nametag.swift

// NSObject Nametag
public var sharedNametagKey : UnsafeMutablePointer<UInt8> = UnsafeMutablePointer.alloc(1)
public extension NSObject {
    public var nametag: String? {
        get {
            return objc_getAssociatedObject(self, &sharedNametagKey) as? String
        }
        set {
            if let newNameTag = newValue {
                // Store unwrapped new value
                objc_setAssociatedObject(self, &sharedNametagKey, newNameTag, .OBJC_ASSOCIATION_RETAIN)
            } else {
                // Remove by assignment to nil
                objc_setAssociatedObject(self, &sharedNametagKey, nil, .OBJC_ASSOCIATION_RETAIN)
            }
        }
    }
}

// Better cast
// Thanks Mike Ash
infix operator --> {}
func --><T, U>(value: T, target: U.Type) -> U? {
    guard sizeof(T.self) == sizeof(U.self) else {return nil}
    return unsafeBitCast(value, target)
}

// Set and get
func getNametag<T>(castable: T) -> String? {
    guard let myObject = castable --> NSObject.self else {return nil}
    return myObject.nametag
}

func setNametag<T>(castable: T, tag: String?) {
    guard let myObject = castable --> NSObject.self else {
        print("Unable to cast")
        return
    }
    myObject.nametag = tag
}

// Original test case
typealias BillTheTypeCat = @convention(block)(item: Int) -> ()
var myBlock: BillTheTypeCat = {_ in print("notSureWhat")}
setNametag(myBlock, tag: "Hello")
print(getNametag(myBlock) ?? "Eh?")
setNametag(myBlock, tag: nil)
print(getNametag(myBlock) ?? "Eh?")