Swift async/await & Actor-powered effectful state-management framework

by iOS Example

? Actomaton

?‍? Actor + ? Automaton = ? Actomaton

Actomaton is Swift async/await & Actor-powered effectful state-management framework inspired by Elm and swift-composable-architecture.

This repository consists of 3 frameworks:

  1. Actomaton: Actor-based effect-handling state-machine at its core. Linux ready.
  2. ActomatonStore: SwiftUI & Combine support
  3. ActomatonDebugging: Helper module to print Action and State (with diffing) per Reducer call.

These frameworks depend on swift-case-paths as Functional Prism library, which is a powerful tool to construct an App-level Mega-Reducer from each screen’s Reducers.

This framework is a successor of the following projects:

Demo App

1. Actomaton (Core)

Example 1-1. Simple Counter

struct State: Sendable {
    var count: Int = 0
}

enum Action: Sendable {
    case increment
    case decrement
}

typealias Environment = Void

let reducer: Reducer<Action, State, Environment>
reducer = Reducer { action, state, environment in
    switch action {
    case .increment:
        state.count += 1
        return Effect.empty
    case .decrement:
        state.count -= 1
        return Effect.empty
    }
}

let actomaton = Actomaton<Action, State>(
    state: State(),
    reducer: reducer
)

@main
enum Main {
    static func main() async {
        assertEqual(await actomaton.state.count, 0)

        await actomaton.send(.increment)
        assertEqual(await actomaton.state.count, 1)

        await actomaton.send(.increment)
        assertEqual(await actomaton.state.count, 2)

        await actomaton.send(.decrement)
        assertEqual(await actomaton.state.count, 1)

        await actomaton.send(.decrement)
        assertEqual(await actomaton.state.count, 0)
    }
}

If you want to do some logging (side-effect), add Effect in Reducer as follows:

reducer = Reducer { action, state, environment in
    switch action {
    case .increment:
        state.count += 1
        return Effect.fireAndForget {
            print("increment")
        }
    case .decrement:
        state.count -= 1
        return Effect.fireAndForget {
            print("decrement and sleep...")
            try await Task.sleep(...) // NOTE: We can use `await`!
            print("I'm awake!")
        }
    }
}

NOTE: There are 5 ways of creating Effect in Actomaton:

  1. No side-effects, but next action only
    • Effect.nextAction
  2. Single async without next action
    • Effect.fireAndForget(id:run:)
  3. Single async with next action
    • Effect.init(id:run:)
  4. Multiple asyncs (i.e. AsyncSequence) with next actions
    • Effect.init(id:sequence:)
  5. Manual cancellation
    • Effect.cancel(id:) / .cancel(ids:)

Example 1-2. Login-Logout (and ForceLogout)

enum State: Sendable {
    case loggedOut, loggingIn, loggedIn, loggingOut
}

enum Action: Sendable {
    case login, loginOK, logout, logoutOK
    case forceLogout
}

// NOTE:
// Use same `EffectID` so that if previous effect is still running,
// next effect with same `EffectID` will automatically cancel the previous one.
//
// Note that `EffectID` is also useful for manual cancellation via `Effect.cancel`.
struct LoginFlowEffectID: EffectIDProtocol {}

struct Environment: Sendable {
    let loginEffect: (userId: String) -> Effect<Action>
    let logoutEffect: Effect<Action>
}

let environment = Environment(
    loginEffect: { userId in
        Effect(id: LoginFlowEffectID()) {
            let loginRequest = ...
            let data = try? await URLSession.shared.data(for: loginRequest)
            if Task.isCancelled { return nil }
            ...
            return Action.loginOK // next action
        }
    },
    logoutEffect: {
        Effect(id: LoginFlowEffectID()) {
            let logoutRequest = ...
            let data = try? await URLSession.shared.data(for: logoutRequest)
            if Task.isCancelled { return nil }
            ...
            return Action.logoutOK // next action
        }
    }
)

