How can I use a Transformable attribute as its actual type in a NSManagedObject subclass? - cocoa

Question

Just starting a project. The data-model file has one entity, which has a single attribute that is Transformable. It's supposed to be a NS/CGRect. I had Xcode create corresponding NSManagedObject subclass files. For "MyThing.swift", I got:
import Foundation
import CoreData
class MyThing: NSManagedObject {
// Insert code here to add functionality to your managed object subclass
}
And I got a "MyThing+CoreDataProperties.swift":
import Foundation
import CoreData
extension MyThing {
#NSManaged var myBounds: NSObject?
}
I want the property to be an actual CGRect, so I need a NSData <-> NSValue <-> NSRect conversion chain. I already have "NSKeyedUnarchiveFromDataTransformer" as the Name under the Attribute Type in Interface Builder. What do I add (and/or change) to pass CGRect values around?
Or do I not do this, and just pass NSValue-wrapped CGRects around instead? (I hope that CoreData will take care of any NSData <-> NSValue conversions.)

Answer 1

As long as your transformable type complies to NSCoding, Core Data will take care of the rest for you. I've used NSAttributedString in my NSManagedObjects, and only changed the CoreData provided id-type that was generated at runtime.
If you want to have a property of your own type, i.e. MyAwesomeObject, then make sure you implement initWithCoder: and encodeWithCoder:.
So, for your case, in order to store CGRects with CoreData, you would need to wrap them in a class that implements the above mentioned methods. This is because CGRect doesn't comply to the NSCoding protocol, and thus cannot be stored directly as a transformable attribute.
Your other option is of course to store x,y,width,height as properties, and either have a transient property wich you compute in awakeFromFetch and awakeFromInsert or just a convenience method.

Answer 2

Because CGRect is not a class but struct i would suggest creating another property which will access the raw variable with proper conversions. And then use only that property.
class MyThing {
var boundsValue : NSValue?
}
extension MyThing {
var bounds : CGRect {
get {
if let value = boundsValue {
value.CGRectValue()
}
return CGRectNull
}
set {
boundsValue = NSValue(CGRect: newValue)
}
}
}