[TCA] enum State をstruct Stateに持たせる場合のpullback方法

問題

The Composable ArchitectureのSwitchStoreでの画面出し分けをする際、enumにStateを持たせる場合、以下のようにpullbackします。

AppCore.swift

enum AppState {
    case firstScreen(FirstScreenState)
    case secondScreen(SecondScreenState)
}

enum AppAction {
    case firstScreenAction(FirstScreenAction)
    case secondScreenAction(SecondScreenAction)
}

struct AppEnvironment { }

let appReducer = Reducer<AppState, AppAction, AppEnvironment>.combine(
    firstScreenReducer
        .pullback(
            state: /AppState.firstScreen,
            action: /AppAction.firstScreenAction,
            environment: { _ in
                .init()
            }
        ),
    
    secondScreenReducer
        .pullback(
            state: /AppState.secondScreen,
            action: /AppAction.secondScreenAction,
            environment: { _ in
                .init()
            }
        ),
    
    Reducer { state, action, _ in
        switch action {
            //...
        }
    }
)

AppView.swift

struct AppView: View {
    let store: Store<AppState, AppAction>
    
    var body: some View {
        SwitchStore(store) {
            CaseLet(state: /AppState.firstScreen, action: AppAction.firstScreenAction) {
                FirstScreen(store: $0)
            }
            
            CaseLet(state: /AppState.secondScreen, action: AppAction.secondScreenAction) {
                SecondScreen(store: $0)
            }
        }
    }
}

しかし以下のように、Stateを持たせたenumをstructのStateに持たせたい場合には、当然ながら同様の方法ではpullbackすることができません。

AppCore.swift

enum AppScreen {
    case firstScreen(FirstScreenState)
    case secondScreen(SecondScreenState)
}

struct AppState {
+    var appScreen: AppScreen = .firstScreen(.init())
}

// 省略...

2重pullbackで解決

そこで、以下のように2重でpullbackすることで解決します。

AppCore.swift

// 省略...

let appReducer = Reducer<AppState, AppAction, AppEnvironment>.combine(
    firstScreenReducer
        .pullback(
-           state: /AppState.firstScreen,
+           state: /AppScreen.firstScreen,
            action: /AppAction.firstScreenAction,
            environment: { _ in
                .init()
            }
        )
+       .pullback(
+           state: \.appScreen,
+           action: /AppAction.self,
+           environment: { $0 }
+       ),
    
    secondScreenReducer
        .pullback(
-           state: /AppState.secondScreen,
+           state: /AppScreen.secondScreen,
            action: /AppAction.secondScreenAction,
            environment: { _ in
                .init()
            }
        )
+       .pullback(
+           state: \.appScreen,
+           action: /AppAction.self,
+           environment: { $0 }
+       ),
    
    // 省略...
)

AppView.swift

struct AppView: View {
    let store: Store<AppState, AppAction>
    
    var body: some View {
-       SwitchStore(store) {
+       SwitchStore(store.scope(state: \.appScreen)) {
-           CaseLet(state: /AppState.firstScreen, action: AppAction.firstScreenAction) {
+           CaseLet(state: /AppScreen.firstScreen, action: AppAction.firstScreenAction) {
                FirstScreen(store: $0)
            }
            
-           CaseLet(state: /AppState.secondScreen, action: AppAction.secondScreenAction) {
+           CaseLet(state: /AppScreen.secondScreen, action: AppAction.secondScreenAction) {
                SecondScreen(store: $0)
            }
        }
    }
}

参考