let reducer = Reducer { action, state, environment in
    switch (action, state) {
    case (.login, .loggedOut):
        state = .loggingIn
        return environment.login(state.userId)

    case (.loginOK, .loggingIn):
        state = .loggedIn
        return .empty

    case (.logout, .loggedIn),
        (.forceLogout, .loggingIn),
        (.forceLogout, .loggedIn):
        state = .loggingOut
        return environment.logout()

    case (.logoutOK, .loggingOut):
        state = .loggedOut
        return .empty

    default:
        return Effect.fireAndForget {
            print("State transition failed...")
        }
    }
}

let actomaton = Actomaton<Action, State>(
    state: .loggedOut,
    reducer: reducer,
    environment: environment
)

@main
enum Main {
    static func test_login_logout() async {
        var t: Task<(), Error>?

        assertEqual(await actomaton.state, .loggedOut)

        t = await actomaton.send(.login)
        assertEqual(await actomaton.state, .loggingIn)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedIn)

        t = await actomaton.send(.logout)
        assertEqual(await actomaton.state, .loggingOut)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedOut)

        XCTAssertFalse(isLoginCancelled)
    }

    static func test_login_forceLogout() async throws {
        var t: Task<(), Error>?

        assertEqual(await actomaton.state, .loggedOut)

        await actomaton.send(.login)
        assertEqual(await actomaton.state, .loggingIn)

        // Wait for a while and interrupt by `forceLogout`.
        // Login's effect will be automatically cancelled because of same `EffectID.
        try await Task.sleep(/* 1 ms */)
        t = await actomaton.send(.forceLogout)

        assertEqual(await actomaton.state, .loggingOut)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedOut)

    }
}

Here we see the notions of EffectID, Environment, and let task: Task<(), Error> = actomaton.send(...)

  • EffectID is for both manual & automatic cancellation of previous running effects. In this example, forceLogout will cancel login‘s networking effect.
  • Environment is useful for injecting effects to be called inside Reducer so that they become replaceable. Environment is known as Dependency Injection (using Reader monad).
  • (Optional) Task<(), Error> returned from actomaton.send(action) is another fancy way of dealing with “all the effects triggered by action“. We can call await task.value to wait for all of them to be completed, or task.cancel() to cancel all. Note that Actomaton already manages such tasks for us internally, so we normally don’t need to handle them by ourselves (use this as a last resort!).

Example 1-3. Timer (using AsyncSequence) and EffectID cancellation

typealias State = Int

enum Action: Sendable {
    case start, tick, stop
}

struct TimerID: EffectIDProtocol {}

struct Environment {
    let timerEffect: Effect<Action>
}

let environment = Environment(
    timerEffect: { userId in
        Effect(id: TimerID(), sequence: {
            AsyncStream<()> { continuation in
                let task = Task {
                    while true {
                        try await Task.sleep(/* 1 sec */)
                        continuation.yield(())
                    }
                }

                continuation.onTermination = { @Sendable _ in
                    task.cancel()
                }
            }
        })
    }
)

let reducer = Reducer { action, state, environment in
    switch action {
    case .start:
        return environment.timerEffect
    case .tick:
        state += 1
        return .empty
    case .stop:
        return Effect.cancel(id: TimerID())
    }
}

let actomaton = Actomaton<Action, State>(
    state: 0,
    reducer: reducer,
    environment: environment
)

@main
enum Main {
    static func test_timer() async {
        assertEqual(await actomaton.state, 0)

        await actomaton.send(.start)

        assertEqual(await actomaton.state, 0)

        try await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 1)

        try await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 2)

        try await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 3)

        await actomaton.send(.stop)

        try await Task.sleep(/* long enough */)
        assertEqual(await actomaton.state, 3,
                    "Should not increment because timer is stopped.")
    }
}

In this example, Effect(id:sequence:) is used for timer effect, which yields Action.tick multiple times.

