Issue deleting cells from UICollectionViewController

Question

I am using UICollectionViewController and I am trying to use deleteItems in a loop of selected items like so:

for item in (collectionView?.indexPathsForSelectedItems)!
        {

            let cell = self.collectionView?.cellForItem(at: item) as! ImageCollectionCell

            deleteLandGradingImage(images: ImagesData(jobNo: self.jobNo, ImageBytes: (self.array[item.item]["imageBytes"] as! String), Username: appDelegate.username)) { result in

                cell.imageView.layer.borderWidth = 0
                self.navigationItem.rightBarButtonItems = [self.editButton]
                self.editButton.isEnabled = true
                self.editButton.title = "Edit"
                self.doneButton.title = "Done"

                self.array.remove(at: item.item)
                self.collectionView?.deselectItem(at: item, animated: true)
                self.collectionView?.deleteItems(at: [item])

            }

        }


deleteLandGradingImage(images: ImagesData(jobNo: self.jobNo, ImageBytes: (self.array[item.item]["imageBytes"] as! String), Username: appDelegate.username)) { result in
}

Is a call to an API that deletes the image inside the cell and removes it from the database.

func deleteLandGradingImage(images: ImagesData, completionHandler:@escaping (_ result:Bool) -> Void) {

        //Define array for returning data

        var returnedResults = Bool()

        //Call API

        WebService().deleteLandGradingImages(images: images)
        {
            (result: Bool) in

            DispatchQueue.main.async {

                //Return our results

                returnedResults = result
                completionHandler(returnedResults)

            }

        }

    }

and here is the method in WebService:

func deleteLandGradingImages(images: ImagesData, completion: @escaping (_ result: Bool) -> Void)
    {

        var jsonDict = [AnyHashable: Any]()

        //Set Lot

        jsonDict["jobNo"] = images.jobNo

        //Set Image

        jsonDict["imageBytes"] = images.ImageBytes

        let jsonData: Data? = try? JSONSerialization.data(withJSONObject: jsonDict, options: .prettyPrinted)

        let urlComponents = NSURLComponents(string: webservice + "DeleteGradingImages");

        //Assign Username

        urlComponents?.user = appDelegate.username;

        //Assign Password

        urlComponents?.password = appDelegate.password;

        //Define our URL String

        let url = urlComponents?.url;

        //Define URL Request

        var request = URLRequest(url: url!)

        //Set Header Values for request

        request.setValue("application/json", forHTTPHeaderField: "Content-Type")

        //Set Header Values for request

        request.setValue("application/json", forHTTPHeaderField: "Accept")

        //Set Request Method to POST

        request.httpMethod = "POST"

        //Set Request Body to JSON Data

        request.httpBody = jsonData

        Alamofire.request(request)
            .responseJSON { response in

                if(response.error != nil)
                {
                    completion(false)
                }
                else
                {
                    let responseString = (String(data: response.data!, encoding: .utf8) != nil) as Bool
                    completion(responseString)
                }

        }
    }

My problem with the loop is that sometimes it works, sometimes I get this error:

Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (1) must be equal to the number of items contained in that section before the update (1), plus or minus the number of items inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).

Sometimes I get this error:

Thread 1: Fatal error: Index out of range

on this line

self.array.remove(at: item.item)

What am I doing wrong?

UPDATE

I got this from an answer:

@IBAction func deleteButtonPressed(_ sender: Any) {

        let group = DispatchGroup()

        let indexPaths = (collectionView?.indexPathsForSelectedItems)!

        var deletes = Array<Dictionary<String, Any>>()

        for item in indexPaths {
            deletes.append(self.array[item.item])
        }

        for item in deletes {

            group.enter()

            deleteLandGradingImage(images: ImagesData(jobNo: self.jobNo, ImageBytes: (item["imageBytes"] as! String), Username: appDelegate.username)) { result in

                let i = self.array.index { (dic) -> Bool in

                    return true

                }
                if i != nil {
                    self.array.remove(at: i!)
                    group.leave()

                }

            }

        }

        group.notify(queue: .main) {
            for item in indexPaths {
                let cell = self.collectionView?.cellForItem(at: item) as! ImageCollectionCell
                cell.imageView.layer.borderWidth = 0
                self.collectionView?.deselectItem(at: item, animated: true)
            }
            self.navigationItem.rightBarButtonItems = [self.editButton]
            self.editButton.isEnabled = true
            self.editButton.title = "Edit"
            self.doneButton.title = "Done"
            self.collectionView?.deleteItems(at: indexPaths)

            if(self.array.count == 0)
            {
                self.imageCollectionDelegate?.ImageCollectionChange(false)
            }

        }

    }

But sometimes I get an error on this line:

let cell = self.collectionView?.cellForItem(at: item) as! ImageCollectionCell

This is the error:

Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value

I noticed this only happens on iPhone and if I select images and scroll through the UICollectionView, then click the delete button.

UPDATE

I have also tried this:

@IBAction func deleteButtonPressed(_ sender: Any) {

        self.createIndicator()

        for item in (collectionView?.indexPathsForSelectedItems)! {

            if let cell = self.collectionView?.cellForItem(at: item) as! ImageCollectionCell? {
                deleteLandGradingImage(images: ImagesData(jobNo: self.jobNo, ImageBytes: (self.array[item.item]["imageBytes"] as! String), Username: appDelegate.username)) { result in

                    cell.imageView.layer.borderWidth = 0

                    self.collectionView?.performBatchUpdates({
                        self.array.remove(at: item.item)
                        self.collectionView?.deselectItem(at: item, animated: true)
                        self.collectionView?.deleteItems(at: [item])
                    }) { completed in
                        self.navigationItem.rightBarButtonItems = [self.editButton]
                        self.editButton.isEnabled = true
                        self.editButton.title = "Edit"
                        self.doneButton.title = "Done"
                        self.stopIndicator()

                    }
                }
            }
        }

    }

But I get this error:

Thread 1: Fatal error: Index out of range

On this line:

self.array.remove(at: item.item)

Answer 1

There are two problems:

1. Updating collectionView in loop which uses collectionView?.indexPathsForSelectedItems
2. Calling collectionView?.deleteItems(at: [item]) too often

Best choice is to perform network requests for all indexes and then remove them from collectionView in one step. You can combine requests in one chain with DispatchGroup.

Some explanation: Removing two items. 1 and 2. Sometimes 1 will be removed first, and after that indexPathsForSelectedItems will contains just one index(1). But loop will try to remove item at indexPath 2. It's a crash.

Sample of using DispatchGroup with your code:

let group = DispatchGroup()
let indexPaths = (collectionView?.indexPathsForSelectedItems)!
var deletes = [<your type>]()
for item in indexPaths {
    deletes.append(array[item.item])
}
// TODO: Block UI with UIActivityIndicatorView
for item in deletes {
    group.enter()
    deleteLandGradingImage(images: ImagesData(jobNo: self.jobNo, ImageBytes: (item["imageBytes"] as! String), Username: appDelegate.username)) { result in
        let i = self.array.index(of: item)!
        self.array.remove(at: i)
        group.leave()
    }
}

group.notify(queue: .main) {
    self.navigationItem.rightBarButtonItems = [self.editButton]
    self.editButton.isEnabled = true
    self.editButton.title = "Edit"
    self.doneButton.title = "Done"
    self.collectionView?.deleteItems(at: indexPaths)
}