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

Question

class Alternative: NSManagedObject {

    @NSManaged var text: String
    @NSManaged var isCorrect: Bool
    @NSManaged var image: NSData
} 

convenience init(text: String, isCorrect: Bool, entity: NSEntityDescription, insertIntoManagedObjectContext context: NSManagedObjectContext!) {
        let alternative = Alternative(entity: entity, insertIntoManagedObjectContext: context) as Alternative
        alternative.text = text
        alternative.isCorrect = isCorrect
        return alternative
}

I want to make a method that lets me initialize new objects with this call:

let newAlternative = Alternative("third platform", True, entityDescription, managedObjectContext)

But i get the error:

Convenience initializer for Alternative must delegate with self.init

What do i need to change in my initalizer to make my example usage work?

Solution 1

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
}

Solution 2

I simply did this with a class function:

class func newInstance(text: String, notes:String, 
                    context: NSManagedObjectContext) -> Item {
    var item = NSEntityDescription.insertNewObjectForEntityForName("Item", 
               inManagedObjectContext: context) as Item
    item.notes = notes
    item.text = text
    return item
}

which you can call like this (almost as pretty):

let item = Item.newInstance(text, notes:notes, context:context)

Solution 3

Swift 3.1 solution:

convenience init(text: String, isCorrect: Bool, image: NSData, moc: NSManagedObjectContext) {
        let entity = NSEntityDescription.entity(forEntityName: "Alternative", in: moc)
        self.init(entity: entity!, insertInto: moc)
        // vars
        self.text = text
        self.isCorrect = isCorrect
        self.image = image
}

Solution 4

You have to call a designated initializer from your convenience initializer. Also, you do not return anything from any initializer.

To fulfill the rules, which are described in Apple's Swift documentation you first need a designated initializer for your subclass, which calls the init() of its superclass, then you can offer a convenience initializer which is only allowed to call a designated initializer from its class declaration.

This would work: (Updated: Taken into account that core data properties marked with @NSManaged are initialized automatically by the runtime. Thanks @Martin R)

init(text: String, isCorrect: Bool, image: NSData, entity: NSEntityDescription,   insertIntoManagedObjectContext context: NSManagedObjectContext!) {
    super.init(entity: entity, insertIntoManagedObjectContext: context)
}

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

View more solutions

Comments

[bogen]

So its not possible to make a designated initializer with Alternative("third platform", True, entityDescription, managedObjectContext) ? My opinion is that it doesnt look as good with Alternative.newInstance(...)

[Martin R]

+1 for not needing to pass an entity description into the initializer.

[Martin R]

Core data properties (marked with @NSManaged) are initialized automatically by the runtime.

[rintaro]

This will cause a problem. see this question: stackoverflow.com/questions/26202346/…

[Mundi]

You can give it another name to your liking instead of newInstance(). Also, you still want named parameters, not just a parameter list, otherwise your code will be less readable. - In summary the class method solution is not really longer or less aesthetic than other initializers.

[Damien]

Quick additional question @martin-r: I am trying to place these into an extension onto NSManagedObject. Do you think this should this be possible, as I'm getting a compile error. It seems to be looking for the NS_DESIGNATED_INITIALIZER from NSManagedObject.h to be used. Any thoughts?

[Martin R]

@Damien: Not without seeing the actual code and the exact error message.

[Martin R]

@Damien: It does not make sense to create a "Person" entity in a NSManagedObject extension. See stackoverflow.com/questions/24834753/… for an alternative (generic) approach.

[Damien]

Doh! You're right! Was working ok up at the "Person" level and I just pushed it down to an extension. I should have thought about it a bit more. Thanks for the hint!

[FredFlinstone]

When you call your func in the example you do not set the context?

[rcat24]

where do you set the context, and how?

[Mundi]

@rcat24 Typically you have the context from your controller / background service etc. That's why this function has the context as an argument.