Persisting VNFeaturePrintObservation to Core Data

Question

I am working with the Photos and CoreML frameworks in Swift to implement some image analysis functionality in an App I'm working on. I require the ability to store the result of VNGenerateImageFeaturePrintRequest(), which has type VNFeaturePrintObservation.

These results need to be persisted for each image so that they can be used later without re-calculating, as it is a slow process. VNFeaturePrintObservation doesn't conform to Codable, so I can't use something like a JSONEncoder which is how I've previously persisted data like this into Core Data.

Is there a way I can persist my VNFeaturePrintObservation objects in Core Data?

Answer 1

I'm not familiar with VNFeaturePrintObservation, but the docs say that

• It's a subclass of VNObservation, and
• VNObservation conforms to NSSecureCoding

That means that VNFeaturePrintObservation also conforms to NSSecureCoding. Any class that conforms to NSSecureCoding can be saved in Core Data by using a Core Data attribute with the type set to "Transformable". Thanks to NSSecureCoding there's no additional work.

That would look like this in the Core Data model editor:

Two things to notice:

• Type is "Transformable"
• Custom class is VNFeaturePrintObservation

If you have Xcode generate your source code, the property will look like this:

@NSManaged public var observation: VNFeaturePrintObservation?

You don't have to do anything special to use this property.

Incidentally, Codable doesn't apply here, because Core Data doesn't use Codable. NSSecureCoding will convert to/from Data automatically, and it's what you need to use with Core Data.