The Composable Architecture

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The Composable Architecture https://github.com/pointfreeco/swift-composable-architecture

一、State

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struct CounterState: Equatable {
  var count = 0
}

二、Action

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enum CounterAction: Equatable {
  case decrementButtonTapped
  case incrementButtonTapped
}

三、Environment

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struct CounterEnvironment {}

四、Reducer

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  • 处理事件以及状态的变化
  • 返回Effect

1. combine

把多个reducer组合成一个

  • reducers的顺序会影响结果

2. pullback

利用local state, action and environment构造全局的reducer

  • 参数：

    • toLocalState: 可写的keyPath，能够从GlobalState中取出LocalState
    • toLocalAction: casePath，能将LocalActioin extract/embed 到GlobalAction.
    • toLocalEnvironment: 函数，能将 GlobalEnvironment转为 LocalEnvironment.

  • 使用：

    • // Global domain that holds a local domain:
struct AppState { var settings: SettingsState, /* rest of state */ }
enum AppAction { case settings(SettingsAction), /* other actions */ }
struct AppEnvironment { var settings: SettingsEnvironment, /* rest of dependencies */ }

// A reducer that works on the local domain:
let settingsReducer = Reducer<SettingsState, SettingsAction, SettingsEnvironment> { ... }

// Pullback the settings reducer so that it works on all of the app domain:
let appReducer: Reducer<AppState, AppAction, AppEnvironment> = .combine(
    settingsReducer.pullback(
        state: \.settings,
        action: /AppAction.settings,
        environment: { $0.settings }
    ),

    /* other reducers */
)
<!--4-->

3. optional

将reducer中 state映射为State?

五、Store

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Store<State, Action>
  • 存储了state和action

1. WithViewStore

可以接受Store，并将其转换为@ObservedObject ViewStore<State, Action>

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var body: some View {
    WithViewStore(self.store) { viewStore in
        HStack {
            Button("−") { viewStore.send(.decrementButtonTapped) }
            Text("\(viewStore.count)")
                .font(Font.body.monospacedDigit())
            Button("+") { viewStore.send(.incrementButtonTapped) }
        }
    }
}

1.1 ViewStore

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@dynamicMemberLookup
public final class ViewStore<State, Action>: ObservableObject

viewStore可以访问到state的属性

原理：将ViewStore声明为dynamicMemberLookup，并实现他的subscript<LocalState>(dynamicMember keyPath: KeyPath<State, LocalState>) -> LocalState方法，即可访问到ViewStore中未定义的成员（若访问到成员不存在，就会返回subscript方法的执行结果）

send

viewStore可以发送一个action到store，store中的reducer会接受并处理

ViewStore不是线程安全的，因此send方法一定要在主线程调用

binding

  • 初始化时传入get函数（获取state）和set函数（发送action）
  • 有关联值的枚举，如果只写关联值以外的部分，可以当做函数来使用：比如 viewStore.binding(get: { $0.text }, send: BindingBasicsAction.textChange) 中BindingBasicsAction.textChange 就是一个输入为 String， 输出为 BindingBasicsAction 实例的函数

2. scope

通过全局的Store获取local Store中的state或者action

  • 参数
    • toLocalState：修改local state
    • fromLocalAction：接收local action
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// Application state made from local states.
struct AppState { var login: LoginState, ... }
struct AppAction { case login(LoginAction), ... }
// A store that runs the entire application.
let store = Store(initialState: AppState(), reducer: appReducer, environment: ())
// Construct a login view by scoping the store to one that works with only login domain.
let loginView = LoginView(
    store: store.scope(
        state: { $0.login },	//取出AppState中的login
        action: { AppAction.login($0) }	//用loginAction生成AppAction
    )
)

3. IfLetStore

判断当前store中的State是不是可选的

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