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Alex Rupérez

Published: 09 Jun 17:25
I starred inamiy/Harvest at GitHub.
🌾 Harvest: Apple's Combine.framework + State Machine, inspired by Redux and Elm.
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🌾 Harvest

Swift 5.1 Build Status

Apple's Combine.framework (from iOS 13) + State Machine, inspired by Elm.

This is a sister library of the following projects:


Xcode 11 (Swift 5.1 / macOS 10.15, iOS 13, ...)


To make a state transition diagram like above with additional effects, follow these steps:

1. Define States and Inputs

// 1. Define `State`s and `Input`s.
enum State {
    case loggedOut, loggingIn, loggedIn, loggingOut

enum Input {
    case login, loginOK, logout, logoutOK
    case forceLogout

2. Define EffectQueue

enum EffectQueue: EffectQueueProtocol {
    case `default`
    case request

    var flattenStrategy: FlattenStrategy {
        switch self {
        case .default: return .merge
        case .request: return .latest

    static var defaultEffectQueue: EffectQueue {

EffectQueue allows additional side-effects (Effect, a wrapper of Publisher) to be scheduled with a specific FlattenStrategy, such as flatMap (.merge), flatMapLatest (.latest), etc. In above case, we want to automatically cancel previous network requests if occurred multiple times, so we also prepare case request queue with .latest strategy.

3. Create EffectMapping (Effect-wise reducer)

// NOTE: `EffectID` is useful for manual effect cancellation, but not used in this example.
typealias EffectID = Never

typealias Harvester = Harvest.Harvester<Input, State>
typealias EffectMapping = Harvester.EffectMapping<EffectQueue, EffectID>
typealias Effect = Harvester.Effect<Input, EffectQueue, EffectID>

// Additional effects while state-transitioning.
let loginOKPublisher = /* show UI, setup DB, request APIs, ..., and send `Input.loginOK` */
let logoutOKPublisher = /* show UI, clear cache, cancel APIs, ..., and send `Input.logoutOK` */
let forceLogoutOKPublisher = /* do something more special, ..., and send `Input.logoutOK` */

let canForceLogout: (State) -> Bool = [.loggingIn, .loggedIn].contains

let mappings: [EffectMapping] = [

  /*  Input   |   fromState => toState     |      Effect       */
  /* ----------------------------------------------------------*/
    .login    | .loggedOut  => .loggingIn  | Effect(loginOKPublisher, queue: .request),
    .loginOK  | .loggingIn  => .loggedIn   | .empty,
    .logout   | .loggedIn   => .loggingOut | Effect(logoutOKPublisher, queue: .request),
    .logoutOK | .loggingOut => .loggedOut  | .empty,

    .forceLogout | canForceLogout => .loggingOut | Effect(forceLogoutOKPublisher, queue: .request)

EffectMapping is Redux's Reducer or Elm's Update pure function that also returns Effect during the state-transition. Note that queue: .request is specified so that those effects will be handled in the same queue with .latest strategy. Instead of writing it as a plain function with pattern-matching, you can also write in a fancy markdown-table-like syntax as shown above.

4. Setup Harvester (state machine)

// Prepare input pipe for sending `Input` to `Harvester`.
let inputs = PassthroughSubject<Input, Never>()

var cancellables: [AnyCancellable] = []

// Setup state machine.
let harvester = Harvester(
    state: .loggedOut,
    input: inputs,
    mapping: .reduce(.first, mappings),  // combine mappings using `reduce` helper
    scheduler: DispatchQueue.main

// Observe state-transition replies (`.success` or `.failure`).
harvester.replies.sink { reply in
    print("received reply = \(reply)")
}.store(in: &cancellables)

// Observe current state changes.
harvester.state.sink { state in
    print("current state = \(state)")
}.store(in: &cancellables)

NOTE: func reduce is declared to combine multiple mappings into one.

5. And let's test!

let send = inputs.send

expect(harvester.state) == .loggedIn    // already logged in
expect(harvester.state) == .loggingOut  // logging out...
// `logoutOKPublisher` will automatically send `Input.logoutOK` later
// and transit to `State.loggedOut`.

expect(harvester.state) == .loggedOut   // already logged out
expect(harvester.state) == .loggingIn   // logging in...
// `loginOKPublisher` will automatically send `Input.loginOK` later
// and transit to `State.loggedIn`.

// 👨🏽 < But wait, there's more!
// Let's send `Input.forceLogout` immediately after `State.loggingIn`.

send(Input.forceLogout)                       // 💥💣💥
expect(harvester.state) == .loggingOut  // logging out...
// `forceLogoutOKublisher` will automatically send `Input.logoutOK` later
// and transit to `State.loggedOut`.

Please notice how state-transitions, effect calls and cancellation are nicely performed. If your cancellation strategy is more complex than just using FlattenStrategy.latest, you can also use Effect.cancel to manually stop specific EffectID.

Note that any sizes of State and Input will work using Harvester, from single state (like above example) to covering whole app's states (like React.js + Redux architecture).

Using Feedback effect model as alternative

Instead of using EffectMapping with fine-grained EffectQueue model, Harvest also supports Feedback system as described in the following libraries:

See inamiy/ReactiveAutomaton#12 for more discussion.

Composable Architecture with SwiftUI

Pull Request #8 introduced HarvestStore and HarvestOptics frameworks for Composable Architecture, especially focused on SwiftUI.

  • HarvestStore: 2-way bindable Store optimized for SwiftUI
  • HarvestOptics: Input & state lifting helpers using FunOptics

See Harvest-SwiftUI-Gallery for the examples.

See also Babylonpartners/ios-playbook#171 for further related discussion.

  • TODO: Write documentation


  1. ReactiveAutomaton (using ReactiveSwift)
  2. RxAutomaton (using RxSwift)
  3. iOSDC 2016 (Tokyo, in Japanese) (2016/08/20)
  4. iOSConf SG (Singapore, in English) (2016/10/20-21)