Helpers to assist using the ObjC runtime to associate both objects and structs in Swift

AssociationHelpers.swift

// Associated wrapper by WeZZard : https://wezzard.com/2015/10/09/associated-object-and-swift-struct/

// Type safe helpers inspired by Tikitu de Jager : https://medium.com/@ttikitu/swift-extensions-can-add-stored-properties-92db66bce6cd#.mx6ekrw16

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)
    }
}

extension NSObject {
    
    func associatedOptionalObject<ValueType: AnyObject>(
        key: UnsafePointer<String>,
        initialiser: () -> ValueType?) -> ValueType? {
            
            if let obj = objc_getAssociatedObject(self, key) as? ValueType {
                return obj
            }
            let obj = initialiser()
            self.associateOptionalObject(key, value:obj)
            return obj
    }

    func associateOptionalObject<ValueType: AnyObject>(key: UnsafePointer<String>, value: ValueType?) {
        
        objc_setAssociatedObject(self, key, value, .OBJC_ASSOCIATION_RETAIN)
    }
    
    func associatedObject<ValueType: AnyObject>(
        key: UnsafePointer<String>,
        initialiser: () -> ValueType) -> ValueType {
            
            if let obj = objc_getAssociatedObject(self, key) as? ValueType {
                return obj
            }
            let obj = initialiser()
            self.associateOptionalObject(key, value:obj)
            return obj
    }
    
    func associateObject<ValueType: AnyObject>(key: UnsafePointer<String>, value: ValueType) {
        
        objc_setAssociatedObject(self, key, value, .OBJC_ASSOCIATION_RETAIN)
    }
    
    func associatedOptionalStruct<ValueType: Any>(
        key: UnsafePointer<String>,
        initialiser: () -> ValueType?) -> ValueType? {
            
            if let obj = objc_getAssociatedObject(self, key) as? AssociatedStruct<ValueType?> {
                return obj.value
            }
            let value = initialiser()
            self.associateOptionalStruct(key, value:value)
            return value
    }
    
    func associateOptionalStruct<ValueType: Any>(key: UnsafePointer<String>, value: ValueType?) {
        
        objc_setAssociatedObject(self, key, AssociatedStruct<ValueType?>(value), .OBJC_ASSOCIATION_RETAIN)
    }

    func associatedStruct<ValueType: Any>(
        key: UnsafePointer<String>,
        initialiser: () -> ValueType) -> ValueType {
            
            if let obj = objc_getAssociatedObject(self, key) as? AssociatedStruct<ValueType> {
                return obj.value
            }
            let value = initialiser()
            self.associateOptionalStruct(key, value:value)
            return value
    }
    
    func associateStruct<ValueType: Any>(key: UnsafePointer<String>, value: ValueType) {
        
        objc_setAssociatedObject(self, key, AssociatedStruct<ValueType>(value), .OBJC_ASSOCIATION_RETAIN)
    }
}

ExampleUsage.swift

let foo = NSNumber(int:5)

private var anOptionalColourKey = "anOptionalColourKey"
private var aColourKey = "aColourKey"
private var anOptionalStringKey = "anOptionalStringKey"
private var aStringKey = "aStringKey"

extension NSNumber {
    
    var anOptionalColour : UIColor? {
        get {
            return self.associatedOptionalObject(&anOptionalColourKey) { return nil as UIColor?  }
        }
        set {
            self.associateOptionalObject(&anOptionalColourKey, value: newValue)
        }
    }
    
    var aColour : UIColor {
        get {
            return self.associatedObject(&aColourKey) {
                return UIColor.blackColor()
            }
        }
        set {
            self.associateObject(&aColourKey, value: newValue)
        }
    }

    var anOptionalString : String? {
        get {
            return self.associatedOptionalStruct(&anOptionalStringKey) {
                return nil as String?
            }
        }
        set {
            self.associateOptionalStruct(&anOptionalStringKey, value: newValue)
        }
    }
    
    var aString : String {
        get {
            return self.associatedStruct(&aStringKey) {
                return ""
            }
        }
        set {
            self.associateStruct(&aStringKey, value: newValue)
        }
        
    }
}

foo.anOptionalColour
foo.anOptionalColour = UIColor.greenColor()
foo.anOptionalColour
foo.anOptionalColour = nil
foo.anOptionalColour

foo.aColour
foo.aColour = UIColor.redColor()
foo.aColour

foo.anOptionalString
foo.anOptionalString = "42"
foo.anOptionalString
foo.anOptionalString = nil
foo.anOptionalString

foo.aString
foo.aString = "Hello World"
foo.aString
foo.aString = "Goodbye World"
foo.aString