UICollectionViewDropDelegate breaks reordering cell mechanics in UICollectionView - ios

Question

I'm trying to implement two collectionViews with drag and drop cells between them. But i faced strange behaviour with reordering cells inside collectionView. Here is minimal code that replicate that behaviour.
Provided code works as expected but then i uncomment collectionView.dropDelegate = self it doesn't work anymore. I tried to find which method of UICollectionViewDropDelegate is called but none of them is called.
class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDropDelegate {
func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
print("in drop")
}
#IBOutlet weak var collectionView: UICollectionView!
var items = [UIColor.red, UIColor.green, UIColor.blue]
override func viewDidLoad() {
super.viewDidLoad()
// collectionView.dropDelegate = self
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return items.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
cell.contentView.layer.backgroundColor = items[indexPath.item].cgColor
return cell
}
func collectionView(_ collectionView: UICollectionView, canMoveItemAt indexPath: IndexPath) -> Bool {
return true
}
func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
let temp = items.remove(at: sourceIndexPath.item)
items.insert(temp, at: destinationIndexPath.item)
}
#IBAction func handleLongGesture(_ gesture: UILongPressGestureRecognizer) {
switch gesture.state {
case .began:
guard let selectedIndexPath = self.collectionView.indexPathForItem(at: gesture.location(in: self.collectionView)) else {
break
}
self.collectionView.beginInteractiveMovementForItem(at: selectedIndexPath)
case .changed:
self.collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: gesture.view!))
case .ended:
self.collectionView.endInteractiveMovement()
default:
self.collectionView.cancelInteractiveMovement()
}
}
}
example without dragDelegate
example with dragDelegate
So this it happens and what i should do to get normal behaviour with dragDelegate?

Answer 1

It's a bit ugly, but I had this problem and the simplest thing I could find that works is to disable the dropDelegate when your gesture recognizer is invoked.
#IBAction func handleLongGesture(_ gesture: UILongPressGestureRecognizer) {
switch gesture.state {
case .began:
guard let selectedIndexPath = self.collectionView.indexPathForItem(at: gesture.location(in: self.collectionView)) else {
break
}
self.collectionView.dropDelegate = nil // Drop delegate behaves VERY badly with beginInteractiveMovementForItem
self.collectionView.beginInteractiveMovementForItem(at: selectedIndexPath)
case .changed:
self.collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: gesture.view!))
case .ended:
self.collectionView.endInteractiveMovement()
self.collectionView.dropDelegate = self
default:
self.collectionView.cancelInteractiveMovement()
self.collectionView.dropDelegate = self
}
}