import Cocoa class CrasherBase: NSView { var hasRect: Bool = false var rect: CGRect? { return hasRect ? CGRect(x: 0, y: 0, width: 50, height: 50) : nil } override func draw(_ dirtyRect: NSRect) { NSEraseRect(dirtyRect) } } class Crasher2: CrasherBase { override var rect: CGRect { return CGRect(x: 0, y: 0, width: 100, height: 100) } override func draw(_ dirtyRect: NSRect) { NSEraseRect(rect) } }