Deleting cells from UICollectionView via NSNotification

Question

I have a simple UICollectionView based app - one UICollectionView and a NSMutableArray based data model for simplicity.

I can delete cells with no problem via the didSelectItemAtIndexPath: delegate method:

-(void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath{
    [self.data removeObjectAtIndex:[indexPath row]];
    [self.collectionView deleteItemsAtIndexPaths:@[indexPath]];
}

However, I'm trying to add a delete option via a UIMenuController in a UICollectionViewCell subclass which is triggered via a UILongPressGestureRecognizer which all works fine and I successfully trigger an NSNotification

-(void)delete:(id)sender{
      NSLog(@"Sending deleteme message");
      [[NSNotificationCenter defaultCenter] postNotificationName:@"DeleteMe!" object:self userInfo:nil];
}

I catch it in my ViewController and call the following method:

-(void)deleteCell:(NSNotification*)note{
       MyCollectionViewCell *cell = [note object];
       NSIndexPath *path = nil;
       if((path = [self.collectionView indexPathForCell:cell]) != nil){
           [self.data removeObjectAtIndex:[path row]];
           [self.collectionView deleteItemsAtIndexPaths:@[path]];
       }
}

And it crashes on the deleteItemsAtIndexPaths: call

-[UICollectionViewUpdateItem action]: unrecognized selector sent to instance 0xee7eb10

I've checked everything obvious - like the object from NSNotification and the indexPath created from the indexPathForCell: call and it all seems totally fine. It seems like I'm calling deleteItemsAtIndexPath: with the same information in both places, but for some reason it fails when it goes via the notification route.

This is the info at the address given in the error:

(lldb) po 0xee7eb10
(int) $1 = 250080016 <UICollectionViewUpdateItem: 0xee7eb10> index path before update (<NSIndexPath 0x9283a20> 2 indexes [0, 0]) index path after update ((null)) action (delete)

Perhaps the index path after update being null is significant...

Any ideas?

Answer 1

I found a crude but working workaround, and it even checks if action is already implemented in a future release (better than a category)

// Fixes the missing action method when the keyboard is visible
#import <objc/runtime.h>
#import <objc/message.h>
__attribute__((constructor)) static void PSPDFFixCollectionViewUpdateItemWhenKeyboardIsDisplayed(void) {
    @autoreleasepool {
    if ([UICollectionViewUpdateItem class] == nil) return; // pre-iOS6.
    if (![UICollectionViewUpdateItem instancesRespondToSelector:@selector(action)]) {
            IMP updateIMP = imp_implementationWithBlock(^(id _self) {});
            Method method = class_getInstanceMethod([UICollectionViewUpdateItem class], @selector(action));
            const char *encoding = method_getTypeEncoding(method);
            if (!class_addMethod([UICollectionViewUpdateItem class], @selector(action), updateIMP, encoding)) {
                NSLog(@"Failed to add action: workaround");
            }
        }
    }
}

Edit: Added check for iOS5.
Edit2: We're shipping that in lots of commercial projects (http://pspdfkit.com) and it works great.

Answer 2

You could pass in the NSIndexPath of the selected cell in the userInfo dictionary of your notification. You could set that in the tag of your custom cell when it is created.

Answer 3

I had the same issue. My app would crash when I tried to insert a cell into my UICollectionView after I had selected text in a UITextField which was placed inside a collectionViewCell. My fix was to resign the first responder before the insertion happened. Since I was using a NSFetchedResultsController to handle the updating I put this at the beginning of controllerWillChangeContent:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [[UIApplication sharedApplication].keyWindow findAndResignFirstResponder];
    ...
}

I found findAndResignFirstResponder from this SO answer