How to detect when user drops an image on a NSImageView with code?

Question

I'm trying to detect when a user drops an image on an NSImageView instantiated by code, not as an Outlet.

The idea is that when the image is released an action occurs. How could I do this with code?

let imageHolder = NSImageView(frame: NSRect(x: 0, y: 0, width: 256, height: 256))
imageHolder.image = NSImage.init(named: "picture")
imageHolder.wantsLayer = true
imageHolder.isEditable = true
view.addSubview(imageHolder)