How to make a designated initializer for NSManagedObject subclass in Swift?

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A convenience initializer must call the designated initializer on self:

convenience init(text: String, isCorrect: Bool, entity: NSEntityDescription, insertIntoManagedObjectContext context: NSManagedObjectContext!) {
    self.init(entity: entity, insertIntoManagedObjectContext: context)
    self.text = text
    self.isCorrect = isCorrect
}

which would be called as 

let newAlternative = Alternative(text: "third platform", isCorrect: true,
     entity: entityDescription, insertIntoManagedObjectContext: managedObjectContext)

In addition, you could also move the creation of the entity description into the convenience initializer instead of passing it as an argument (as motivated by
Mundi’s answer):

convenience init(text: String, isCorrect: Bool, insertIntoManagedObjectContext context: NSManagedObjectContext!) {
    let entity = NSEntityDescription.entityForName("Alternative", inManagedObjectContext: context)!
    self.init(entity: entity, insertIntoManagedObjectContext: context)
    self.text = text
    self.isCorrect = isCorrect
}

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