Some UICollectionViewDropDelegateMethods get not called - swift

Question

I have an iPad application where you can drag images from Safari into a collection view. I also want to to drag and drop within the collection view to rearrange the collection View cells.
The drag and drop from safari is working. Dragging a cell within the collection view is also possible, but dropping is not possible. I took a look at the UICollectionViewDropDelegate methods "canLoadObjects", "sessionDidUpdate" and "performDrop" which get called in this order. I made another simple collection view app with drag and drop, so I know what to do.
I recognized that in the simple app, when I drag the cell onto another place "canLoadObjects" get called. I confirmed that with a breakpoint. In the other app this does not happen. Instead only "performDrop" gets called, but not "canLoadObjects" and "sessionDidUpdate". I set the delegate correctly (otherwise "performDrop would not get called either).
Does anybody know why some delegate methods doesn't get called? Thanks!
import UIKit
class ImageGalleryViewController: UIViewController {
var cellWidthScale: CGFloat = 1.0
var currentCellSize = CGSize()
var imageFetcher : ImageFetcher!
var loadAllImages = false
var tappedCellIndex = 0
#IBOutlet weak var collectionView: UICollectionView!{
didSet{
collectionView.delegate = self
collectionView.dataSource = self
collectionView.dragInteractionEnabled = true
collectionView.dragDelegate = self
collectionView.dropDelegate = self
collectionView.addGestureRecognizer(UIPinchGestureRecognizer(target: self, action: #selector(adjustCellWidth(with:))))
}
}
var userIsPinching = false
var imageDataTuple : [(NSURL?, CGSize)]? {
didSet{
if loadAllImages == true{
collectionView?.reloadData()
loadAllImages = false
}
}
}
override func viewDidLoad() {
super.viewDidLoad()
collectionView.register(UINib(nibName: "ImageCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "ImageCell")
}
#objc func adjustCellWidth(with recognizer: UIPinchGestureRecognizer){
switch recognizer.state {
case .changed:
userIsPinching = true
cellWidthScale *= recognizer.scale
recognizer.scale = 1.0
collectionView.collectionViewLayout.invalidateLayout()
default: break
}
}
}
//MARK: UICollectionViewDelegate
extension ImageGalleryViewController: UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout{
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return imageDataTuple?.count ?? 0
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ImageCell", for: indexPath)
if let imageCell = cell as? ImageCollectionViewCell{
imageCell.imageView.image = nil
imageCell.spinner.startAnimating()
if let url = imageDataTuple?[indexPath.row].0{
imageCell.imageUrl = url
//never fetch data on the main queue
DispatchQueue.global(qos: .userInitiated).async {
let urlContent = try? Data(contentsOf: (url as URL).imageURL)
//update the UI always on the main queue
DispatchQueue.main.async {
//check if the url is really the one we want to show
if let imageData = urlContent, url == imageCell.imageUrl, let image = UIImage(data: imageData){
imageCell.spinner.stopAnimating()
imageCell.imageView.image = image
}
else{
imageCell.spinner.stopAnimating()
imageCell.imageView.image = UIImage(systemName: "xmark.octagon.fill")
imageCell.imageView.tintColor = UIColor.red
imageCell.imageView.sizeToFit()
}
}
}
}
}
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
if let checkedImageData = imageDataTuple{
let imageHeightToWidth = checkedImageData[indexPath.row].1.height/checkedImageData[indexPath.row].1.width
let calculatedWidth = (self.view.frame.width/4) * cellWidthScale
currentCellSize = CGSize(width: calculatedWidth, height: calculatedWidth * imageHeightToWidth)
return currentCellSize
}
return CGSize.zero
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
tappedCellIndex = indexPath.item
performSegue(withIdentifier: "ShowImage", sender: self)
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "ShowImage"{
if let destVC = segue.destination as? DetailImageViewController{
