Effects immediately firing instead of after store.receive call in test

[acosmicflamingo]

I have an appDelegateReducer that looks like this (might be familiar for isoword developers ;) ):

let appDelegateReducer = Reducer<UserSettings, AppDelegateAction, AppDelegateEnvironment> { state, action, environment in
  switch action {

  case .didFinishLaunching:
    return Effect(value: AppDelegateAction.userSettingsLoaded)

  case .userSettingsLoaded:
    return environment.setUserInterfaceStyle(state.colorScheme.userInterfaceStyle)
    // NB: This is necessary because UIKit needs at least one tick of the run loop before we
    //     can set the user interface style.
      .subscribe(on: environment.mainQueue)
      .fireAndForget()
  }
}

Simple, right? So I'm wondering why when I run a test to make sure that the setUserInterfaceStyle function is called, it seems to be executed right when store.send is called and not after store.receive is called:

  func testAppDelegateFeature() {
    let mainQueue = DispatchQueue.test
    var didSetUserInterfaceStyle = false
    let environment = update(AppDelegateEnvironment.failing) {
      $0.mainQueue = mainQueue.eraseToAnyScheduler()
      $0.setUserInterfaceStyle = { _ in didSetUserInterfaceStyle = true; return .none }
    }
    let store = TestStore(
      initialState: .init(),
      reducer: appDelegateReducer,
      environment: environment
    )

    store.send(.didFinishLaunching)
    XCTAssertFalse(didSetUserInterfaceStyle)   // Test fails (didSetuserInterfaceStyle unexpectedly true)
    store.receive(.userSettingsLoaded)
    mainQueue.advance()                        // Since .userSettingsLoaded effect is subscribed to mainQueue
    XCTAssertTrue(didSetUserInterfaceStyle).   // Where I expect didSetuserInterfaceStyle to actually be true
}

Perhaps I'm misunderstanding something about how effects are run? Or maybe the compiler is over-optimizing something?

[acosmicflamingo]

Whoops! For anyone who encounters the same problem, it turns out it's because I was not setting the 'setUserInterfaceStyle' closure correctly in my test:

let environment = update(AppDelegateEnvironment.failing) {
  $0.mainQueue = mainQueue.eraseToAnyScheduler()

  // Wrong
  $0.setUserInterfaceStyle = { _ in didSetUserInterfaceStyle = true; return .none }

  // Right
  $0.setUserInterfaceStyle = { _ in
    .fireAndForget {
      didSetUserInterfaceStyle = true
    }
  }
}

I also noticed that they do this test in SettingsFeature (which is how I found the answer). I suppose by adding it as a simple closure, it may have the same kind of behavior as Combine's Future objects, where it just fires immediately. Adding the functionality within the .fireAndForget could be equivalent to enclosing a Future object within a Defer closure. Since I had watched so many PointFree videos, my memory is now a blur and I can't vividly recall what the explanation was, but I think I remember Brandon saying if you aren't careful with how you set your enclosures, they may be called much sooner than you 'd expect.