swift-composable-architecture-mirror/Examples/CaseStudies/SwiftUICaseStudies/02-SharedState-UserDefaults.swift

import ComposableArchitecture
import SwiftUI

private let readMe = """
  This screen demonstrates how multiple independent screens can share state in the Composable \
  Architecture through user defaults (i.e. "app storage"). Each tab manages its own state, and \
  could be in separate modules, but changes in one tab are immediately reflected in the other, and \
  all changes are persisted to use defaults.

  This tab has its own state, consisting of a count value that can be incremented and decremented, \
  as well as an alert value that is set when asking if the current count is prime.
  """

@Reducer
struct SharedStateUserDefaults {
  enum Tab { case counter, profile }

  @ObservableState
  struct State: Equatable {
    var currentTab = Tab.counter
    var counter = CounterTab.State()
    var profile = ProfileTab.State()
  }

  enum Action {
    case counter(CounterTab.Action)
    case profile(ProfileTab.Action)
    case selectTab(Tab)
  }

  var body: some Reducer<State, Action> {
    Scope(state: \.counter, action: \.counter) {
      CounterTab()
    }

    Scope(state: \.profile, action: \.profile) {
      ProfileTab()
    }

    Reduce { state, action in
      switch action {
      case .counter, .profile:
        return .none
      case let .selectTab(tab):
        state.currentTab = tab
        return .none
      }
    }
  }
}

struct SharedStateUserDefaultsView: View {
  @Bindable var store: StoreOf<SharedStateUserDefaults>

  var body: some View {
    TabView(selection: $store.currentTab.sending(\.selectTab)) {
      CounterTabView(
        store: store.scope(state: \.counter, action: \.counter)
      )
      .tag(SharedStateUserDefaults.Tab.counter)
      .tabItem { Text("Counter") }

      ProfileTabView(
        store: store.scope(state: \.profile, action: \.profile)
      )
      .tag(SharedStateUserDefaults.Tab.profile)
      .tabItem { Text("Profile") }
    }
    .navigationTitle("Shared State Demo")
  }
}

extension SharedStateUserDefaults {
  @Reducer
  struct CounterTab {
    @ObservableState
    struct State: Equatable {
      @Presents var alert: AlertState<Action.Alert>?
      @Shared(.count) var count = 0
    }

    enum Action {
      case alert(PresentationAction<Alert>)
      case decrementButtonTapped
      case incrementButtonTapped
      case isPrimeButtonTapped

      enum Alert: Equatable {}
    }

    var body: some Reducer<State, Action> {
      Reduce { state, action in
        switch action {
        case .alert:
          return .none

        case .decrementButtonTapped:
          state.$count.withLock { $0 -= 1 }
          return .none

        case .incrementButtonTapped:
          state.$count.withLock { $0 += 1 }
          return .none

        case .isPrimeButtonTapped:
          state.alert = AlertState {
            TextState(
              isPrime(state.count)
                ? "👍 The number \(state.count) is prime!"
                : "👎 The number \(state.count) is not prime :("
            )
          }
          return .none
        }
      }
      .ifLet(\.$alert, action: \.alert)
    }
  }

  @Reducer
  struct ProfileTab {
    @ObservableState
    struct State: Equatable {
      @Shared(.count) var count = 0
    }

    enum Action {
      case resetStatsButtonTapped
    }

    var body: some Reducer<State, Action> {
      Reduce { state, action in
        switch action {
        case .resetStatsButtonTapped:
          state.$count.withLock { $0 = 0 }
          return .none
        }
      }
    }
  }
}

private struct CounterTabView: View {
  @Bindable var store: StoreOf<SharedStateUserDefaults.CounterTab>

  var body: some View {
    Form {
      Text(template: readMe, .caption)

      VStack(spacing: 16) {
        HStack {
          Button {
            store.send(.decrementButtonTapped)
          } label: {
            Image(systemName: "minus")
          }

          Text("\(store.count)")
            .monospacedDigit()

          Button {
            store.send(.incrementButtonTapped)
          } label: {
            Image(systemName: "plus")
          }
        }

        Button("Is this prime?") { store.send(.isPrimeButtonTapped) }
      }
    }
    .buttonStyle(.borderless)
    .alert($store.scope(state: \.alert, action: \.alert))
  }
}

private struct ProfileTabView: View {
  let store: StoreOf<SharedStateUserDefaults.ProfileTab>

  var body: some View {
    Form {
      Text(
        template: """
          This tab shows the count from the previous tab, and it is capable of resetting the count \
          back to 0.

          This shows that it is possible for each screen to model its state in the way that makes \
          the most sense for it, while still allowing the state and mutations to be shared \
          across independent screens.
          """,
        .caption
      )

      VStack(spacing: 16) {
        Text("Current count: \(store.count)")
        Button("Reset") { store.send(.resetStatsButtonTapped) }
      }
    }
    .buttonStyle(.borderless)
  }
}

extension SharedKey where Self == AppStorageKey<Int> {
  fileprivate static var count: Self {
    appStorage("sharedStateDemoCount")
  }
}

#Preview {
  SharedStateUserDefaultsView(
    store: Store(initialState: SharedStateUserDefaults.State()) { SharedStateUserDefaults() }
  )
}

/// Checks if a number is prime or not.
private func isPrime(_ p: Int) -> Bool {
  if p <= 1 { return false }
  if p <= 3 { return true }
  for i in 2...Int(sqrtf(Float(p))) {
    if p % i == 0 { return false }
  }
  return true
}