Dropping into NSBox subclass doesn't work - swift

Question

I have created NSBox subclass to implement drag-and-drop. I followed this tutorial. Code looks like this:
import Cocoa
class EditCard: NSBox, NSDraggingSource, NSPasteboardItemDataProvider, NSPasteboardWriting {
var value: String = "0"
let supportedTypes: [NSPasteboard.PasteboardType] = [.string]
func highlight() {
self.borderColor = NSColor.controlAccentColor
self.borderWidth = 2.0
}
func unhighlight() {
self.borderColor = NSColor.clear
self.borderWidth = 0.0
}
override func draggingEntered(_ sender: NSDraggingInfo) -> NSDragOperation {
let canReadPasteboardObjects = sender.draggingPasteboard.canReadObject(forClasses: [NSString.self], options: nil)
if canReadPasteboardObjects {
highlight()
return .copy
}
return NSDragOperation()
}
override func performDragOperation(_ sender: NSDraggingInfo) -> Bool {
sender.draggingDestinationWindow?.orderFrontRegardless()
guard let pasteboardObjects = sender.draggingPasteboard.readObjects(forClasses: [NSString.self], options: nil), pasteboardObjects.count > 0 else { return false }
pasteboardObjects.forEach { (object) in
//Add both views to group
}
sender.draggingDestinationWindow?.orderFrontRegardless()
return true
}
override func draggingEnded(_ sender: NSDraggingInfo) {
unhighlight()
}
override func draggingExited(_ sender: NSDraggingInfo?) {
unhighlight()
}
override func concludeDragOperation(_ sender: NSDraggingInfo?) {
highlight()
}
override func prepareForDragOperation(_ sender: NSDraggingInfo) -> Bool {
return true
}
func draggingSession(_ session: NSDraggingSession, sourceOperationMaskFor context: NSDraggingContext) -> NSDragOperation {
return .copy
}
func pasteboard(_ pasteboard: NSPasteboard?, item: NSPasteboardItem, provideDataForType type: NSPasteboard.PasteboardType) {
switch type {
case .string:
guard let indexData = value.data(using: .utf8) else { return }
item.setData(indexData, forType: type)
default: break
}
}
func writableTypes(for pasteboard: NSPasteboard) -> [NSPasteboard.PasteboardType] {
var types = [NSPasteboard.PasteboardType]()
types.append(.string)
return types
}
func pasteboardPropertyList(forType type: NSPasteboard.PasteboardType) -> Any? {
return nil
}
override func mouseDragged(with event: NSEvent) {
let pasteboardItem = NSPasteboardItem()
pasteboardItem.setDataProvider(self, forTypes: [.string])
let pdfData = self.dataWithPDF(inside: self.bounds)
let imageFromPDF = NSImage(data: pdfData)
let draggingItem = NSDraggingItem(pasteboardWriter: pasteboardItem)
draggingItem.setDraggingFrame(self.bounds, contents: imageFromPDF)
beginDraggingSession(with: [draggingItem], event: event, source: self)
}
override func awakeFromNib() {
self.wantsLayer = true
self.registerForDraggedTypes(supportedTypes)
}
}
I want to have this view to work both ways. Dragging this view works, but other views of the same subclass don't highlight when putting dragged view over it. When I try to print() on any function that accepts dragging (draggingEntered(), performDragOperation(), etc) nothing prints. Can somebody help me?