How to have stored properties in Swift, the same way I had on Objetive-C?

I am switching an application from Objective-C to Swift, which I have a couple of categories with stored properties, for example:

@interface UIView (MyCategory)

- (void)alignToView:(UIView *)view
          alignment:(UIViewRelativeAlignment)alignment;
- (UIView *)clone;

@property (strong) PFObject *xo;
@property (nonatomic) BOOL isAnimating;

@end

As Swift extensions don't accept stored properties like these, I don't know how to maintain the same structure as the Objc code. Stored properties are really important for my app and I believe Apple must have created some solution for doing it in Swift.

As said by jou, what I was looking for was actually using associated objects, so I did (in another context):

import Foundation
import QuartzCore
import ObjectiveC

extension CALayer {
    var shapeLayer: CAShapeLayer? {
        get {
            return objc_getAssociatedObject(self, "shapeLayer") as? CAShapeLayer
        }
        set(newValue) {
            objc_setAssociatedObject(self, "shapeLayer", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
        }
    }

    var initialPath: CGPathRef! {
        get {
            return objc_getAssociatedObject(self, "initialPath") as CGPathRef
        }
        set {
            objc_setAssociatedObject(self, "initialPath", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
        }
    }
}

But I get an EXC_BAD_ACCESS when doing:

class UIBubble : UIView {
    required init(coder aDecoder: NSCoder) {
        ...
        self.layer.shapeLayer = CAShapeLayer()
        ...
    }
}

Any ideas?