The Application Kit, usually called AppKit, is a graphical user interface toolkit for macOS. It comprises a collection of Objective-C classes and protocols that can be used to build an application for the Mac. These classes can also be used in Swift through its Objective-C bridge. Xcode has built-in functionality for developing a Cocoa application using AppKit, including the ability to visually design user interfaces with Interface Builder. AppKit, along with Foundation and Core Data, is part of Cocoa. Accordingly, it relies heavily on patterns like reference types, delegation, notifications, target–action, and model–view–controller. Most of the applications bundled with macOS—for example, the Finder, TextEdit, Calendar, and Preview–use AppKit to provide their user interface. While AppKit is primarily associated with macOS, it was originally developed for use on NeXTSTEP (and, later, OPENSTEP). As in Foundation, its classes and protocols still use the “NS” prefix as a result of this heritage. An open-source clone of AppKit is maintained as part of GNUstep for various other Unix-based OSes. iOS, iPadOS, and tvOS use a framework called UIKit, which is derived from AppKit and uses many similar structures.