get-mock.swift

// Beware: this is "pseudocode", which is not guaranteed to compile. ;-)

import CoreData

protocol CoreDataProtocol {
  var managedObjectContext: NSManagedObjectContext? { get }
}

class Card: NSManagedObject, CoreDataProtocol {
  // conforms by default
}

struct CardMock: CoreDataProtocol {
  let managedObjectContext: NSManagedObjectContext?
  var card: CardThing? {
    if let _ = _managedObjectContext {
      // we have one, return a card
      return CardThing()
    } else {
      // it is nil, return nil
      return nil
    }
  }
  
  init(managedObjectContext context: NSManagedObjectContext?) {
    managedObjectContext = context
  }
}

XCTAssertNil(CardMock(managedObjectContext: nil).card)
XCTAssertNotNil(CardMock(managedObjectContext: NSManagedObjectContext()).card)