Swift initialize Self in a static methods

Question

Having problem getting this to work, inside User.provideInstance I'm unable to initialize Self and return. Any thoughts?

extension NSManagedObject {
    public convenience init(managedObjectContext: NSManagedObjectContext) {
        let entity = NSEntityDescription.entityForName(String(self.dynamicType), inManagedObjectContext: managedObjectContext)!
        self.init(entity: entity, insertIntoManagedObjectContext: managedObjectContext)
    }
}

public protocol Deserializable {
    static func provideInstance(json: [NSObject: AnyObject]) -> Self
}

@objc(User) public class User: NSManagedObject, Deserializable {

    public static func provideInstance(json: [NSObject: AnyObject]) -> Self {
       let context = DIContainer.instance.resolve(CoreDataStack.self).managedObjectContext
       let instance = self.init(managedObjectContext: context)
       return instance
    }
}

Error is on let instance = self.init(managedObjectContext: context):

Constructing an object of class type Self with a metatype value must use a required initializer

What error do you get?

jtbandes
– jtbandes

2016-06-11 21:00:48 +00:00
Commented Jun 11, 2016 at 21:00 — jtbandes
– jtbandes, Commented Jun 11, 2016 at 21:00
@jtbandes updated question and included error

aryaxt
– aryaxt

2016-06-11 21:03:06 +00:00
Commented Jun 11, 2016 at 21:03 — aryaxt
– aryaxt, Commented Jun 11, 2016 at 21:03

Sulthan · Accepted Answer · 2016-06-11 21:47:09Z

1

Why bother with initializers?

extension NSManagedObject {
    class func provide(managedObjectContext managedObjectContext: NSManagedObjectContext) -> Self {
        let entity = NSEntityDescription.entityForName(String(self.dynamicType), inManagedObjectContext: managedObjectContext)!
        return self.init(entity: entity, insertIntoManagedObjectContext: managedObjectContext)
    }
}

public protocol Deserializable {
    static func provideInstance(json: [NSObject: AnyObject]) -> Self
}

@objc(User) public class User: NSManagedObject, Deserializable {

    public static func provideInstance(json: [NSObject: AnyObject]) -> Self {
        let context = ...
        let instance = self.provide(managedObjectContext: context)
        return instance
    }
}

The problem with initializers is that they are not always inherited, unless they are required. And you cannot create a required initializer in an extension.

answered Jun 11, 2016 at 21:47

Sulthan

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2 Comments

user4282669 Over a year ago

How is this any different from what I posted?

Sulthan Over a year ago

I have no forced casts there.

score 1 · Accepted Answer · 2016-06-11 21:44:36Z

1

As the error said, you need a required initializer to create an object, incase you subclass. Add final before your class to prevent this error.

Try this if you need to subclass:

@objc(User) public class User: NSManagedObject, Deserializable {

private static func pInstance<T>(json: [NSObject: AnyObject]) -> T {
    let context = DIContainer.instance.resolve(CoreDataStack.self).managedObjectContext
    let copy: NSManagedObject = NSManagedObject(managedObjectContext: context)
    let instance = copy as! T
    return instance
}

public static func provideInstance(json: [NSObject: AnyObject]) -> Self {
        return pInstance(json)
    }
}

Basically, create a helper function to create an instance of T based on the base class, NSManagedObject. Then use it the actual provideInstance function with the inferred type Self.

edited Jun 11, 2016 at 21:44

answered Jun 11, 2016 at 21:25

user4282669

1 Comment

aryaxt Over a year ago

If I add final it means I won't be able to create a subclass of User anymore

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