Prototype.swift

class Product : NSObject, NSCopying {
    
    private(set) var name:String
    private var stockLevelBackingValue:Int = 0
    
    init(name:String, stockLevel:Int) {
        self.name = name
        super.init()
        self.stockLevel = stockLevel
    }
    
    var stockLevel:Int {
        get {
            return stockLevelBackingValue
        }
        set {
            stockLevelBackingValue = max(0, newValue)
        }
    }
    
    func copyWithZone(zone: NSZone) -> AnyObject {
        return Product(name: self.name, stockLevel: self.stockLevel)
    }
}
var handler = { (p:Product) in
    print("Change: \(p.name) \(p.stockLevel) items in stock")
}
let logger = Logger<Product>(callback: handler)
logger.logItem(Product(name: "Kayak", stockLevel: 10))