Need generic FetchedResultsController builder (Swift)

Question

I created a method to build an frc:

private func buildFRC<T:NSManagedObject>(entity: T, sortKey: String) 
    -> NSFetchedResultsController<T>? {

    let fetchRequest: NSFetchRequest = T.fetchRequest()
    let sortDescriptor1 = NSSortDescriptor(key: sortKey, ascending: true)
    fetchRequest.sortDescriptors = [sortDescriptor1]

    searchContext.reset()

    var frc: NSFetchedResultsController<T>? =
        NSFetchedResultsController<T>(
        fetchRequest:            fetchRequest as! NSFetchRequest<T>,
        managedObjectContext:   searchContext,
        sectionNameKeyPath:     nil,   
        cacheName:              nil)   
    frc!.delegate = self                

    try? frc!.performFetch()
    return frc
}

I want to call something like this from within a closure:

self.frc = self.buildFRC(entity: ObjectName, sortKey: "trackName")

but I'm getting this error:

"Cannot convert value of type 'ObjectName.Type' to expected argument type 'NSManagedObject'".

Yet, ObjectName is the class name of an NSManagedObject. I tried myself but eventually I just keep chasing errors in a circle.

score 2 · Accepted Answer · answered Nov 16 '18 at 23:50

Your function declaration doesn't mean quite what you think it does.

private func buildFRC<T:NSManagedObject>(entity: T, sortKey: String) -> NSFetchedResultsController<T>?

This means that T must be a subclass of NSManagedObject, and that the first argument must be an instance of T. When you call it like this

self.frc = self.buildFRC(entity: ObjectName, sortKey: "trackName")

...you're passing in the subclass as the first argument, when your declaration expects an instance.

It's not hard to fix, because you don't need to include T as an argument. In general, Swift generics don't need you to pass the type as an argument-- the type comes from how the function is used. Drop that argument and rewrite the declaration as

private func buildFRC<T:NSManagedObject>(sortKey: String) -> NSFetchedResultsController<T>? {

Then call the function with something like

self.frc: NSFetchedResultsController<ObjectName>? = self.buildFRC(sortKey: "trackName")

Swift will figure out that T represents ObjectName in that call and the code will work.

On a tangential note, your call to searchContext.reset() is kind of dangerous and probably not needed. If you fetch some objects from the context and then call this function later on, the reset will cause all of those previously fetched objects to become invalid. Using them would crash your app.

Thanks for your thoughtful replay. All is working now. I realize I need to study generics a bit more. — , Nov 17 '18 at 02:28
Unsure about reset: I am working directly from objects in contexts, so I figured a reset would be okay. Are you suggesting I batch delete them all instead? — , Nov 17 '18 at 02:41
Resetting and deleting are very different operations with very different effects. Resetting is unsafe unless no previously fetched objects are in memory. For example if your call this function twice, any objects from the first call should no longer be referenced anywhere in code. It’s likely to be a dangerous call unless you have a very clear understanding of its effects. — Tom Harrington, Nov 17 '18 at 03:51

Need generic FetchedResultsController builder (Swift)

1 Answers1