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ViewsWeird delete items animation when using basic UICollectionView with Flow Layout
I ran into a problem when using the simplest UICollectionView and UICollectionViewFlowLayout.
The collection itself works fine, but when the cell is removed, there are problems with the animation.
Here is a code example that demonstrates the problem:
class Cell: UICollectionViewCell { }
class MyViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
var items = Array(repeating: [UIColor.red, .blue, .yellow, .cyan, .magenta, .green], count: 100).flatMap { $0 }
lazy var collectionView: UICollectionView = {
let collectionViewLayout = UICollectionViewFlowLayout()
collectionViewLayout.scrollDirection = .vertical
collectionViewLayout.minimumLineSpacing = 10
collectionViewLayout.minimumInteritemSpacing = 10
let collectionView = UICollectionView(frame: .zero, collectionViewLayout: collectionViewLayout)
collectionView.translatesAutoresizingMaskIntoConstraints = false
collectionView.register(Cell.self, forCellWithReuseIdentifier: "Cell")
collectionView.delegate = self
collectionView.dataSource = self
return collectionView
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(collectionView)
NSLayoutConstraint.activate([
.init(item: collectionView, attribute: .leading, relatedBy: .equal, toItem: view, attribute: .leading, multiplier: 1, constant: 0),
.init(item: collectionView, attribute: .trailing, relatedBy: .equal, toItem: view, attribute: .trailing, multiplier: 1, constant: 0),
.init(item: collectionView, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 0),
.init(item: collectionView, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: 0),
])
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
items.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell
cell.backgroundColor = items[indexPath.item]
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
CGSize(width: UIScreen.main.bounds.width, height: 300)
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
items.remove(at: indexPath.item)
collectionView.deleteItems(at: [indexPath])
}
}
The problem itself looks like this (in slow motion):
You can notice how the next cell disappears for the duration of the animation, and at the end of it it appears.
Question: what am I doing wrong? This is a very basic use of a collection and I'm confused by these issues.
2 answers
Answer question0
It seems like there is a problem with your array. It's created dynamically and has conflicts with UICollectionView's implementation.
Try replacing your array with the following static array.
var items = [UIColor.red, .blue, .yellow, .cyan, .magenta, .green, .red, .blue, .yellow, .cyan, .magenta, .green]
Here is what it looks like after changing it.
0
From the rest of the answers, the reason for this glitch became clear - there are problems with the deletion animation if some of the new cells are off the screen.
I solved the problem by adding layout invalidation to the animation block:
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
collectionView.performBatchUpdates({
self.items.remove(at: indexPath.item)
collectionView.deleteItems(at: [indexPath])
collectionView.collectionViewLayout.invalidateLayout()
}, completion: nil)
}