Create CoreData object in tests with a simple init - ios

Question

I have a set of NSManagedObject subclasses which are used by the ClassToBeTested.
The ClassToBeTested operates just on a few properties of the NSManagedObject subclasses and doesn't need relationships or the whole CoreData stack.
Can I somehow use the same objects in tests by just creating them in a regular way:
let template = CoreDataClass()
template.name = randomString(length: 40) // This fails!
templates.append(template)
Currently it fails with error:
failed: caught "NSInvalidArgumentException", "-[CoreDataClass
setTemplate_name:]: unrecognized selector sent to instance
0x600000af4c40"

Answer 1

Although I get a different error (did not call designated initializer) when I try to do that, in either case, the answer to your question is: No, you cannot do that.
But with NSPersistentContainer nowadays, it is easy to use a singleton in-memory Core Data Stack for such testing. Include your data model in your test bundle, then put this in your test's global scope:
var sharedTestContext: NSManagedObjectContext = {
// Swift is smart enough to execute this only once.
let container = NSPersistentContainer(name: "<YourDataModelName>")
let description = NSPersistentStoreDescription()
description.type = NSInMemoryStoreType
container.persistentStoreDescriptions = [description]
container.loadPersistentStores { (description, error) in
if let error = error {
fatalError("Failed to load store for test: \(error)")
}
}
return container.newBackgroundContext()
}()
And define a special managed object initializer for testing like this:
/**
Initializes a managed object for testing
- important: This assumes your managed object subclass name is the same
as its entity name.
*/
public extension NSManagedObject {
convenience init(testContext: NSManagedObjectContext?) {
let context = testContext ?? sharedTestContext
/* The following contraption is necessary to avoid warning:
"Multiple NSEntityDescriptions claim the NSManagedObject subclass"
For explanation see:
https://stackoverflow.com/questions/51851485/multiple-nsentitydescriptions-claim-nsmanagedobject-subclass */
let name = String(describing: type(of: self))
let entity = NSEntityDescription.entity(forEntityName: name, in: context)!
self.init(entity: entity, insertInto: context)
}
}
Now you can create your test object thus:
let template = CoreDataClass(testContext: nil)