UICollectionview - blink when move item - swift

Question

I want to reorder my cells in my UICollectionView. But when I drop my item, the "cellForItemAt" method is called and this will cause the cell to flash (See image below).
What should I do to avoid this behavior ?
Thank you in advance for your help.
class ViewController: UIViewController {
#IBOutlet weak var collectionView: UICollectionView!
private let cellIdentifier = "cell"
private let cells = [""]
private var longPressGesture: UILongPressGestureRecognizer!
override func viewDidLoad() {
super.viewDidLoad()
longPressGesture = UILongPressGestureRecognizer(target: self, action: #selector(self.handleLongGesture(gesture:)))
collectionView.addGestureRecognizer(longPressGesture)
}
//Selectors
#objc func handleLongGesture(gesture: UILongPressGestureRecognizer) {
switch(gesture.state) {
case .began:
guard let selectedIndexPath = collectionView.indexPathForItem(at: gesture.location(in: collectionView)) else {
break
}
collectionView.beginInteractiveMovementForItem(at: selectedIndexPath)
case .changed:
collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: gesture.view!))
case .ended:
collectionView.endInteractiveMovement()
default:
collectionView.cancelInteractiveMovement()
}
}
}
// MARK: - UICollectionViewDataSource
extension ViewController: UICollectionViewDataSource {
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 10
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellIdentifier, for: indexPath)
return cell
}
func collectionView(_ collectionView: UICollectionView, canMoveItemAt indexPath: IndexPath) -> Bool {
return true
}
func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
}
}
// MARK: - UICollectionViewDelegateFlowLayout
extension ViewController: UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: 100, height: 100)
}
}

Answer 1

You need to call endInteractiveMovement in perfomBatchUpdates.
But whenever endInteractiveMovement triggered, cellForRow called. So cell will be refreshed and new cell will added(check with random color extension). To secure that, you need to save selectedCell in variable. And return that cell when endInteractiveMovement called.
Declare currentCell in ViewController
var isEnded: Bool = true
var currentCell: UICollectionViewCell? = nil
Store selected cell in variable when gesture began & call endInteractiveMovement in performBatchUpdates.
So, your handleLongGesture func look like below:
//Selectors
#objc func handleLongGesture(gesture: UILongPressGestureRecognizer) {
switch(gesture.state) {
case .began:
guard let selectedIndexPath = collectionView.indexPathForItem(at: gesture.location(in: collectionView)) else {
break
}
isEnded = false
//store selected cell in currentCell variable
currentCell = collectionView.cellForItem(at: selectedIndexPath)
collectionView.beginInteractiveMovementForItem(at: selectedIndexPath)
case .changed:
collectionView.updateInteractiveMovementTargetPosition(gesture.location(in: gesture.view!))
case .ended:
isEnded = true
collectionView.performBatchUpdates({
self.collectionView.endInteractiveMovement()
}) { (result) in
self.currentCell = nil
}
default:
isEnded = true
collectionView.cancelInteractiveMovement()
}
}
Also need to change cellForRow
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
if currentCell != nil && isEnded {
return currentCell!
} else {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellIdentifier, for: indexPath)
cell.backgroundColor = .random
return cell
}
}
TIP
Use random color extension for better testing
extension UIColor {
public class var random: UIColor {
return UIColor(red: CGFloat(drand48()), green: CGFloat(drand48()), blue: CGFloat(drand48()), alpha: 1.0)
}
}
EDIT
If you have multiple sections.
Lets take array of array
var data: [[String]] = [["1","2"],
["1","2","3","4","5","6","7"],
["1","2","3","4","5","6","7","8","9","10","11","12","13","14","15"]]
Then you need to maintain data when reordering
func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
print("\(sourceIndexPath) -> \(destinationIndexPath)")
let movedItem = data[sourceIndexPath.section][sourceIndexPath.item]
data[sourceIndexPath.section].remove(at: sourceIndexPath.item)
data[destinationIndexPath.section].insert(movedItem, at: destinationIndexPath.item)
}

Answer 2

You can try to call
collectionView.reloadItems(at: [sourceIndexPath, destinationIndexPath])
right after all your updates (drag and drop animation) are done.
For example call it after performBatchUpdates. It will remove blinking.