Example to capture the NSView screenshot in Mac App

ScreenshotViewController.swift

import Cocoa

class ScreenshotViewController: NSViewController {
    override func prepareForSegue(segue: NSStoryboardSegue, sender: AnyObject?) {
        if let viewController = segue.destinationController as? PreviewViewController{
            viewController.image = getScreenshot()
        }
    }
    
    func getScreenshot() -> NSImage {
        let rep = self.view.bitmapImageRepForCachingDisplayInRect(self.view.bounds)!
        self.view.cacheDisplayInRect(self.view.bounds, toBitmapImageRep: rep)
        let image: NSImage = NSImage()
        image.addRepresentation(rep)
        return image
    }
}