Associated Objects (from https://github.com/chbo297/CCZoomTransition)

AssociatedObjects.swift

// http://nshipster.com/swift-objc-runtime/
extension UIViewController {
    private struct AssociatedKeys {
        static var DescriptiveName = "nsh_DescriptiveName"
    }

    var descriptiveName: String? {
        get {
            return objc_getAssociatedObject(self, &AssociatedKeys.DescriptiveName) as? String
        }

        set {
            if let newValue = newValue {
                objc_setAssociatedObject(
                    self,
                    &AssociatedKeys.DescriptiveName,
                    newValue as NSString?,
                    .OBJC_ASSOCIATION_RETAIN_NONATOMIC
                )
            }
        }
    }
}

// https://github.com/chbo297/CCZoomTransition/blob/master/CCZoomTransition/CCZoomTransitionVCExtension.swift
extension UIViewController {
    private struct AssociatedKey {
        static var ZoomTransitioner = "cc_zoomTransitioner"
    }
    
    private var cc_transitioner : CCZoomTransitioner?{
        get {
            if let transitioner = objc_getAssociatedObject(self, &AssociatedKey.ZoomTransitioner) as? CCZoomTransitioner {
                return transitioner
            } else {
                return nil
            }
        }
        
        set {
            objc_setAssociatedObject(self, &AssociatedKey.ZoomTransitioner, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            // nonatomic - fast, not thread safe. atomic - deafult behavior, thread safe.
            // http://stackoverflow.com/a/10753201/3502608
        }
    }
}

extension UIView {
    
//    public var dance: Dance {
//        get {
//            return DanceExtensionStoredPropertyHandler.associatedObject(base: self, key: &DanceExtensionStoredPropertyHandler.danceKey) {
//                return Dance(dancingView: self) // initial value - is reference type so it won't change
//            }
//        }
//        set {
//            DanceExtensionStoredPropertyHandler.associateObject(base: self, key: &DanceExtensionStoredPropertyHandler.danceKey, value: newValue)
//        }
//    }
    
    fileprivate struct DanceAssociatedKey {
        static var dance = "dance_key"
    }
    
    public var dance: Dance {
        get {
            if let danceInstance = objc_getAssociatedObject(self, &DanceAssociatedKey.dance) as? Dance {
                return danceInstance
            } else {
                let newDanceInstance = Dance(dancingView: self)
                objc_setAssociatedObject(self, &DanceAssociatedKey.dance, newDanceInstance, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
                return newDanceInstance
            }
        }
        
//        set {
//            objc_setAssociatedObject(self, &DanceAssociatedKey.dance, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
//        }
    }
    
}

// Bool

fileprivate struct PushMenuAssociatedKey {
        static var isEnabledKey = "push_menu_enabled_key"
    }
    
    public var isPushMenuEnabled: Bool {
        get {
            if let isPushMenuEnabledInstance = objc_getAssociatedObject(self, &PushMenuAssociatedKey.isEnabledKey) as? Bool {
                return isPushMenuEnabledInstance
            } else {
                let newIsPushMenuEnabledInstance = false
                objc_setAssociatedObject(self, &PushMenuAssociatedKey.isEnabledKey, newIsPushMenuEnabledInstance, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
                return newIsPushMenuEnabledInstance
            }
        }
        
        set {
            objc_setAssociatedObject(self, &PushMenuAssociatedKey.isEnabledKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            
            if newValue {
                // ...
            } else {
                // ...
            }
        }
    }