Generic NSFetchedResultsController with generic NSFetchRequest results in an error - ios

Question

When trying to set up a generic fetched results controller I get Cannot convert value of type 'NSFetchRequest<T>' to expected argument type 'NSFetchRequest<_>', Insert ' as! NSFetchRequest<_> when initializing the controller.
fileprivate var fetchedResultsController: NSFetchedResultsController<T>!
guard let request: NSFetchRequest<T> = T.fetchRequest() as? NSFetchRequest<T> else {
assertionFailure("Can't set up NSFetchRequest")
return
}
request.sortDescriptors = [NSSortDescriptor(key: key, ascending: ascending)]
fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: dataStore.viewContext, sectionNameKeyPath: nil, cacheName: nil)
do {
try fetchedResultsController.performFetch()
} catch let error {
DDLogError("Error fetching entities: \(error)")
}
Anyone knows what's going on here?

Answer 1

For FRC you should explicitly specify that it have a type T.
Also, there is not enough of context from your code snippet, so it's unclear how did you define this class. However, here is how I think it should look like:
import CoreData
class Model<T> where T: NSManagedObject {
fileprivate var context: NSManagedObjectContext! // assume that somewhere during initialization we setting context
fileprivate lazy var fetchedResultscontroller: NSFetchedResultsController<T> = { [weak self] in
guard let this = self else {
fatalError("lazy property has been called after object has been descructed")
}
guard let request = T.fetchRequest() as? NSFetchRequest<T> else {
fatalError("Can't set up NSFetchRequest")
}
request.sortDescriptors = [NSSortDescriptor(key: "key", ascending: true)]
let tmp = NSFetchedResultsController<T>(fetchRequest: request, managedObjectContext: this.context, sectionNameKeyPath: nil, cacheName: nil)
return tmp
}()
}