Uicollectionview + Nsfetchedresultscontroller in Swift 3

UICollectionView + NSFetchedResultsController in Swift 3

Thank you Pawel, your Gist was very helpful. I forked it and updated it to fully support Swift 3 and iOS10. Since it doesn't solved the problem with updating the UICollectionView from a background thread.

The error is caused by updating the UICollectionView from a background thread.

This application is modifying the autolayout engine from a background thread after the engine was accessed from the main thread.

To get a combination of NSFetchedResultsController and UICollectionView working you have to wrap every update of the collectionView object inside

DispatchQueue.main.async {
    // i.e. insertion of new items                          
    this.collectionView!.insertItems(at: [newIndexPath!])
}

Here is a link to the full updated Gist

NSFetchedResultsContollerDelegate for CollectionView

Combining a fetched results controller with a collection view is a bit tricky.
The problem is explained in

• http://ashfurrow.com/blog/how-to-use-nsfetchedresultscontroller-with-uicollectionview

If you're looking for how to get around the
NSInternalInconsistencyException runtime exception with
UICollectionView, I have an example on GitHub detailing how to queue
updates from the NSFetchedResultsControllerDelegate.

The problem is that the existing UITableView class uses beginUpdates
and endUpdates to submit batches to the table view. UICollectionView
has a new performBatchUpdates: method, which takes a block parameter
to update the collection view. That's sexy, but it doesn't work well
with the existing paradigm for NSFetchedResultsController.

Fortunately, that article also provides a sample implementation:

• https://github.com/AshFurrow/UICollectionView-NSFetchedResultsController

From the README:

This is an example of how to use the new UICollectionView with
NSFetchedResultsController. The trick is to queue the updates made
through the NSFetchedResultsControllerDelegate until the controller
finishes its updates. UICollectionView doesn't have the same
beginUpdates and endUpdates that UITableView has to let it work easily
with NSFetchedResultsController, so you have to queue them or you get
internal consistency runtime exceptions.

Using UICollectionView with CoreData and NSFetchedResultsController

Sounds like an issue I had a while ago, when I was using objective C.

Check out

UICollectionView Assertion failure

I had to do a work around to get my FRC to work with the collection view.

Seems like you found the same git as me, here is the fix that solves the collection view issue.

https://github.com/AshFurrow/UICollectionView-NSFetchedResultsController/issues/13

Reorder cells in UICollectionView + NSFetchedResultsController

As you've found, you can't just tell NSFetchedResultsController to change the order of objects in its fetchedObjects array. That array is sorted based on your sort descriptor, and is read-only.

So you have two possible options:

1. Change the property you're using in the sort descriptor, so that the new sort result will match the result of the drag and drop operation. Save changes, and NSFetchedResultsController delegate methods will fire. You've changed the order and your fetched results controller will reflect that in its fetchedObjects array.
2. Don't use NSFetchedResultsController, just do a fetch and save the results in your own array. Re-order the items in your own array but don't change anything that Core Data cares about.

UICollectionView Cells Handled by NSFetchedResultsController not Repopulating Images

I experienced an issue in the NSFetchedResultsController behaviour. If NSPredicate is set on its fetch request, the delegate methods are not called.

Try to remove the predicate to see if you have this problem too.

I'm trying to see if it's a bug in Xcode, or some unhanded error related to faults, or if the new query token system is used in the NSFetchedResultsController class and we need to do something about it.

NSFetchedResultsController per cell in a collectionView

Since we require separate queries in each cell, we ended up setting separate NSFetchedResultsController in each cell when it's dequeued. And then have the NSFetchedResultsController set as nil when prepareForReuse.

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