Cocoa consists of the Foundation Kit, Application Kit, and Core Data frameworks, as included by the Cocoa.h header file, and the libraries and frameworks included by those, such as the C standard library and the Objective-C runtime.[1]
Cocoa applications are typically developed using the development tools provided by Apple, specifically Xcode (formerly Project Builder) and Interface Builder (now part of Xcode), using the programming languagesObjective-C or Swift. However, the Cocoa programming environment can be accessed using other tools. It is also possible to write Objective-C Cocoa programs in a simple text editor and build it manually with GNU Compiler Collection (GCC) or Clang from the command line or from a makefile.
For end users, Cocoa applications are those written using the Cocoa programming environment. Such applications usually have a familiar look and feel, since the Cocoa programming environment provides a lot of common UI elements (such as buttons, scroll bars, etc.), and automates many aspects of an application to comply with Apple's human interface guidelines.
Cocoa continues the lineage of several software frameworks (mainly the App Kit and Foundation Kit) from the NeXTSTEP and OpenStep programming environments developed by NeXT in the 1980s and 1990s. Apple acquired NeXT in December 1996, and subsequently went to work on the Rhapsody operating system that was to be the direct successor of OpenStep. It was to have had an emulation base for classic Mac OS applications, named Blue Box. The OpenStep base of libraries and binary support was termed Yellow Box. Rhapsody evolved into Mac OS X, and the Yellow Box became Cocoa. Thus, Cocoa classes begin with the letters NS, such as NSString or NSArray. These stand for the original proprietary term for the OpenStep framework, NeXTSTEP.[2]
Much of the work that went into developing OpenStep was applied to developing Mac OS X, Cocoa being the most visible part. However, differences exist. For example, NeXTSTEP and OpenStep used Display PostScript for on-screen display of text and graphics, while Cocoa depends on Apple's Quartz (which uses the Portable Document Format (PDF) imaging model, but not its underlying technology). Cocoa also has a level of Internet support, including the NSURL and WebKitHTML classes, and others, while OpenStep had only rudimentary support for managed network connections via NSFileHandle classes and Berkeley sockets.
The API toolbox was originally called “Yellow Box” and was renamed to Cocoa - a name that had been already trademarked by Apple. Apple's Cocoa trademark had originated as the name of a multimedia project design application for children. The name was intended to evoke "Java for kids", as it ran embedded in web pages.[3] The original "Cocoa" program was discontinued following the return of Steve Jobs to Apple. At the time, Java was a big focus area for the company, so “Cocoa” was used as the new name for “Yellow Box” because, in addition to the native Objective-C usage, it could also be accessed from Java via a bridging layer.[4] Even though Apple discontinued support for the Cocoa Java bridge, the name continued and was even used for the Cocoa Touch API.
Memory management
One feature of the Cocoa environment is its facility for managing dynamically allocated memory. Foundation Kit's NSObject class, from which most classes, both vendor and user, are derived, implements a reference counting scheme for memory management. Objects that derive from the NSObject root class respond to a retain and a release message, and keep a retain count. A method titled retainCount exists, but contrary to its name, will usually not return the exact retain count of an object. It is mainly used for system-level purposes. Invoking it manually is not recommended by Apple.
A newly allocated object created with alloc or copy has a retain count of one. Sending that object a retain message increments the retain count, while sending it a release message decrements the retain count. When an object's retain count reaches zero, it is deallocated by a procedure similar to a C++ destructor. dealloc is not guaranteed to be invoked.
Starting with Objective-C 2.0, the Objective-C runtime implemented an optional garbage collector, which is now obsolete and deprecated in favor of Automatic Reference Counting (ARC). In this model, the runtime turned Cocoa reference counting operations such as "retain" and "release" into no-ops. The garbage collector does not exist on the iOS implementation of Objective-C 2.0. Garbage collection in Objective-C ran on a low-priority background thread, and can halt on Cocoa's user events, with the intention of keeping the user experience responsive. The legacy garbage collector is still available on Mac OS X version 10.13, but no Apple-provided applications use it.
In 2011, the LLVM compiler introduced Automatic Reference Counting (ARC), which replaces the conventional garbage collector by performing static analysis of Objective-C source code and inserting retain and release messages as necessary.
Main frameworks
Cocoa consists of three Objective-C object libraries called frameworks. Frameworks are functionally similar to shared libraries, a compiled object that can be dynamically loaded into a program's address space at runtime, but frameworks add associated resources, header files, and documentation. The Cocoa frameworks are implemented as a type of bundle, containing the aforementioned items in standard locations.
