least ugly way to add a property to a class extension

gistfile1.txt

private var fooKey = 0

extension UIViewController {

    var foo: Bool {
        set {
            objc_setAssociatedObject(self, &fooKey, NSNumber(value: newValue), .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
        
        get {
            return objc_getAssociatedObject(self, &fooKey) as? Bool ?? false
        }
    }

}