spin-example-combine.swift

// the State of the feedback loop
struct Levels {
    let left: Int
    let right: Int
}

// the transition requests of the feedback loop
enum Event {
    case increaseLeft
    case decreaseLeft 
    case increaseRight
    case decreaseRight
}

// the side effects of the feedback loop
func leftEffect(inputLevels: Levels) -> AnyPublisher<Event, Never> {
    // this is the stop condition to our Spin
    guard inputLevels.left != inputLevels.right else { return Empty().eraseToAnyPublisher() }

    // this is the regulation for the left level
    if inputLevels.left < inputLevels.right {
        return Just(.increaseLeft).eraseToAnyPublisher()
    }  else {
        return Just(.decreaseLeft).eraseToAnyPublisher()
    }
}

func rightEffect(inputLevels: Levels) -> AnyPublisher<Event, Never> {
    // this is the stop condition to our Spin
    guard inputLevels.left != inputLevels.right else { return Empty().eraseToAnyPublisher() }

    // this is the regulation for the right level
    if inputLevels.right < inputLevels.left {
        return Just(.increaseRight).eraseToAnyPublisher()
    }  else {
        return Just(.decreaseRight).eraseToAnyPublisher()
    }
}

// the reducer of the feedback loop
func levelsReducer(currentLevels: Levels, event: Event) -> Levels {

	guard currentLevels.left != currentLevels.right else { return currentLevels }

	switch event {
	    case .decreaseLeft:
	        return Levels(left: currentLevels.left-1, right: currentLevels.right)
	    case .increaseLeft:
	        return Levels(left: currentLevels.left+1, right: currentLevels.right)
	    case .decreaseRight:
	        return Levels(left: currentLevels.left, right: currentLevels.right-1)
	    case .increaseRight:
	        return Levels(left: currentLevels.left, right: currentLevels.right+1)
	}
}

// The builder way to build the Spin
let levelsSpin = Spinner
	.initialState(Levels(left: 10, right: 20))
	.feedback(Feedback(effect: leftEffect))
	.feedback(Feedback(effect: rightEffect))
	.reducer(Reducer(levelsReducer))

// The declarative way to build the Spin
let levelsSpin = CombineSpin(initialState: Levels(left: 10, right: 20),
                             reducer: Reducer(levelsReducer)) {
    Feedback(effect: leftEffect)
    Feedback(effect: rightEffect)
}

// to make a reactive stream from the Spin
AnyPublisher.stream(from: levelsSpin)