NSCollectionViewDelegate methods aren't getting called - swift

Question

I have an NSCollectionView where I would like to show 6 color wells representing a small color palette. I've set it to have a delegate. I've registered the types that it can drag and drop, but the delegate methods for it aren't getting called.
Here's the class that's both my data source and delegate:
class TileView: NSView, NSCollectionViewDataSource, NSCollectionViewDelegate {
var currentPalette: ColorPalette = .grayscale
#IBOutlet var colorPalettePanel: NSPanel? = nil
#IBOutlet var colorCollection: NSCollectionView? = nil
override func awakeFromNib() {
colorCollection?.registerForDraggedTypes([NSPasteboard.PasteboardType.color])
colorCollection?.setDraggingSourceOperationMask(NSDragOperation.move, forLocal: true)
}
override func draw(_ dirtyRect: NSRect) {
let context = NSGraphicsContext.current?.cgContext
draw(into: context)
}
// MARK: - Collection View Data Source Methods
func collectionView(_ collectionView: NSCollectionView, numberOfItemsInSection section: Int) -> Int {
return 6
}
func collectionView(_ collectionView: NSCollectionView, itemForRepresentedObjectAt indexPath: IndexPath) -> NSCollectionViewItem {
let colorIndex = indexPath.last
let result = ColorCollectionViewItem(nibName: "ColorCollectionViewItem", bundle: Bundle.main, colorIndex: colorIndex!)
let color = gCustomPalette6 [ colorIndex! ]
result.colorWell?.color = NSColor(cgColor: color)!
return result
}
// MARK: - Collection View Delegate Methods
func collectionView(_ collectionView: NSCollectionView, validateDrop draggingInfo: NSDraggingInfo, proposedIndex proposedDropIndex: UnsafeMutablePointer<Int>, dropOperation proposedDropOperation: UnsafeMutablePointer<NSCollectionView.DropOperation>) -> NSDragOperation {
print("dragging Info = \(draggingInfo), proposed drop operation = \(proposedDropOperation)")
return NSDragOperation.move
}
func collectionView(_ collectionView: NSCollectionView, acceptDrop draggingInfo: NSDraggingInfo, index: Int, dropOperation: NSCollectionView.DropOperation) -> Bool {
if dropOperation == .before {
return true
}
return false
}
func collectionView(_ collectionView: NSCollectionView, canDragItemsAt indexes: IndexSet, with event: NSEvent) -> Bool {
return true
}
func collectionView(_ collectionView: NSCollectionView, draggingSession session: NSDraggingSession, willBeginAt screenPoint: NSPoint, forItemsAt indexPaths: Set<IndexPath>) {
print("dragging session = \(session)")
}
}
I've set the delegate in Interface Builder, but none of the delegate methods are ever called.
My delegate object is also the data source and the data source methods all get called as expected. The data source is also set up in Interface Builder. What else do I need to do?