncErrorWrapperHelper
package:exec_pointers/asset_details.dart Repository.saveToStorage
package:exec_pointers/asset_details.dart 64:22 Asset.markAsViewed
test/asset_details_test.dart 57:9 main.<fn>.<fn>
This test failed after it had already completed. Make sure to use [expectAsync]
or the [completes] matcher when testing async code.

Answer 1

This code is tightly coupled to implementation concerns that make testing it in isolation difficult.
It should be refactored to follow a more SOLID design with explicit dependencies that can be replaced when testing in isolation (unit testing)
For example
class Asset {
Asset({Repository repository}) {
this.repository = repository;
}
final Repository repository;
bool viewed;
void markAsViewed() {
viewed = true;
repository.saveToStorage();
}
//...
}
That way when testing a mock/stub of the dependency can be used to avoid any unwanted behavior.
// Create a Mock Repository using the Mock class provided by the Mockito package.
// Create new instances of this class in each test.
class MockRepository extends Mock implements Repository {}
main() {
test('mark as viewed', () {
final repo = MockRepository();
// Use Mockito to do nothing when it calls the repository
when(repo.saveToStorage())
.thenAnswer((_) async => { });
final subject = Asset(repo);
expect(subject.viewed, false);
subject.markAsViewed();
expect(subject.viewed, true);
//
verify(repo.saveToStorage());
});
}
The test should now be able to be exercised without unexpected behavior from the dependency.
Reference An introduction to unit testing
Reference Mock dependencies using Mockito
Reference mockito 4.1.1