as of 2020 even in Xcode 11.3.1 it is possible.. class PreviewViewController: NSViewController, QLPreviewingController { 		override var nibName: NSNib.Name? {         return NSNib.Name("PreviewViewController")     }     override func loadView() {         super.loadView() 				/* ... */     } 		func preparePreviewOfFile(at url: URL, completionHandler handler: @escaping (Error?) -> Void) {         /* Add the supported content types to the QLSupportedContentTypes array in the Info.plist of the extension.         // Perform any setup necessary in order to prepare the view.         // Call the completion handler so Quick Look knows that the preview is fully loaded.         // Quick Look will display a loading spinner while the completion handler is not called. */         handler(nil)     } } and you don't need to load any swift dylibs.. And as far i understand Eskimo's explaining.. that circumstance is also valid for objective-c. You could end up with different librarys running at the same time.. but even tho, there is no problem doing so. Thats what all apps do more or less. Just go the way Xcode offers you. Build & Run, be happy