UICollectionView + NSFetchedResultsController Swift 3 / iOS 10

UICollectionView + NSFetchedResultsController.swift

    var _fetchedResultsController: NSFetchedResultsController<Entity>? = nil
    var blockOperations: [BlockOperation] = []

    var fetchedResultController: NSFetchedResultsController<Entity> {
        if _fetchedResultsController != nil {
            return _fetchedResultsController!
        }
        
        let fetchRequest: NSFetchRequest<Entity> = Entity.fetchRequest()
        let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).managedObjectContext!
        
        fetchRequest.predicate = NSPredicate(format: "...")
        
        // sort by item text
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "...", ascending: true)]
        let resultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)
        
        resultsController.delegate = self;
        _fetchedResultsController = resultsController
        
        do {
            try _fetchedResultsController!.performFetch()
        } catch {
            let nserror = error as NSError
            fatalError("Unresolved error \(nserror)")
        }        
        return _fetchedResultsController!
    } 
    
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
        
        if type == NSFetchedResultsChangeType.insert {
            print("Insert Object: \(newIndexPath)")
            
            if (collectionView?.numberOfSections)! > 0 {
                
                if collectionView?.numberOfItems( inSection: newIndexPath!.section ) == 0 {
                    self.shouldReloadCollectionView = true
                } else {
                    blockOperations.append(
                        BlockOperation(block: { [weak self] in
                            if let this = self {
                                DispatchQueue.main.async {
                                    this.collectionView!.insertItems(at: [newIndexPath!])
                                }
                            }
                            })
                    )
                }
                
            } else {
                self.shouldReloadCollectionView = true
            }
        }
        else if type == NSFetchedResultsChangeType.update {
            print("Update Object: \(indexPath)")
            blockOperations.append(
                BlockOperation(block: { [weak self] in
                    if let this = self {
                        DispatchQueue.main.async {
                            
                            this.collectionView!.reloadItems(at: [indexPath!])
                        }
                    }
                    })
            )
        }
        else if type == NSFetchedResultsChangeType.move {
            print("Move Object: \(indexPath)")
            
            blockOperations.append(
                BlockOperation(block: { [weak self] in
                    if let this = self {
                        DispatchQueue.main.async {                          
                            this.collectionView!.moveItem(at: indexPath!, to: newIndexPath!)
                        }
                    }
                    })
            )
        }
        else if type == NSFetchedResultsChangeType.delete {
            print("Delete Object: \(indexPath)")
            if collectionView?.numberOfItems( inSection: indexPath!.section ) == 1 {
                self.shouldReloadCollectionView = true
            } else {
                blockOperations.append(
                    BlockOperation(block: { [weak self] in
                        if let this = self {
                            DispatchQueue.main.async {                     
                                this.collectionView!.deleteItems(at: [indexPath!])
                            }
                        }
                        })
                )
            }        
        }
    }
    
    public func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {       
        if type == NSFetchedResultsChangeType.insert {
            print("Insert Section: \(sectionIndex)")     
            blockOperations.append(
                BlockOperation(block: { [weak self] in
                    if let this = self {
                        DispatchQueue.main.async {               
                            this.collectionView!.insertSections(NSIndexSet(index: sectionIndex) as IndexSet)
                        }
                    }
                    })
            )
        }
        else if type == NSFetchedResultsChangeType.update {
            print("Update Section: \(sectionIndex)")
            blockOperations.append(
                BlockOperation(block: { [weak self] in
                    if let this = self {
                        DispatchQueue.main.async {           
                            this.collectionView!.reloadSections(NSIndexSet(index: sectionIndex) as IndexSet)
                        }
                    }
                    })
            )
        }
        else if type == NSFetchedResultsChangeType.delete {
            print("Delete Section: \(sectionIndex)")  
            blockOperations.append(
                BlockOperation(block: { [weak self] in
                    if let this = self {
                        DispatchQueue.main.async {          
                            this.collectionView!.deleteSections(NSIndexSet(index: sectionIndex) as IndexSet)
                        }
                    }
                    })
            )
        }
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        
        // Checks if we should reload the collection view to fix a bug @ http://openradar.appspot.com/12954582
        if (self.shouldReloadCollectionView) {
            DispatchQueue.main.async {
                self.collectionView.reloadData();
            }
        } else {
            DispatchQueue.main.async {
                self.collectionView!.performBatchUpdates({ () -> Void in
                    for operation: BlockOperation in self.blockOperations {
                        operation.start()
                    }
                    }, completion: { (finished) -> Void in
                        self.blockOperations.removeAll(keepingCapacity: false)
                })
            }
        }
    }
    
    deinit {
        for operation: BlockOperation in blockOperations {
            operation.cancel()
        }
        blockOperations.removeAll(keepingCapacity: false)
    }

This is life saver! I've spent three weeks hunting this bug. Thank you @nor0x!

Why is there no animation when inserting, updating, or deleting cells into the collation? What is shouldReloadCollectionView?

The reason why there are no animations in this case is, because shouldReloadCollectionView will be set to true when the first element is inserted. It will never be set back to false. So all those nice block operations will actually never be executed. Instead, in all cases (insert, update, move, delete) a complete reload is performed on the UICollectionView, and this happens without animation.

Please help, i'm getting this error
'Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (6) must be equal to the number of items contained in that section before the update (7), plus or minus the number of items inserted or deleted from that section (0 inserted, 0 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'

I am having the same issue as tobitech. CollectionView with 2 sections, 0, and 1. Section 0 contains a header and one cell that never changes. Section 1 consists of 0 or more cells. Inserts and deletes only happen for Section 1. IndexPath in controller controller: didChange anObject: at indexPath: IndexPath? for type: newIndexPath: always returns [0,0] as indexPath for inserts and deletes, leading to essentially the same error as above because the change is not to the cell at [0,0] but always to a cell at [1,X]_ Thoughts?

what is shouldReloadCollectionView ?

The whole point of having NSFetchedResultsController is to avoid reload and have automatic updates. Isn't it? Am I missing something?