destVC.imageUrl = imageDataTuple![tappedCellIndex].0
}
}
}
}
//MARK: UICollectionViewDropDelegate
extension ImageGalleryViewController: UICollectionViewDropDelegate{
//1. Can the dropDelegate handle the object?
func dropInteraction(_ interaction: UIDropInteraction, canHandle session: UIDropSession) -> Bool {
//requirement: only drop with image and url should be handled
return session.canLoadObjects(ofClass: NSURL.self)
}
//2. update the session. Return a propsal in which manner the object schould be dropped (.copy; .move...)
func dropInteraction(_ interaction: UIDropInteraction, sessionDidUpdate session: UIDropSession) -> UIDropProposal {
//Es wird überprüft, ob der collectionView auf dem gedroppt werden soll der ist, von dem das DragItem stammt
let isSelf = (session.localDragSession?.localContext as? UICollectionView) == collectionView
let dropOperation: UIDropOperation = isSelf ? .move : .copy
//intent: Soll eine neue Cell erstellt werden oder in die eingefügt werden, über der man sich gerde befindet?
return UICollectionViewDropProposal(operation: dropOperation, intent: .insertAtDestinationIndexPath)
}
//3. perform the actual drop. update UI and the model
func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
let destinationIntexPath = coordinator.destinationIndexPath ?? IndexPath(row: 0, section: 0)
for item in coordinator.items{
//Is the drop from the same collectionView or external? internal --> != nil
if let sourceIndexPath = item.sourceIndexPath{
if let url = item.dragItem.localObject as? NSURL{
collectionView.performBatchUpdates {
imageDataTuple?.remove(at: sourceIndexPath.item)
imageDataTuple?.insert((url, currentCellSize), at: destinationIntexPath.item)
collectionView.deleteItems(at: [sourceIndexPath])
collectionView.insertItems(at: [destinationIntexPath])
}
coordinator.drop(item.dragItem, toItemAt: destinationIntexPath)
}
}
//external --> nil
else{
var dataTupel: (NSURL?, CGSize?) {
didSet{
DispatchQueue.main.async {
if let controllerTitel = self.title, let presentImageSize = dataTupel.1, let presentURL = dataTupel.0{
if ImageGalleryModel.imageURLs[controllerTitel] != nil {
ImageGalleryModel.imageURLs[controllerTitel]!.append((presentURL, presentImageSize))
print(ImageGalleryModel.imageURLs)
self.imageDataTuple = ImageGalleryModel.imageURLs[controllerTitel]!
self.collectionView.insertItems(at: [IndexPath(item: ImageGalleryModel.imageURLs[controllerTitel]!.count - 1, section: 0)])
}
}
}
}
}
item.dragItem.itemProvider.loadObject(ofClass: NSURL.self) { provider, error in
dataTupel.0 = provider as? NSURL
}
item.dragItem.itemProvider.loadObject(ofClass: UIImage.self) { provider, error in
if let receivedImage = provider as? UIImage{
dataTupel.1 = receivedImage.size
}
}
}
}
}
}
//MARK: UICollectionViewDragDelegate
extension ImageGalleryViewController: UICollectionViewDragDelegate{
//Proveide inital drag items
func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
return dragItem(at: indexPath)
}
//Provide drag items which were added while dragging
func collectionView(_ collectionView: UICollectionView, itemsForAddingTo session: UIDragSession, at indexPath: IndexPath, point: CGPoint) -> [UIDragItem] {
session.localContext = collectionView
return dragItem(at: indexPath)
}
private func dragItem(at indexPath: IndexPath) -> [UIDragItem]{
if let image = (collectionView.cellForItem(at: indexPath) as? ImageCollectionViewCell)?.imageView.image{
let dragItem = UIDragItem(itemProvider: NSItemProvider(object: image))
dragItem.localObject = image
return [dragItem]
}
return []
}
}
//MARK: New selected Gallery
extension ImageGalleryViewController: GalleryListViewControllerDelegate{
func gallerySelected(name: String) {
loadAllImages = true
imageDataTuple = ImageGalleryModel.imageURLs[name]
self.title = name
}
}

Answer 1

The problem was that the auto correction chose delegate methods of the UIDropInteraction-delegate and not from the UICollectionViewDropDelegate. Just picked the correct methods and everything works as intended.