How to integrate _AppStorage with TCA?

[tkunst891]

The @Dependency.AppStorage implementation seems to me very helpful in combination with the UserDefaults dependency - thanks for that! But it seems not obvious to me how to use it in a TCA application.

I tried to include a @Dependency.AppStorage value in a reducer's State, but it does not compile, because it is not Equatable:

struct State: Equatable {
  @Dependency.AppStorage("mySetting") var mySetting = 100
  // ... other state

This will give a compile error. But it does not seem to make sense to make AppStorage conform to Equatable - I do not see how to properly define the == operator. There will never be two instances with differing values.

Also I do not understand how I am expected to update an AppStorage value in a reducer. TCA discourages executing sideeffects directly in the reduce function, and setting a UserDefault is a sideeffect. So you would need to use an effect (.fireAndForget), but such an effect can not change the state.

As a third issue it is not trivial to observe a changing UserDefault and react to changes in a reducer.

Anybody have suggestions or has already successfully used _AppStorageDependency in a TCA application?

[tgrapperon]

Hey @tkunst891! In my mind, you would use it in the reducer and use its async sequence to update a local value. The sequence always emit on "connection", so you can use it to set your initial value. So you'd produce early a Effect like:

.run { send in
  for await p in self.$pref {
    await send(.pref(p)) // An action that sets `pref` in your state.
  }
}

If you only want the value without observing it, then I agree that the side effect is not optimal. If you want to avoid it, you can also fish the value in a .task and emit an action that sets it.

DependencyAdditions was written with vanilla SwiftUI and server-side Swift in mind. It is likely that you can write a special reducer that would provide a binding to the value. This seems interesting. I'll think about it.

[tgrapperon]

That being said, this feature is experimental, so suggestions are welcome!

[tkunst891]

Thanks for answering. I currently can't think of an improvement to your library which would not involve a dependency on swift-composable-architecture (which your library should of course not have, it should remain to be usable without TCA). For my own use, I have created a small helper package to let me work with @Dependency.AppStorage: https://github.com/tkunst891/dependency-appstorage-tca.git.