Example 1-4. EffectQueue

enum Action: Sendable {
    case fetch(id: String)
    case _didFetch(Data)
}

struct State: Sendable {} // no state

struct Environment: Sendable {
    let fetch: @Sendable (_ id: String) async throws -> Data
}

struct DelayedEffectQueue: EffectQueueProtocol {
    // First 3 effects will run concurrently, and other sent effects will be suspended.
    var effectQueuePolicy: EffectQueuePolicy {
        .runOldest(maxCount: 3, .suspendNew)
    }

    // Adds delay between effect start. (This is useful for throttling / deboucing)
    var effectQueueDelay: EffectQueueDelay {
        .random(0.1 ... 0.3)
    }
}

let reducer = Reducer<Action, State, Environment> { action, state, environment in
    switch action {
    case let .fetch(id):
        return Effect(queue: DelayedEffectQueue()) {
            let data = try await environment.fetch(id)
            return ._didFetch(data)
        }
    case let ._didFetch(data):
        // Do something with `data`.
        return .empty
    }
}

let actomaton = Actomaton<Action, State>(
    state: State(),
    reducer: reducer,
        environment: Environment(fetch: { /* ... */ })
)

await actomaton.send(.fetch(id: "item1"))
await actomaton.send(.fetch(id: "item2")) // min delay of 0.1
await actomaton.send(.fetch(id: "item3")) // min delay of 0.1 (after item2 actually starts)
await actomaton.send(.fetch(id: "item4")) // starts when item1 or 2 or 3 finishes

Above code uses a custom DelayedEffectQueue that conforms to EffectQueueProtocol with suspendable EffectQueuePolicy and delays between each effect by EffectQueueDelay.

See EffectQueuePolicy for how each policy takes different queueing strategy for effects.

/// `EffectQueueProtocol`'s buffering policy.
public enum EffectQueuePolicy: Hashable, Sendable
{
    /// Runs `maxCount` newest effects, cancelling old running effects.
    case runNewest(maxCount: Int)

    /// Runs `maxCount` old effects with either suspending or discarding new effects.
    case runOldest(maxCount: Int, OverflowPolicy)

    public enum OverflowPolicy: Sendable
    {
        /// Suspends new effects when `.runOldest` `maxCount` of old effects is reached until one of them is completed.
        case suspendNew

        /// Discards new effects when `.runOldest` `maxCount` of old effects is reached until one of them is completed.
        case discardNew
    }
}

For convenient EffectQueueProtocol protocol conformance, there are built-in sub-protocols:

/// A helper protocol where `effectQueuePolicy` is set to `.runNewest(maxCount: 1)`.
public protocol Newest1EffectQueueProtocol: EffectQueueProtocol {}

/// A helper protocol where `effectQueuePolicy` is set to `.runOldest(maxCount: 1, .discardNew)`.
public protocol Oldest1DiscardNewEffectQueueProtocol: EffectQueueProtocol {}

/// A helper protocol where `effectQueuePolicy` is set to `.runOldest(maxCount: 1, .suspendNew)`.
public protocol Oldest1SuspendNewEffectQueueProtocol: EffectQueueProtocol {}

so that we can write in one-liner: struct MyEffectQueue: Newest1EffectQueueProtocol {}

Example 1-5. Reducer composition

Actomaton-Gallery provides a good example of how Reducers can be combined together into one big Reducer using Reducer.combine.

In this example, swift-case-paths is used as a counterpart of WritableKeyPath, so if we use both, we can easily construct Mega-Reducer without a hussle.

(NOTE: CasePath is useful when dealing with enums, e.g. enum Action and enum Current in this example)

enum Root {} // just a namespace

extension Root {
    enum Action: Sendable {
        case changeCurrent(State.Current?)

        case counter(Counter.Action)
        case stopwatch(Stopwatch.Action)
        case stateDiagram(StateDiagram.Action)
        case todo(Todo.Action)
        case github(GitHub.Action)
    }

    struct State: Equatable, Sendable {
        var current: Current?

