NavigationView {
    // 第一个 view 为左侧 sidebar
    List {
        // 点击 link 会 push 到 destination；如果是分屏下，destination 会显示在 detailView。
        NavigationLink(destination: CityView(city: city)) {
            Text(city.description)
        }
    }
    // 第二个 view 为右侧 detail view
    Image(systemName: "cloud.sun").font(.largeTitle)
}

class Store: ObservableObject { // 1
    @Published var appState = AppState() // 1
    
    // 2
    static func reduce(state: AppState, action: AppAction) -> (AppState, AppCommand?) {
        var appState = state
        var appCommand: AppCommand?
        
        switch action {
            // case ...   
        }
        
        // 3, 4
        return (appState, appCommand)
    }
}

struct FindAppCommand: AppCommand {

    func execute(in store: Store) {
        // await ...
        store.dispatch(action) // 5
    }
}

struct FindRequest: AppRequest {
    
    var publisher: AnyPublisher {
        URLSession.shared
            .dataTaskPublisher(for: url)
            .map { $0.data }
            .decode(type: Find.self, decoder: JSONDecoder())
            .mapError { AppError.networkingFailed($0) }
            .eraseToAnyPublisher()
    }
}

@Environment(\.horizontalSizeClass) var horizontalSizeClass: UserInterfaceSizeClass?

var body: some View {
    if horizontalSizeClass == .compact {
        VStack {
            // ...
        }
    } else {
        HStack {
            // ...
        }
    }
}

extension AppRequest {
    var appid: String { "" }
}