Foundation Kit (Foundation), first appeared in Enterprise Objects Framework on NeXTSTEP 3.[5] It was developed as part of the OpenStep work, and subsequently became the basis for OpenStep's AppKit when that system was released in 1994. On macOS, Foundation is based on Core Foundation. Foundation is a generic object-oriented library providing string and value manipulation, containers and iteration, distributed computing, event loops (run loops), and other functions that are not directly tied to the graphical user interface. The "NS" prefix, used for all classes and constants in the framework, comes from Cocoa's OPENSTEP heritage, which was jointly developed by NeXT and Sun Microsystems.
Application Kit (AppKit) is directly descended from the original NeXTSTEP Application Kit. It contains code programs can use to create and interact with graphical user interfaces. AppKit is built on top of Foundation, and uses the same NS prefix.
Core Data is the object persistence framework included with Foundation and Cocoa and found in Cocoa.h.[1]
A key part of the Cocoa architecture is its comprehensive views model. This is organized along conventional lines for an application framework, but is based on the Portable Document Format (PDF) drawing model provided by Quartz. This allows creating custom drawing content using PostScript-like drawing commands, which also allows automatic printer support and so forth. Since the Cocoa framework manages all the clipping, scrolling, scaling and other chores of drawing graphics, the programmer is freed from implementing basic infrastructure and can concentrate on the unique aspects of an application's content.
The Smalltalk teams at Xerox PARC eventually settled on a design philosophy that led to easy development and high code reuse. Named model–view–controller (MVC), the concept breaks an application into three sets of interacting object classes:
Model classes represent problem domain data and operations (such as lists of people/departments/budgets; documents containing sections/paragraphs/footnotes of stylized text).
View classes implement visual representations and affordances for human-computer interaction (such as scrollable grids of captioned icons and pop-up menus of possible operations).
Controller classes contain logic that surfaces model data as view representations, maps affordance-initiated user actions to model operations, and maintains state to keep the two synchronized.
Cocoa's design is a fairly, but not absolutely strict application of MVC principles. Under OpenStep, most of the classes provided were either high-level View classes (in AppKit) or one of a number of relatively low-level model classes like NSString. Compared to similar MVC systems, OpenStep lacked a strong model layer. No stock class represented a "document," for instance. During the transition to Cocoa, the model layer was expanded greatly, introducing a number of pre-rolled classes to provide functionality common to desktop applications.
In Mac OS X 10.3, Apple introduced the NSController family of classes, which provide predefined behavior for the controller layer. These classes are considered part of the Cocoa Bindings system, which also makes extensive use of protocols such as Key-Value Observing and Key-Value Binding. The term 'binding' refers to a relationship between two objects, often between a view and a controller. Bindings allow the developer to focus more on declarative relationships rather than orchestrating fine-grained behavior.
With the arrival of Mac OS X 10.4, Apple extended this foundation further by introducing the Core Data framework, which standardizes change tracking and persistence in the model layer. In effect, the framework greatly simplifies the process of making changes to application data, undoing changes when necessary, saving data to disk, and reading it back in.
In providing framework support for all three MVC domains, Apple's goal is to reduce the amount of boilerplate or "glue" code that developers have to write, freeing up resources to spend time on application-specific features.
Late binding
In most object-oriented languages, calls to methods are represented physically by a pointer to the code in memory. This restricts the design of an application since specific command handling classes are needed, usually organized according to the chain-of-responsibility pattern. While Cocoa retains this approach for the most part, Objective-C's late binding opens up more flexibility.
Under Objective-C, methods are represented by a selector, a string describing the method to call. When a message is sent, the selector is sent into the Objective-C runtime, matched against a list of available methods, and the method's implementation is called. Since the selector is text data, this lets it be saved to a file, transmitted over a network or between processes, or manipulated in other ways. The implementation of the method is looked up at runtime, not compile time. There is a small performance penalty for this,[6] but late binding allows the same selector to reference different implementations.
By a similar token, Cocoa provides a pervasive data manipulation method called key-value coding (KVC).[7] This allows a piece of data or property of an object to be looked up or changed at runtime by name. The property name acts as a key to the value. In traditional languages, this late binding is impossible. KVC leads to great design flexibility. An object's type need not be known, yet any property of that object can be discovered using KVC. Also, by extending this system using something Cocoa terms key-value observing (KVO), automatic support for undo-redo is provided.
Late static binding is a variant of binding somewhere between static and dynamic binding. The binding of names before the program is run is called static (early); bindings performed as the program runs are dynamic (late or virtual).
