From WWDC 2012 Session 245 (translated to Swift):
let viewToCapture = self.window!.contentView!
let rep = viewToCapture.bitmapImageRepForCachingDisplay(in: viewToCapture.bounds)!
viewToCapture.cacheDisplay(in: viewToCapture.bounds, to: rep)
let img = NSImage(size: viewToCapture.bounds.size)
img.addRepresentation(rep)
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…