AssociatedObject.swift

public func associatedObject<ValueType: Any>(base: AnyObject, key: UnsafePointer<UInt8>, initializer: () -> ValueType)
    -> ValueType {
    // if there is already an associated value returns it
    if let associated = objc_getAssociatedObject(base, key) as? ValueType {
        return associated
    }

    // associated value not found, initializes closure, makes the association and returns it
    let associated = initializer()
    associateObject(base: base, key: key, value: associated)

    return associated
}

public func associateObject<ValueType: Any>(base: AnyObject, key: UnsafePointer<UInt8>, value: ValueType) {
    objc_setAssociatedObject(base, key, value, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}

// Usage example

// Add a `someProperty` property to all UIViews

private struct AssociatedKeys {
    static var myKey: UInt8 = 0
}

extension UIView {
    var someProperty: Int {
        return associatedObject(base: self, key: &AssociatedKeys.myKey, initializer: { () -> Int in
            return 0
        })
    }
}

print(view.someProperty) // prints 0
view.someProperty = 1
print(view.someProperty) // prints 1