Swift 3 - Program crashes when trying to perform fetch request

Question

currently, I'm trying to fetch an entity of type song on one of my view controllers. this is the code that's relevant, that I have:

import CoreData

class TimerScreenVC: UIViewController, NSFetchedResultsControllerDelegate {

var songController: NSFetchedResultsController<Song>!


override function viewDidLoad() {
   super.viewdidLoad()
   attemptSongFetch()
}


func attemptSongFetch() {
    let fetchRequest: NSFetchRequest<Song> = Song.fetchRequest()
    let controller = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    let sortByTitle = NSSortDescriptor(key: "title", ascending: true)
    fetchRequest.sortDescriptors = [sortByTitle]
    songController = controller
    do {
        try songController.performFetch()
    } catch {
        let error = error as NSError
        print("\(error)")
    }


}


    func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    print("called will change")
}
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    print("called did change")
}
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
    switch (type) {
    case .insert:
        print("has been called")
    default:
        print("has been called")
    }

}
}

however, when I load this view controller, I'm faced with the error "terminating with uncaught exception of type NSException". I can make the error go away and the program works fine if I comment out attemptSongFetch() in viewDidLoad(), but I need to call that function.

Also I have the exact same function, attemptSongFetch(), with the exact same code on another ViewController & that one isn't crashing. Any ideas? Any help would be greatly appreciated.

update so here's the error, it's telling me that I need to set the sort description, which is strange because it's already defined?:

017-02-20 15:48:21.006 Alarm Clock[10433:158613] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'An instance of NSFetchedResultsController requires a fetch request with sort descriptors' libc++abi.dylib: terminating with uncaught exception of type NSException (lldb)

Don't know if it is the proble, but your fetch controller don't have delegate — CZ54
– CZ54, Commented Feb 20, 2017 at 14:47
"terminating with uncaught exception of type NSException" it not a a specific error. Include the full error log correctly. — user5890979
– user5890979, Commented Feb 20, 2017 at 14:59
see the comments on the question here and also see its answers — mfaani
– mfaani, Commented Feb 20, 2017 at 15:03
@Chandan context is defined in the app delegate, it's essentially a shortcut variable defined as the appdelegate.persistentcontainer.viewContext — Spencer Milanak
– Spencer Milanak, Commented Feb 20, 2017 at 20:40

vadian · Accepted Answer · 2017-02-20 21:08:07Z

5

The error message is pretty clear:

An instance of NSFetchedResultsController requires a fetch request with sort descriptors

At the moment you are creating the NSFetchedResultsController there are no sort descriptors (yet). Just reorder the lines:

let fetchRequest: NSFetchRequest<Song> = Song.fetchRequest()
let sortByTitle = NSSortDescriptor(key: "title", ascending: true)
fetchRequest.sortDescriptors = [sortByTitle]
let controller = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)

answered Feb 20, 2017 at 21:08

vadian

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1 Comment

Spencer Milanak Over a year ago

I'm not sure how I missed that, but you answered my question. Thanks!

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