Counters.swift

struct Counter {
    var value = 0
    
    enum Event { case Increment, Decrement }
    
    mutating func handle(event: Event) {
        switch event {
        case .Increment: value += 1
        case .Decrement: value -= 1
        }
    }
    
    struct Actions {
        let dispatch: Event -> ()
        
        func increment() { dispatch(.Increment) }
        func decrement() { dispatch(.Decrement) }
    }
}

struct TwoCounters {
    var left = Counter()
    var right = Counter()
    
    enum Event { case Left(Counter.Event), Right(Counter.Event) }
    
    mutating func handle(event: Event) {
        switch event {
        case .Left(let event): left.handle(event)
        case .Right(let event): right.handle(event)
        }
    }
    
    struct Actions {
        let left: Counter.Actions
        let right: Counter.Actions
        let both: Counter.Actions
        
        init(dispatch: Event -> ()) {
            self.left = Counter.Actions { dispatch(.Left($0)) }
            self.right = Counter.Actions { dispatch(.Right($0)) }
            self.both = Counter.Actions { event in
                dispatch(.Left(event))
                dispatch(.Right(event))
            }
        }
    }
}

var twoCounters = TwoCounters()
let actions = TwoCounters.Actions { twoCounters.handle($0) }

print(twoCounters)
actions.left.increment()
print(twoCounters)
actions.right.increment()
print(twoCounters)
actions.both.decrement()
print(twoCounters)