How to “downcast” an array of [NSManageObject] to the selected entity? - ios

Question

I am developing an App using CoreData.
I have 5 entities (A, B, C, D, E) and 90% of the attributes of each entity are common. 10% are different.
I created an array of [NSManageObject] to store the fetchRequest of the selected entity. Until that point everything seems to work.
Here is my fetchRequest code. First of all I made an extension of NSManagedObjectContext:
extension NSManagedObjectContext {
func fetchMOs (_ entityName: String, sortBy: [NSSortDescriptor]? = nil, predicate: NSPredicate? = nil) throws -> [NSManagedObject] {
let request = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
request.returnsObjectsAsFaults = false
request.predicate = predicate
request.sortDescriptors = sortBy
return try! self.fetch(request) as! [NSManagedObject]
}
}
Then I request the data by calling to this function within the context:
documentArray = try! context.fetchMOs(requestedEntity!, sortBy: requestedSortBy, predicate: requestedPredicate)
The problem comes when I want to access the attributes of the selected entity to work with them.
For example, I selected entity A and the fetchRequest stored the data in the array [NSManageObject]. Now I want to print one attribute of the entity:
[NSManageObject].attribute1 <— but this is incorrect.
It should be: [A].attribute1
I guess I could do a switch statement to downcast and work with the data:
Switch entitySelected {
Case “A”:
arrayOfMO as! [A]
arrayOfMO.attribute1 ......
And so on for each entity. But it seems to me that there should be a more clean way of doing that.
The data stored in the entities are Strings and what I want to do with it is just to fill labels and buttons in a tableViewController.
Thanks.

Answer 1

Try next:
extension NSManagedObjectContext {
func fetchMOs<T: NSManagedObject>(sortBy: [NSSortDescriptor]? = nil, predicate: NSPredicate? = nil) throws -> [T] {
let request = NSFetchRequest<T>(entity: T.entity())
request.returnsObjectsAsFaults = false
request.predicate = predicate
request.sortDescriptors = sortBy
return try? self.fetch(request) ?? []
}
}
Then you can get
let results: [SomeMoClass] = moc.fetchMOs<SomeMoClass>()