NSCollectionView, like NSTableView/NSOutlineView, will reuse existing views
instead of recreating them when it can to speed up the UI. This is most
commonly used during scrolling where views that scroll out of view will be
reused with new data and scrolled into view. It will call -prepareForReuse (See
NSCollectionVIewElement protocol) on the NSCollectionViewItem (and I think the
view too) so you can reset them back to their post -init state. I’d look into
implementing that and see if that helps.
Also, -reloadData will trigger a series of NSCollectionViewDelegate methods to
be called which will dictate what appears in the view after reload. Set some
breakpoints in -collectionView:numberOfItemsInSection: &
-collectionView:itemForRepresentedObjectAtIndexPath: and make sure you are
returning expected values.
With that said, -reloadData is a pretty big hammer and can kill performance
with large view hierarchies. It’s better to use API like
-performBatchUpdates:completionHandler: and related methods to
insert/move/remove views as needed.
Hope that helps & good luck.
—Rob
> On Jan 8, 2023, at 2:03 PM, Arved von Brasch via Cocoa-dev
> <cocoa-dev@lists.apple.com> wrote:
>
> Hello list,
>
> I’ve encountered a bug for the life of me I can’t work out. I’m restricted to
> supporting Mojave (10.14) for the time being, so it is possible this has been
> fixed / not an issue on later releases. This is a Core Data backed
> application, with several Xib loaded views that are swapped in and out based
> on the selection of a segmented control in the toolbar. The basic issue is
> that when I call ‘reloadData()’ on a CollectionView, all the items disappear
> (this typically happens if the user switches to a different view and then
> back again, as, while previously loaded views are kept in memory, they are
> reloaded upon switching back to the foreground - ie. this is not tabs, but
> separate loads of Xib file Views). If I put a break point in an override of
> ‘viewWillDisappear()` in my NSCollectionViewItem subclass, I can see the
> backtrace indicates that NSCollectionView has decided to reuse the
> CollectionViewItem but not issued a new one:
>
> * NSCollectionView reloadData
> * _NSCollectionViewCore reloadData
> * _NSDictionaryM enumerateKeysAndObjectsWithOptions:usingBlock:
> * __35-_NSCollectionViewCore reloadData]_block_invoke
> * NSCollectionViewCore _reuseCell:notifyDidEndDisplaying:
> * _reuseViewWithReuseQueue
> * NSCollectionViewItem _setHiddenForReuse:
> * NSView(NSInternal) _setHidden:setNeedsDisplay:
> * NSView _recursiveViewWillDisappearBeacuseHidden
> * NSViewController _sendViewWillDisappear
> * @objc CollectionViewItem.viewWillDisappear()
> * CollectionViewItem.viewWillDisappear()
>
> In the NSCollectionView itself, I can see the items are removed from the
> ‘visibleItems()’ array, but the items themselves still seems to be known
> about by the collection view, as they will remain in the
> ‘selectionIndexPaths:’ if they were previously selected.
>
> What’s really weird is that because we have multiple top level views, a lot
> of the code is reused on different displays, including the NSArrayController
> subclass that serves as the Delegate and DataSource for this
> NSCollectionView. It works perfectly fine in the other case. The main
> difference between these two top levels views, is that in the one that works,
> the Collection View is slaved to a Table View (ie the collection view shows
> the items in a relationship to the selection in the table view), where as in
> the one that doesn’t work, the behaviour is inverted (the selection in the
> collection view is the data source for a slaved table view). To be clear,
> though, both are displaying completely different Core Data entities. I don’t
> think I’ve missed anything, so as far as I can tell both NSCollectionViews
> are configured identically in the two XIBs.
>
> The last weird thing is that the problem only seems to manifest if the
> application has previously been built. If I Make Clean first, the resulting
> behaviour is as I would expect. If I close and reopen the application, the
> behaviour is then broken. The backtrace is common to both situations, so the
> problem is that something is preventing the Collection View from reissuing a
> CollectionViewItem for the objects. I’m not sure where to look for that side,
> other than to say that ‘viewWillAppear()’ is not called when the problem is
> occurring, and does when it isn't.
