AssociatedStruct.swift

/*
get {
    return (objc_getAssociatedObject(self, &ourObserverItemKey)
        as? AssociatedStruct<SomeStructType>)
        .map {$0.value}
}
set {
    objc_setAssociatedObject(self,
        &ourObserverItemKey,
        newValue.map
            { AssociatedStruct<SomeStructType>($0) },
        .OBJC_ASSOCIATION_RETAIN)
}
*/

public final class AssociatedStruct<T>: NSObject, NSCopying {
    public typealias Type = T
    public let value: Type
    
    public init(_ value: Type) { self.value = value }
    
    public func copyWithZone(zone: NSZone) -> AnyObject {
        return self.dynamicType.init(value)
    }
}

extension AssociatedStruct where T: NSCopying {
    public func copyWithZone(zone: NSZone) -> AnyObject {
        return self.dynamicType.init(value.copyWithZone(zone) as! Type)
    }
}