Deleting a row in UITableView causes NSInternalInconsistencyException with computed property

Question

I have a computed property as an instance variable:

var favorites: [PFObject]
    get { return Array(ObjectManager.favorites) }
}

where ObjectManager.favorites is a Set static var on the ObjectManager class. Below are my table view methods:

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    return 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return favorites.count
}

override func tableView(tableView: UITableView, commitEditingStyle editingStyle: UITableViewCellEditingStyle, forRowAtIndexPath indexPath: NSIndexPath) {
    if editingStyle == .Delete {

        //delete it from the original set
        ObjectManager.removeFavoriteObject(favorites[indexPath.row])

        tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)


        println(favorites.count)
        // Delete the row from the data source

    }

    //If there are no longer any fav objects, then there is no need to display the Edit button.
    if favorites.isEmpty {
        self.navigationItem.rightBarButtonItem = nil
    }
}

This results in the following exception being trigger when I call deleteRowsAtIndexPaths.

2015-06-08 21:00:35.320 MedConf[56258:707923] *** Assertion failure in -[UITableView _endCellAnimationsWithContext:], /SourceCache/UIKit_Sim/UIKit-3347.44/UITableView.m:1623
2015-06-08 21:00:35.396 MedConf[56258:707923] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of rows in section 0.  The number of rows contained in an existing section after the update (1) must be equal to the number of rows contained in that section before the update (1), plus or minus the number of rows inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
*** First throw call stack:
(
    0   CoreFoundation                      0x00000001114aec65 __exceptionPreprocess + 165
    1   libobjc.A.dylib                     0x0000000110defbb7 objc_exception_throw + 45
    2   CoreFoundation                      0x00000001114aeaca +[NSException raise:format:arguments:] + 106
    3   Foundation                          0x0000000110a0498f -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
    4   UIKit                               0x0000000111b30c53 -[UITableView _endCellAnimationsWithContext:] + 12678
    5   UIKit                               0x0000000121f59f1b -[UITableViewAccessibility deleteRowsAtIndexPaths:withRowAnimation:] + 48
    6   MedConf                             0x000000010f1cebb3 _TFC7MedConf28FavoritesTableViewController9tableViewfS0_FTCSo11UITableView18commitEditingStyleOSC27UITableViewCellEditingStyle17forRowAtIndexPathCSo11NSIndexPath_T_ + 195
    7   MedConf                             0x000000010f1ced87 _TToFC7MedConf28FavoritesTableViewController9tableViewfS0_FTCSo11UITableView18commitEditingStyleOSC27UITableViewCellEditingStyle17forRowAtIndexPathCSo11NSIndexPath_T_ + 87
    8   UIKit                               0x0000000111b56226 -[UITableView animateDeletionOfRowWithCell:] + 132
    9   UIKit                               0x0000000111b353fd __52-[UITableView _swipeActionButtonsForRowAtIndexPath:]_block_invoke + 72
    10  UIKit                               0x0000000111a54da2 -[UIApplication sendAction:to:from:forEvent:] + 75
    11  UIKit                               0x0000000111b6654a -[UIControl _sendActionsForEvents:withEvent:] + 467
    12  UIKit                               0x0000000111b65919 -[UIControl touchesEnded:withEvent:] + 522
    13  UIKit                               0x0000000111dffa10 _UIGestureRecognizerUpdate + 9487
    14  UIKit                               0x0000000111aa1686 -[UIWindow _sendGesturesForEvent:] + 1041
    15  UIKit                               0x0000000111aa22b2 -[UIWindow sendEvent:] + 666
    16  UIKit                               0x0000000111a68581 -[UIApplication sendEvent:] + 246
    17  UIKit                               0x0000000111a75d1c _UIApplicationHandleEventFromQueueEvent + 18265
    18  UIKit                               0x0000000111a505dc _UIApplicationHandleEventQueue + 2066
    19  CoreFoundation                      0x00000001113e2431 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    20  CoreFoundation                      0x00000001113d82fd __CFRunLoopDoSources0 + 269
    21  CoreFoundation                      0x00000001113d7934 __CFRunLoopRun + 868
    22  CoreFoundation                      0x00000001113d7366 CFRunLoopRunSpecific + 470
    23  GraphicsServices                    0x0000000113603a3e GSEventRunModal + 161
    24  UIKit                               0x0000000111a53900 UIApplicationMain + 1282
    25  MedConf                             0x000000010f1debf7 main + 135
    26  libdyld.dylib                       0x0000000113bff145 start + 1
    27  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

I'm not sure what the cause of the exception is, as it gets thrown even if I comment out line where I remove the object from the data source: ObjectManager.removeFavoriteObject(favorites[indexPath.row]). Below is the removeFavoriteObject method.

static func removeFavoriteObject( objectToRemove: PFObject ) {

    if contains(favorites, objectToRemove) {
        if let favObject = objectToRemove as? Attendee {
            favObject.isFavorite = false
        } else if let favObject = objectToRemove as? Event {
            favObject.isFavorite = false
        } else if let favObject = objectToRemove as? Session {
            favObject.isFavorite = false
        }
    }

    favorites.remove(objectToRemove)
}

Answer 1

try passing index of object to that removeFavoriteObject static method:

ObjectManager.removeFavoriteObject(indexPath.row)

and as you are passing element index from favourites array, it must be there. So no need to check using 'if'.

static func removeFavoriteObject( index: Int ) {

   var objectToRemove = favorites[index]
   if let favObject = objectToRemove as? Attendee {
       favObject.isFavorite = false
   }
   else if let favObject = objectToRemove as? Event {
       favObject.isFavorite = false
   }
   else if let favObject = objectToRemove as? Session {
       favObject.isFavorite = false
   }

   favorites.removeAtIndex(index)
}

Answer 2

tableview.beginUpdates()
tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
tableview.endUpdates()

Hope this helps

Answer 3

Try this

static func removeFavoriteObject( objectToRemove: PFObject ) {

if contains(favorites, objectToRemove) {
    favorites.remove(objectToRemove)
    if let favObject = objectToRemove as? Attendee {
        favObject.isFavorite = false
    } else if let favObject = objectToRemove as? Event {
        favObject.isFavorite = false
    } else if let favObject = objectToRemove as? Session {
        favObject.isFavorite = false
    }
}
}