>
> I’m fairly convinced that the problem is in the Xib file. I have tried
> removing everything else except the NSCollectionView and a few buttons and
> the problem doesn’t seem to manifest in that case. I haven’t tried bringing
> in one additional view or controller at a time to see when it eventually
> breaks, because there are far too many for that to be a good debugging path.
> The view with the working CollectionView is significantly simpler, if that
> matters. (I suppose a solution could be to take the Collection View out into
> its own Xib file and load into the view manually, or create it
> programatically, neither of which is particularly attractive.) Doing a
> side-by-side comparison of the configuration of Collection View in each Xib
> file and associated Controllers only shows expected differences (e.g.
> entities and spacing sizes).
>
> I’ve been banging my head agains this for several days now, so would
> appreciate any guidance towards a solution,
>
> Arved
>
> P.S. The code is fairly straightforward, I think:
>
> class FlowLayout: NSCollectionViewFlowLayout {
>
> init(withSize size: NSSize, edge: CGFloat, spacing: CGFloat) {
> super.init()
> itemSize = size
> estimatedItemSize = NSZeroSize
> sectionInset = NSEdgeInsetsMake(edge, edge, edge, edge)
> minimumInteritemSpacing = spacing
> minimumLineSpacing = spacing
> }
>
> override func layoutAttributesForElements(in rect: NSRect) ->
> [NSCollectionViewLayoutAttributes] {
> // Excluded for brevity
> return leftAlignedAttributes
> }
>
> override func shouldInvalidateLayout(forBoundsChange newBounds: NSRect) ->
> Bool {
> return true
> }
>
> }
>
> class CollectionViewItem: NSCollectionViewItem {
>
> fileprivate var prefSize: NSSize
>
> required override init(nibName nibNameOrNil: NSNib.Name?, bundle
> nibBundleOrNil: Bundle?) {
> prefSize = NSMakeSize(0, 0)
> super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
> }
>
> override func viewDidLoad() {
> super.viewDidLoad()
> view.wantsLayer = true
> view.layer?.cornerRadius = 8.0
> }
>
> override var isSelected: Bool {
> didSet {
> super.isSelected = isSelected
>
> if isSelected {
> view.layer?.backgroundColor =
> NSColor.selectedControlColor.cgColor
> } else {
> view.layer?.backgroundColor = NSColor.clear.cgColor
> }
> }
> }
>
> override var preferredContentSize: NSSize {
> get {
> return prefSize
> }
> set(newValue) {
> self.prefSize = newValue
> }
> }
>
> }
>
> protocol CollectionViewDelegate : NSCollectionViewDelegate {
> func collectionViewPreviewSelectedItem()
> }
>
> class CollectionView : NSCollectionView {
>
> override func keyDown(with event: NSEvent) {
> if event.characters == " " {
> if let _ = self.delegate as? CollectionViewDelegate {
> (self.delegate as?
> CollectionViewDelegate)?.collectionViewPreviewSelectedItem()
> }
> } else {
> super.keyDown(with: event)
> }
> }
>
> }
>
>
> // Abstract parent class
>
> class CollectionViewController: NSArrayController,
> NSCollectionViewDataSource, NSCollectionViewDelegateFlowLayout,
> CollectionViewDelegate, ObjectControllerExtensions {
>
> var itemSize: NSSize {
> get {
> return NSZeroSize
> }
> }
>
> var interfaceID: NSUserInterfaceItemIdentifier {
> get {
> return .init("")
> }
> }
>
> @IBOutlet weak var mainCollection: NSCollectionView!
> @IBOutlet weak var sourceView: NSView!
