ContentView.swift

import SwiftUI
import ComposableArchitecture

struct AppReducer: Reducer {
    
    struct State: Equatable {
        @PresentationState var alert: AlertState<Action.Alert>?
    }
    
    enum Action {
        case alertButtonTapped
        case alert(PresentationAction<Alert>)
        enum Alert: Equatable {
            case confirmButtonTapped
        }
    }
    
    var body: some Reducer<State, Action> {
        Reduce { state, action in
            switch action {
            case .alertButtonTapped:
                state.alert = AlertState {
                    TextState("Alert")
                } actions: {
                    ButtonState(action: .send(.confirmButtonTapped)) {
                        TextState("OK")
                    }
                    ButtonState(role: .cancel) {
                        TextState("やめとく")
                    }
                }
                return .none
                
            case .alert:
                return .none
            }
        }
        .ifLet(\.alert, action: /Action.alert)
    }
}

struct ContentView: View {
    let store: StoreOf<AppReducer> = .init(initialState: AppReducer.State(), reducer: { AppReducer() })
    
    var body: some View {
        WithViewStore(store, observe: { $0 }) {viewStore in
            VStack {
                Button("Alert") {
                    viewStore.send(.alertButtonTapped)
                }
            }
            .alert(store: store.scope(state: \.$alert, action: { .alert($0) }))
        }
    }