NSCollectionView Custom Layout Odd Behavior

I am experimenting with my own custom layout in NSCollectionView on macOS. Whenever I run my app, I get a layout that is definitely not what I have computed (verified this through lots of calls to print()). The cells are shifted down by some seemingly arbitrary amount.

However, when I widen the window, the collection view automatically corrects itself, and the layout is what I expect. See the gif demonstration and my custom layout code. What is going on?

class MyLayout: NSCollectionViewLayout {        var attr: [NSCollectionViewLayoutAttributes] = []        override var collectionViewContentSize: NSSize {          get {              if self.collectionView?.frame == nil {                  return .zero              }              return NSSize(width: CGFloat(self.collectionView?.frame.width ?? 0), height: CGFloat(self.collectionView?.numberOfSections ?? 0) * 75)          }      }        override func prepare() {          super.prepare()            print("prepare() \(String(describing: collectionView?.frame))")          self.attr = []          let height = CGFloat(75)            for s in 0..<(collectionView?.numberOfSections ?? 0) {                let width = CGFloat((collectionView?.frame.width ?? 0)) / CGFloat((collectionView?.numberOfItems(inSection: s) ?? 1))                  for i in 0..<(collectionView?.numberOfItems(inSection: s) ?? 0) {                    let attr = NSCollectionViewLayoutAttributes(forItemWith: IndexPath(arrayLiteral: s, i))                  var shiftedOrigin = collectionView?.frame.origin ?? .zero                  shiftedOrigin = shiftedOrigin.applying(CGAffineTransform(translationX: width * CGFloat(i), y: CGFloat(s) * height))                  attr.frame = NSRect(origin: shiftedOrigin, size: NSSize(width: width, height: height))                  print(attr.frame)                  self.attr.append(attr)              }          }      }        override func layoutAttributesForElements(in rect: NSRect) -> [NSCollectionViewLayoutAttributes] {          print("layout attributes for elements IN \(rect)")            var returnAttr: [NSCollectionViewLayoutAttributes] = []          for attr in attr {              if rect.intersects(attr.frame) {                  returnAttr.append(attr)              }          }          collectionView?.updateConstraints()          return returnAttr      }        override func layoutAttributesForItem(at indexPath: IndexPath) -> NSCollectionViewLayoutAttributes? {          print("layout attributes for elements AT \(indexPath)")            for attr in attr {              if attr.indexPath == indexPath {                  return attr              }          }            return nil      }        override func shouldInvalidateLayout(forBoundsChange newBounds: NSRect) -> Bool {          true      }  }    

https://stackoverflow.com/questions/67411691/nscollectionview-custom-layout-odd-behavior May 06, 2021 at 12:04PM