>
> override func awakeFromNib() {
> super.awakeFromNib()
> mainCollection.register(NSNib.init(nibNamed: interfaceID.rawValue,
> bundle: nil), forItemWithIdentifier: interfaceID)
> mainCollection.collectionViewLayout = FlowLayout(withSize: itemSize,
> edge: 5, spacing: 10)
> mainCollection.registerForDraggedTypes([.fileURL, .png, .tiff])
> mainCollection.setDraggingSourceOperationMask(.every, forLocal: true)
> mainCollection.setDraggingSourceOperationMask(.delete, forLocal: false)
> sourceView.wantsLayer = true
> }
>
> func numberOfSections(in collectionView: NSCollectionView) -> Int {
> return 1
> }
>
> func collectionView(_ collectionView: NSCollectionView,
> numberOfItemsInSection section: Int) -> Int {
> let objects = arrangedObjects as! Array<NSManagedObject>
> return objects.count
> }
>
> func collectionView(_ collectionView: NSCollectionView,
> itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
> let item = mainCollection.makeItem(withIdentifier: interfaceID, for:
> indexPath) as! CollectionViewItem
> let objects = arrangedObjects as! Array<NSManagedObject>
> item.representedObject = objects[indexPath.item]
> return item
> }
>
> func collectionView(_ collectionView: NSCollectionView, layout
> collectionViewLayout: NSCollectionViewLayout, sizeForItemAt indexPath:
> IndexPath) -> NSSize {
> return itemSize
> }
>
> func collectionView(_ collectionView: NSCollectionView, didSelectItemsAt
> indexPaths: Set<IndexPath>) {
> let objects = arrangedObjects as! Array<NSManagedObject>
> let selected = indexPaths.map({path in
> return objects[path.item]
> })
> setSelectedObjects(selected)
> }
>
> func collectionView(_ collectionView: NSCollectionView, didDeselectItemsAt
> indexPaths: Set<IndexPath>) {
> let objects = arrangedObjects as! Array<NSManagedObject>
> let selected = indexPaths.map({path in
> return objects[path.item]
> })
> removeSelectedObjects(selected)
> }
>
> // Removed Drag and Drop support code for brevity
>
> }
>
> // Working Subclass
>
> class ImageItemController: CollectionViewController {
>
> override var interfaceID: NSUserInterfaceItemIdentifier {
> get {
> return .init("ImageItemView")
> }
> }
>
> override var itemSize: NSSize {
> get {
> return NSMakeSize(200, 200)
> }
> }
>
> override func awakeFromNib() {
> super.awakeFromNib()
> sortDescriptors = [NSSortDescriptor(key: #keyPath(ImageItem.filename),
> ascending: true)]
> }
>
> override func add(_ sender: Any?) {
> // Removed for brevity
> }
>
> override func remove(_ sender: Any?) {
> // Removed for brevity
> }
>
> override func collectionView(_ collectionView: NSCollectionView, layout
> collectionViewLayout: NSCollectionViewLayout, sizeForItemAt indexPath:
> IndexPath) -> NSSize {
> let objects = arrangedObjects as! Array<NSManagedObject>
> var size = (objects[indexPath.item].value(forKey:
> #keyPath(ImageItem.image)) as? NSImage)?.size ?? itemSize
> let ratio = size.height / size.width
> size.width = itemSize.height / ratio
> size.height = itemSize.height
> return size
> }
>
> // Code removed for brevity (displaying a popover when the space bar is
> pressed to show a larger version of the image)
>
> }
>
> // Failing Subclass
>
> class MediaCollectionController: CollectionViewController {
>
> override var interfaceID: NSUserInterfaceItemIdentifier {
> get {
> return .init("MediaCollectionView")
> }
> }
>
> override var itemSize: NSSize {
> get {
> return NSMakeSize(146, 206)
> }
> }
>
> override func awakeFromNib() {
> super.awakeFromNib()
> sortDescriptors = [NSSortDescriptor(key:
> #keyPath(MediaCollection.title), ascending: true)]
> }
>
> override func add(_ sender: Any?) {
> // Removed for brevity
> }
>
> override func remove(_ sender: Any?) {
> // For testing purposes, changed this function to do nothing but reload
> the collection view
> mainCollection.reloadData()
> }
>
> }
>
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