CasePathable crashes on get

[Otbivnoe]

I have a list with trips where each row has own reducer and state. For some reasons this line ForEach(store.scope(state: \.[case: \.trips], action: \.trips)) causes the crash:

Library version: 1.11.2

My simplified code of the list:

struct TripsContent: Reducer {

    @ObservableState
    @CasePathable
    @dynamicMemberLookup
    enum State: Equatable {
        // other cases
        case trips(IdentifiedArrayOf<TripListItem.State>)
    }

    @CasePathable
    @dynamicMemberLookup
    enum Action {
        case trips(IdentifiedActionOf<TripListItem>)
    }

    var body: some Reducer<State, Action> {
        Reduce { state, action in
            return .none
        }
        .forEach(\.[case: \.trips], action: \.trips) {
            TripListItem()
        }
    }
}

struct TripsContentView: View {

    let store: StoreOf<TripsContent>

    var body: some View {
        switch store.state {
        case .trips:
            tripsList()
        }
    }

    private func tripsList() -> some View {
        ForEach(store.scope(state: \.[case: \.trips], action: \.trips)) { store in
            TripListItemView(store: store)
        }
    }
}

and a cell:

struct TripListItem: Reducer {
    
    @ObservableState
    struct State: Identifiable, Equatable {
        var id: String { name }
        let name: String
    }

    @CasePathable
    enum Action {
        case onTap
    }

    var body: some Reducer<State, Action> {
        EmptyReducer()
    }
}

struct TripListItemView: View {

    let store: StoreOf<TripListItem>

    var body: some View {
        Button {
            store.send(.onTap)
        } label: {
            Text(store.name)
                .font(.system(size: 26))
        }
    }
}

What am I doing wrong? 🤔

[mbrandonw]

Hi @Otbivnoe, what version of Xcode/Swift are you using? Recent versions have improved their ability to do availability checking, and so should have prevented you from writing code that would get you into that unavailable getter. I would try Xcode 16 beta 3 to see if it now rejects this code.

[Otbivnoe]

Hey! I'm using Xcode 15.2 (Swift 5.9.2).

[Otbivnoe]

Installed the Version 15.4 and still no any helpful warnings... Can you help with the code change to get rid of this crash?

[ChloeMayhewELT]

Hi @Otbivnoe,

Rather than \.[case: \.trips], you can just use \.trips.

[mbrandonw]

Hey! I'm using Xcode 15.2 (Swift 5.9.2).

Ok, that would explain it. This code is not valid in Swift 6 becomes they fixed the availability checking:

What you are attempting here is quite complex. An enum of identified arrays powering multiple ForEachs. For some complex, nested situations, we highly recommend making a property TCA domain for each layer.

So, you could have a TripsList feature to encapsulate the behavior of a list of trips, and it would be the one that uses the .forEach operator:

@Reducer
struct TripsList {
  @ObservableState
  struct State {
    var trips: IdentifiedArrayOf<TripListItem.State>
  }
  enum Action {
    case trips(IdentifiedActionOf<TripListItem>)
  }
  var body: some ReducerOf<Self> {
    Reduce<State, Action> { state, action in
        .none
    }
    .forEach(\.trips, action: \.trips) {
      TripListItem()
    }
  }
}

And then TripsContent would hold onto this domain in its enums, rather than the identified domain directly, and it would use the ifCaseLet operator to specify which reducer runs on which case:

struct TripsContent: Reducer {
  @ObservableState
  @CasePathable
  @dynamicMemberLookup
  enum State {
    // other cases
    case trips(TripsList.State)
  }

  @CasePathable
  enum Action {
    case trips(TripsList.Action)
  }

  var body: some Reducer<State, Action> {
    Reduce { state, action in
      return .none
    }
    .ifCaseLet(\.trips, action: \.trips) {
      TripsList()
    }
  }
}

And then the view would need to do something similar. It uses if let to unwrap the store in a particular case of the enum, and then ForEach to derive a store for each element in the collection:

@ViewBuilder
private func tripsList() -> some View {
  if let store = store.scope(state: \.trips, action: \.trips) {
    ForEach(store.scope(state: \.trips, action: \.trips)) { store in
      TripListItemView(store: store)
    }
  }
}

In general the tools work a lot better together when you just have a full feature at each level instead of taking shortcuts.

[Otbivnoe]

@mbrandonw Thanks for the detailed explanation!