htc1_block10.swift

protocol Observer {
    func stateWasChanged(with newState: State)
}

struct State {
    var value: Int?

    static func reduce(state: State, event: Event) -> State {
        var state = state
        switch event {
        case .changeValue(let newValue):
            state.value = newValue
        }
        return state
    }
}

enum Event {
    case changeValue(newValue: Int?)
}

class Store {
    var state: State = State() {
        didSet {
            observers.forEach { observer in
                observer.stateWasChanged(with: state)
            }
        }
    }

    var observers: [Observer] = []

    func accept(event: Event) {
        state = State.reduce(state: state, event: event)
    }
}