Where Do I Register a Valuetransformer in Swift

Where do I register a ValueTransformer in Swift?

You are right, you can register your value transformers in the AppDelegate. If you want something that closer resembles ObjectiveC's +initialize you can use lazy initialization of a class variable. E.g:

class AppDelegate: NSObject, NSApplicationDelegate {

    static let doInitialize: Void = {
        // register transformers here
    }()

    override init() {
        super.init()
        AppDelegate.doInitialize
    }
}

This pattern should also work for classes other than the AppDelegate if you want to keep the transformers things closer to the classes that actually use them.

Where do I register a ValueTransformer in Swift?

You are right, you can register your value transformers in the AppDelegate. If you want something that closer resembles ObjectiveC's +initialize you can use lazy initialization of a class variable. E.g:

class AppDelegate: NSObject, NSApplicationDelegate {

    static let doInitialize: Void = {
        // register transformers here
    }()

    override init() {
        super.init()
        AppDelegate.doInitialize
    }
}

This pattern should also work for classes other than the AppDelegate if you want to keep the transformers things closer to the classes that actually use them.

Custom NSValueTransformer in xcode 6 with swift

After you initialise newTransformer you should also include the line:

NSValueTransformer.setValueTransformer(newTransformer, forName: "myTransformer")

Then in your Interface Builder you should use myTransformer instead of newTransformer under the Value Transformer dropdown.

In a SwiftUI lifecycle app, where exactly should I register a CoreData transformerValue?

Put it into init, like

class PersistenceController {
   static let shared = PersistenceController()

   init() {
      UIColorValueTransformer.register()       // << here !!
  
      // ... other init code
   }

   // ... other code
}

CoreData Transformable: Custom Transformer never gets called - NSKeyedArchiver is used instead

You need to register your ValueTransformer before you use it.

ValueTransformer.setValueTransformer(MyTransformer(), forName: NSValueTransformerName(rawValue: "MyTransformer"))
let fetch: NSFetchRequest<Entity> = Entity.fetchRequest()

For more info refer docs.

CoreData: ValueTransformer functions are not called

You haven't overridden the correct methods of ValueTransformer. Your methods are:

func transformedValue(value: String?) -> NSData?
func reverseTransformedValue(value: NSData?) -> String?

The correct methods are:

func transformedValue(_ value: Any?) -> Any?
func reverseTransformedValue(_ value: Any?) -> Any?

The big hint that you're implementing the wrong methods is that you didn't need to add the override keyword.

BTW, this expression:

encoding: String.Encoding(rawValue: String.Encoding.utf8.rawValue))!

can be replaced with:

encoding: .uf8

It would also likely be better to replace your return "nil" with return nil; it's a String?, so it can be nil if things go wrong.

Core Data not automatically calling value transformer when getting / setting attribute directly in code

(From the comment above:) The inverse transformer is called when you save the context,
not when you set an attribute.

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