        // Current screen (NOTE: enum, so only 1 screen will appear)
        enum Current: Equatable {
            case counter(Counter.State)
            case stopwatch(Stopwatch.State)
            case stateDiagram(StateDiagram.State)
            case todo(Todo.State)
            case github(GitHub.State)
        }
    }

    // NOTE: `contramap` is also called `pullback` in swift-composable-architecture.
    static var reducer: Reducer<Action, State, Environment> {
        Reducer.combine(
            Counter.reducer
                .contramap(action: /Action.counter)
                .contramap(state: /State.Current.counter)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            Todo.reducer
                .contramap(action: /Action.todo)
                .contramap(state: /State.Current.todo)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            StateDiagram.reducer
                .contramap(action: /Action.stateDiagram)
                .contramap(state: /State.Current.stateDiagram)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            Stopwatch.reducer
                .contramap(action: /Action.stopwatch)
                .contramap(state: /State.Current.stopwatch)
                .contramap(state: \State.current)
                .contramap(environment: { $0.stopwatch }),

            GitHub.reducer
                .contramap(action: /Action.github)
                .contramap(state: /State.Current.github)
                .contramap(state: \State.current)
                .contramap(environment: { $0.github })
        )
    }
}

To learn more about CasePath, visit the official site and tutorials:

2. ActomatonStore (SwiftUI)

Store (from ActomatonStore.framework) provides a thin wrapper of Actomaton to work seamlessly in SwiftUI world. It will be stored as a @StateObject of the App’s view.

For example, from Actomaton-Gallery:

struct AppView: View {
    @StateObject
    private var store: Store<Root.Action, Root.State, Roote.Environment> = .init(
        state: Root.State(...),
        reducer: Root.reducer,
        environment: Root.Environment(...)
    )

    init() {}

    var body: some View {
        // IMPORTANT: Pass `Store.Proxy` to children.
        RootView(store: self.store.proxy)
    }
}

struct RootView: View {
    // IMPORTANT: `Store.Proxy`, not `Store` itself.
    private let store: Store<Root.Action, Root.State, Root.Environment>.Proxy

    init(store: Store<Root.Action, Root.State, Root.Environment>.Proxy) {
        self.store = store
    }

    var body: some View {
        return VStack {
            NavigationView {
                List {
                    navigationLink(example: allExamples[0])
                    navigationLink(example: allExamples[1])
                    navigationLink(example: allExamples[2])
                    navigationLink(example: allExamples[3])
                    navigationLink(example: allExamples[4])
                }
                .navigationBarTitle(Text("? Actomaton Gallery ?️"), displayMode: .large)
            }
        }
    }

    private func navigationLink(example: Example) -> some View {
        NavigationLink(
            destination: example.exampleView(store: self.store)
                .navigationBarTitle(
                    "\(example.exampleTitle)",
                    displayMode: .inline
                ),
            isActive: self.store.current
                .stateBinding(onChange: Root.Action.changeCurrent)
                .transform(
                    get: { $0?.example.exampleTitle == example.exampleTitle },
                    set: { _, isPresenting in
                        isPresenting ? example.exampleInitialState : nil
                    }
                )
        ) {
            HStack(alignment: .firstTextBaseline) {
                example.exampleIcon
                    .frame(width: 44)
                Text(example.exampleTitle)
            }
            .font(.body)
            .padding(5)
        }
    }
}

Here, the most important part is that the topmost container view of the app will hold Store as a single source of truth, and for the child views, Store.Proxy will be passed instead, so that we don’t duplicate multiple Stores but Binding and Store.proxy.send (sending message to topmost Store) functionalities are still available.

This (and overall) architecture is inherited from Harvest which has a different Store structure compared to swift-composable-architecture.

See following information for more details:

Store.Proxy.stateBinding

To interact with SwiftUI’s State-binding-based event handling, ActomatonStore provides Store.Proxy.stateBinding hook methods so that direct state changes can be converted into action dispatches (indirection) This indirection allows us to build more sophisticated UI architecture including better testing.

References

License

MIT

GitHub

View Github