Example of using Objective-C associated objects as the backing store for properties added via a Swift extension.

UICollectionViewLayoutAttributes+CustomCell.swift

private var stringPropertyAssociationKey: UInt8 = 0
private var intPropertyAssociationKey: UInt8 = 0

// This is a replacement for a custom UICollectionViewLayoutAttributes subclass since
// layoutAttributesForElementsInRect() is crashing when a subclass is registered.
// https://devforums.apple.com/message/1120429#1120429
//
// http://stackoverflow.com/questions/25426780/swift-extension-stored-properties-alternative#answer-25428013
// http://stackoverflow.com/questions/24133058/is-there-a-way-to-set-associated-objects-in-swift#answer-25428409
extension UICollectionViewLayoutAttributes {

    var stringProperty: String {
        get {
            let optionalObject: AnyObject? = objc_getAssociatedObject(self, &stringPropertyAssociationKey)
            if let object: AnyObject = optionalObject {
                return object as! String
            }
            return ""
        }
        set(newValue) {
            objc_setAssociatedObject(self, &stringPropertyAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))
        }
    }

    var intProperty: Int {
        get {
            let optionalObject: AnyObject? = objc_getAssociatedObject(self, &intPropertyAssociationKey)
            if let object: AnyObject = optionalObject {
                return object as! Int
            }
            return 0
        }
        set(newValue) {
            objc_setAssociatedObject(self, &intPropertyAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))
        }
    }

}