Can’t instantiate ViewState from a TestStore when constructed using BindingViewStore

[rhysm94]

When writing tests, it’s possible to test ViewState if it is constructed directly from your feature state. e.g.

// Inside Feature
struct State: Equatable {
  var text = “”
  var isLoadingOneThing = false
  var isLoadingAnotherThing = false
}

// Inside View
struct ViewState: Equatable {
  var text: String
  var isLoading: Bool

  init(state: Feature.State) {
    self.text = text
    self.isLoading = state.isLoadingOneThing || state.isLoadingAnotherThing
  }
}

When writing tests, it’s easy to assert on your ViewState:

await testStore.send(.someAction) {
  $0.isLoadingOneThing = true
}

XCTAssertTrue(ViewState(testStore.state).isLoading))

However, when using the new Binding features of TCA along with ViewState, if the ViewState is initialised using a BindingViewStore, it’s no longer possible to write tests against the ViewState.
e.g.

// Inside Feature
struct State: Equatable {
  @BindingState var text = “”
  var isLoadingOneThing = false
  var isLoadingAnotherThing = false
}

// Inside View
struct ViewState: Equatable {
  @BindingViewState var text: String
  var isLoading: Bool

  init(store: BindingViewStore<Feature.State>) {
    self._text = store.$text
    self.isLoading = state.isLoadingOneThing || state.isLoadingAnotherThing
  }
}

I can’t find a way to derive or construct a BindingViewStore from the TestStore, so it’s no longer possible to write the above test code, as we can’t construct our ViewStore any longer.

I tried forking TCA and writing an extension on TestStore where Action: BindableAction to write something which exposed a BindingViewStore, but I couldn’t quite get the types to line up for the Action type, so I think I was doing something wrong there 😅

Is there any chance that this is something that could be added, or are there any recommendations on alternative approaches? Ideally I’d like to avoid adding computed properties to my state and then assign them in the ViewState, but I guess this approach could work as a fallback.

Thanks!

[stephencelis]

@rhysm94 Thanks for starting the discussion. This is definitely a gap that ought to be filled, but as we haven't spent a ton of time testing view state transformations we don't know if our API design would be as informed. Do you have any suggestions? How would you like to test your view state?

The simplest solution I could imagine is adding a bindingViewStore property, or maybe simply bindings, or some other name, to TestStore:

let store = TestStore(…)

await store(.someAction) {
  $0.isLoadingOneThing = true
}
XCTAssertTrue(ViewState(store.bindings).isLoading))

We've also talked about adding an XCTAssertDifference helper to Custom Dump, so you could maybe even someday get some exhaustivity (in a bit of a nested soup):

await XCTAssertDifference(store.bindings) {
  await store(.someAction) {
    $0.isLoadingOneThing = true
  }
} changes: {
  $0.isLoading = true
}

Any other ideas come to mind?

[rhysm94]

@stephencelis That bindingViewStore extension (or bindings!) is exactly what I envisioned and tried implementing 😀

I think I just got a bit stuck trying to work out how to transform the Action type.

I like the XCTAssertDifference as well, for exhaustivity!

[stephencelis]

@rhysm94 We have the basic tool sketched out here: #2394

Wanna take it for a spin and report back?

[rhysm94]

@stephencelis Hey, sorry for not responding sooner! I tested this out, and it works perfectly 😁
Exactly what I needed, thank you!