How can I create instances of managed object subclasses in a NSManagedObject Swift extension?

by

(Updated for Swift 3/4 now. Solutions for earlier Swift versions
can be found in the edit history.)

You can use unsafeDowncast to cast the return value
of NSEntityDescription.insertNewObject() to Self
(which is the type on which the method is actually called):

extension NSManagedObject {
    class func create(in context: NSManagedObjectContext) -> Self {
        let classname = entityName()
        let object = NSEntityDescription.insertNewObject(forEntityName: classname, into: context)
        return unsafeDowncast(object, to: self)
    }

    // Returns the unqualified class name, i.e. the last component.
    // Can be overridden in a subclass.
    class func entityName() -> String {
        return String(describing: self)
    }
}

Then

let obj = YourEntity.createInContext(context)

works and the compiler infers the type of obj correctly as YourEntity.

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