Best practices for async testing

[cybrox]

Hi! First of all, thanks for maintaining dart-statemachine!

I was wondering, how one would best go about testing objects that implement a state machine?
Transitions caused by method calls are pretty simple to test, however, I'm stuck at testing timeout transitions.

I tried using fake_async but it does not seem to work properly in this case.

This is an example test code I tried:

// this is all inside a fake_async block

        test("transitions back to unregistered after 10 minutes", () async {
          final completer = Completer<void>();

          final accountFuture = Account.create();

          accountFuture.then((account) {
            final registerFuture = account.register();
            registerFuture.then((_) {
              // By calling .register(), the state machine will transition to .registered
              // The .registered state has a timeout transition of 10 minutes, after
              // which it will transition back to .unregistered

              // This fails, as the state is still .registered
              expect(account.status, equals(AccountStatus.unregistered));

              a.elapse(const Duration(minutes: 100));

              // This fails, as the state is still .registered, despite 100 minutes having passed(?)
              expect(account.status, equals(AccountStatus.unregistered));

              completer.complete();
            });
          });

          // Prevent test from exiting early
          await completer.future;
        });

Any help would be greatly appreciated, as the plugin is perfect for building complex, time sensitive state machines but testing them has yielded no usable results for me after hours of trying.

[renggli]

One thing that looks suspicious is the position of the comment // this is all inside a fake_async block. The fake_async block needs to be inside the test, not around the test, see these examples.

Otherwise I have a hard time understanding what your test is attempting to do. A minimal reproducible example would help me debug the problem and propose a solution.

[cybrox]

Thanks for the reply. I did create a minimal example for testing and it works fine - unlike my actual code.

I will close this and re-open if I find any issue actually related to state-machine.
For now, it looks like I am doing something wrong with the creation of mock streams. (Using Mocktail)

For anyone else stumbling upon this, this is my working example code:

// Class file
import 'package:statemachine/statemachine.dart';

class TestEntity {
  final TestEntityState _state = TestEntityState();

  TestEntity() {
    this._state.registered.onTimeout(const Duration(minutes: 10), () {
      this._state.unregistered.enter();
    });

    this._state.machine.start();

    this._state.unregistered.enter();
  }

  void register() {
    this._state.registered.enter();
  }

  TestEntityStatus get currentState {
    return this._state.currentState;
  }
}

enum TestEntityStatus {
  unknown,
  unregistered,
  registered,
}

class TestEntityState {
  Machine machine = Machine<TestEntityStatus>();

  late State unknown;
  late State unregistered;
  late State registered;

  TestEntityState() {
    this.unknown = this.machine.newState(TestEntityStatus.unknown);
    this.unregistered = this.machine.newState(TestEntityStatus.unregistered);
    this.registered = this.machine.newState(TestEntityStatus.registered);
  }

  TestEntityStatus get currentState {
    final stateObj = this.machine.current as State<TestEntityStatus>?;
    return stateObj?.identifier ?? TestEntityStatus.unknown;
  }
}

// Test file
import 'package:fake_async/fake_async.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:kochmobile/entities/test_entity.dart';

void main() {
  test("enters default state on creation", () {
    final testEntity = TestEntity();
    expect(testEntity.currentState, TestEntityStatus.unregistered);
  });

  test("switches to registered when calling function", () {
    final testEntity = TestEntity();
    testEntity.register();
    expect(testEntity.currentState, TestEntityStatus.registered);
  });

  test("switches back to unregistered after timeout", () {
    fakeAsync((a) {
      final testEntity = TestEntity();
      testEntity.register();
      expect(testEntity.currentState, TestEntityStatus.registered);

      a.elapse(const Duration(minutes: 12));

      expect(testEntity.currentState, TestEntityStatus.unregistered);
    });
  });
}

[cybrox]

In case anyone finds this helpful, I since figured out what happened...

I was using State.onStream for multiple states in my state machine and I used the following code in test files main() function:

  mockAccountService = MockAccountService();

  StreamController<NativeEvent>? accountEvents;

  setUp(() {
    reset(mockAccountService);

    accountEvents = StreamController<NativeEvent>(sync: true);
    when(() => mockAccountService.events).thenAnswer((_) => accountEvents!.stream);
  });

  tearDown(() {
    accountEvents!.close();
    accountEvents = null;
  });

  test("...", () {
    // can now simply use accountEvents!.add(); to simulate inputs via stream
  });

Now, since accountEvents is not a broadcast stream controller, in the constructor of my object, the second State.onStream threw a Stream already listened to exception which was caught by statemachine. So I did not immediately realize this.

This in turn caused TransitionError on all states except the ones created before the second State.onStream, since they never got set up.

Changing the dummy stream to a .broadcast stream controller fixed it. 🎉

accountEvents = StreamController<NativeEvent>.broadcast(sync: true);