CODEWITHSUNDEEP
cocoacore-dataswiftnsmanagedobject
cfischer
cfischer

Reputation: 24912

Adding a stored property to a subclass of NSManagedObject in Swift

If I try to add a stored property to a subclass in NSManagedObject in Swift, without providing it with a default value (I'll do that in an initialiser, mind you), I get this error:

Stored property 'foo' requires an initial value or should be @NSManaged

The code is as follows:

class Thing : NSManagedObject{
    var foo : String
    var bar : String

    init(foo: String, bar : String){

        // blah, blah...
    }
}

What's the reason for enforcing this? Why the heck can't I initialise in an initialiser????

Upvotes: 1

Views: 1348

Answers (1)

Rob Napier
Rob Napier

Reputation: 299345

EDIT: The below answer applies to a wide variety of situations, and is related for this, but does not exactly address the NSManagedObject situation. In the case of NSManagedObject, an object can be loaded from the persistent store and initialized without calling your special init. Swift doesn't know what it should assign foo and bar in those cases, so requires some default values (rather than just using final or required as you could do in other subclassing situations cases).

So the correct question is: what would you expect Core Data to do with foo and bar when it loads this object out of the data store?

Because the compiler cannot prove that all subclasses will implement or call init(foo,bar). If a subclass did not implement that initializer, then foo or bar may not be initialized.

You can resolve this many ways. You can provide defaults. You can make the values explicitly unwrapped optionals (making their default nil). You can make the values optional. You can declare this initializer required so that all subclasses must implement it. Or you can declare Thing to be final so that it cannot be subclassed.

Upvotes: 5

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