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AppKit
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<p>AppKit (formally Application Kit) is a <a href="/facts/Graphical_user_interface/wBlQFWDP">graphical user interface</a> <a href="/facts/Widget_toolkit/v8wM8wwy">toolkit</a>. It initially served as the UI framework for <a href="/facts/NeXTSTEP/eL0bymga">NeXTSTEP</a>. Along with <a href="/facts/Foundation_Kit/LcnWzx5R">Foundation</a> and <a href="/facts/Display_PostScript/Kt6Pj1D0">Display PostScript</a>, it became one of the core parts of the <a href="/facts/OpenStep/RTKQxPR1">OpenStep</a> specification of APIs. Later, AppKit and Foundation became part of <a href="/facts/Cocoa_(API)/p10p5aFI">Cocoa</a>, the <a href="/facts/Objective-C/5mM557BW">Objective-C</a> API framework of <a href="/facts/MacOS/Y93vVpTm">macOS</a>. <a href="/facts/GNUstep/LaIT4jYx">GNUstep</a>, <a href="/facts/GNU/HlAKKBdt">GNU</a>'s implementation of the OpenStep/Cocoa API, also contains an implementation of the AppKit API. </p><p>AppKit comprises a collection of <a href="/facts/Objective-C/5mM557BW">Objective-C</a> <a href="/facts/Class_(computer_programming)/zvzkSAtE">classes</a> and <a href="/facts/Protocol_(object-oriented_programming)/jDMpXoLN">protocols</a> that can be used to build an application in OpenStep/Cocoa. These classes can also be used in <a href="/facts/Swift_(programming_language)/A7qHQC32">Swift</a> through its Objective-C bridge. <a href="/facts/Xcode/glURdIVT">Xcode</a> has built-in functionality for developing a Cocoa application using AppKit, including the ability to visually design user interfaces with <a href="/facts/Interface_Builder/IMK6EyVq">Interface Builder</a>. It relies heavily on patterns like <a href="/facts/Reference_type/0xLHHokA">reference types</a>, <a href="/facts/Delegation_(object-oriented_programming)/tnJuJUzK">delegation</a>, <a href="/facts/Publish%25E2%2580%2593subscribe_pattern/d2aH1etM">notifications</a>, <a href="/facts/Target%25E2%2580%2593action/VjqBAAWd">target–action</a>, and <a href="/facts/Model%25E2%2580%2593view%25E2%2580%2593controller/vRbMXjMm">model–view–controller</a>. A sign of the NeXTSTEP heritage, AppKit's classes and protocols still use the "NS" prefix. </p><p>Most of the applications bundled with macOS—for example, the <a href="/facts/Finder_(software)/LD3gHLDo">Finder</a>, <a href="/facts/TextEdit/x3nW9TDD">TextEdit</a>, <a href="/facts/Calendar_(Apple)/XWMVpdgq">Calendar</a>, and <a href="/facts/Preview_(macOS)/YxbMjLvd">Preview</a>—use AppKit to provide their user interface. </p><p><a href="/facts/MacOS/Y93vVpTm">macOS</a>, <a href="/facts/IOS/mK5cggH5">iOS</a>, <a href="/facts/IPadOS/fClrmH7D">iPadOS</a>, and <a href="/facts/TvOS/jDMqvvfI">tvOS</a> also support other UI frameworks, including <a href="/facts/UIKit/GUGI8gsU">UIKit</a>, which is derived from AppKit and uses many similar structures, and <a href="/facts/SwiftUI/dRdmzY13">SwiftUI</a>, a <a href="/facts/Swift_(programming_language)/A7qHQC32">Swift</a>-only declarative UI framework. </p><p>Prior to <a href="/facts/MacOS_Catalina/B2RE5fGd">macOS Catalina</a>, macOS also supported <a href="/facts/Carbon_(API)/KAIwFWI2">Carbon</a>, a UI framework derived from the <a href="/facts/Macintosh_Toolbox/Pgvfu771">Macintosh Toolbox</a>. </p>