Anonymous-ish classes (closure?) in Swift for NSFetchedResultsController - ios

Question

Some context:
I've been away from iOS programming for 5+ years and boy howdy have things changed. Just to make it more exciting, I'm trying Swift at the same time.
I'm trying to build a relatively simple iOS app using the Master-Detail App template and am at the point where I'm trying to add sections to the data. My (Core Data) data model is pretty simple at this point - some Locations (these are the sections) each of which contain some Containers. These are linked in the obvious way (a Location have a one-to-many reference to Containers, and Containers have a one-to-one reference to its Location).
I suspect that I'm heading to a place where I'm going to want the fetchedResultsController.object(at: indexPath) to return the Container corresponding with the indexPath but I'm also going to want the fetchedResultsController.sections[section] to return a Location.
The code
This is pretty much the code that comes from the app template (with minor modifications to use my Location type as the generic ResultType for NSFetchedResultsController - that might might be a mistake; maybe it should be a Container or even a NSManagedObject - we'll get to that in a minute).
var fetchedResultsController: NSFetchedResultsController<Location> {
if _fetchedResultsController != nil {
return _fetchedResultsController!
}
let fetchRequest: NSFetchRequest<Location> = Location.fetchRequest()
// Set the batch size to a suitable number.
fetchRequest.fetchBatchSize = 20
// Edit the sort key as appropriate.
let sortDescriptor = NSSortDescriptor(key: "name", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.managedObjectContext!, sectionNameKeyPath: nil, cacheName: "Master")
aFetchedResultsController.delegate = self
_fetchedResultsController = aFetchedResultsController
do {
try _fetchedResultsController!.performFetch()
} catch {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
return _fetchedResultsController!
}
My questions
I'm new to Swift and I'm struggling to understand what the code that the Xcode template created for me. So I have a few questions:
What do we call this pattern? Anonymous classes (apparently not)? Closures? Knowing what it's called will help me do a better job of searching for related answers instead of wasting your time on these noob questions.
What is it actually defining? What I suspect it's doing is defining an implementation for init and returning an instance of NSFetchedResultsController that has all the default implementations. If that's not completely right, help me understand it a little better (pointing at something to read is also helpful).
How should I go about overriding methods of NSFetchedResultsController when using this pattern? Or is that something where I need to create a real subclass.
What type should I be using as the generic ResultType and why? This is a little off topic, but what the heck, maybe you'll take pity on me and bump me along another step of the journey.

Answer 1

It's basically called a computed property.
It checks if an instance of NSFetchedResultsController exists. If not it creates one and configures it.
There is nothing to override. However you should implement the delegate methods.
The generic type is correct. It avoids a lot of type casting.
I doubt this is the code which comes from the app template. The Swift equivalent of the objective-c-ish pattern with the instance variable and the nil-check is a lazy instantiated property. The body is called once the first time the property is accessed.
lazy var fetchedResultsController : NSFetchedResultsController<Location> = {
let fetchRequest: NSFetchRequest<Location> = Location.fetchRequest()
// Set the batch size to a suitable number.
fetchRequest.fetchBatchSize = 20
// Edit the sort key as appropriate.
let sortDescriptor = NSSortDescriptor(key: "name", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
let controller = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.managedObjectContext!, sectionNameKeyPath: nil, cacheName: "Master")
controller.delegate = self
do {
try controller.performFetch()
} catch {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
return controller
}()
To add sections you have to specify the sectionNameKeyPath parameter and add an appropriate second sort descriptor.
The table view data source methods should look like
func numberOfSections(in tableView: UITableView) -> Int {
return self.fetchedResultsController.sections?.count ?? 0
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return self.fetchedResultsController.sections?[section].numberOfObjects ?? 0
}