Rich objects
One of the most useful features of Cocoa is the powerful base objects the system supplies. As an example, consider the Foundation classes NSString and NSAttributedString, which provide Unicodestrings, and the NSText system in AppKit, which allows the programmer to place string objects in the GUI.
NSText and its related classes are used to display and edit strings. The collection of objects involved permit an application to implement anything from a simple single-line text entry field to a complete multi-page, multi-column text layout schema, with full professional typography features such as kerning, ligatures, running text around arbitrary shapes, rotation, full Unicode support, and anti-aliasedglyph rendering. Paragraph layout can be controlled automatically or by the user, using a built-in "ruler" object that can be attached to any text view. Spell checking is automatic, using a system-wide set of language dictionaries. Unlimited undo/redo support is built in. Using only the built-in features, one can write a text editor application in as few as 10 lines of code. With new controller objects, this may fall towards zero. When extensions are needed, Cocoa's use of Objective-C makes this a straightforward task. Objective-C includes the concept of "categories," which allows modifying existing class "in-place". Functionality can be accomplished in a category without any changes to the original classes in the framework, or even access to its source. In other common languages, this same task requires deriving a new subclass supporting the added features, and then replacing all instances of the original class with instances of the new subclass.
Implementations and bindings
The Cocoa frameworks are written in Objective-C. Java bindings for the Cocoa frameworks (termed the Java bridge) were also made available with the aim of replacing Objective-C with a more popular language[8] but these bindings were unpopular among Cocoa developers and Cocoa's message passing semantics did not translate well to a statically-typed language such as Java.[9] Cocoa's need for runtime binding means many of Cocoa's key features are not available with Java. In 2005, Apple announced that the Java bridge was to be deprecated, meaning that features added to Cocoa in macOS versions later than 10.4 would not be added to the Cocoa-Java programming interface.
Originally, AppleScript Studio could be used to develop simpler Cocoa applications.[11] However, as of Snow Leopard, it has been deprecated. It was replaced with AppleScriptObjC, which allows programming in AppleScript, while using Cocoa frameworks.[12]
A Ruby language implementation named MacRuby, which removes the need for a bridge mechanism, was formerly developed by Apple, while Nu is a Lisp-like language that uses the Objective-C object model directly, and thus can use the Cocoa frameworks without needing a binding.
Other implementations
There are also open source implementations of major parts of the Cocoa framework, such as GNUstep and Cocotron,[16] which allow cross-platform Cocoa application development to target other operating systems, such as Microsoft Windows and Linux.
NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT Computer, founded by Steve Jobs, in the late 1980s and early 1990s and was initially used for its range of proprietary workstation computers such as the NeXTcube. It was later ported to several other computer architectures.
OpenStep is an object-oriented application programming interface (API) specification developed by NeXT. It provides a framework for building graphical user interfaces (GUIs) and developing software applications. OpenStep was designed to be platform-independent, allowing developers to write code that could run on multiple operating systems, including NeXTSTEP, Windows NT, and various Unix-based systems. It has influenced the development of other GUI frameworks, such as Cocoa for macOS, and GNUstep.
GNUstep is a free software implementation of the Cocoa Objective-C frameworks, widget toolkit, and application development tools for Unix-like operating systems and Microsoft Windows. It is part of the GNU Project.
Carbon was one of two primary C-based application programming interfaces (APIs) developed by Apple for the macOS operating system. Carbon provided a good degree of backward compatibility for programs that ran on Mac OS 8 and 9. Developers could use the Carbon APIs to port (“carbonize”) their “classic” Mac applications and software to the Mac OS X platform with little effort, compared to porting the app to the entirely different Cocoa system, which originated in OPENSTEP. With the release of macOS 10.15 Catalina, the Carbon API was officially discontinued and removed, leaving Cocoa as the sole primary API for developing macOS applications.
The Enterprise Objects Framework, or simply EOF, was introduced by NeXT in 1994 as a pioneering object-relational mapping product for its NeXTSTEP and OpenStep development platforms. EOF abstracts the process of interacting with a relational database by mapping database rows to Java or Objective-C objects. This largely relieves developers from writing low-level SQL code.
The Foundation Kit, or just Foundation for short, is an Objective-C framework in the OpenStep specification described by NeXT Computer, Inc.. It provides basic classes such as wrapper classes and data structure classes. This framework uses the prefix NS. It is also part of Cocoa and of the Swift standard library.
WebObjects is a discontinued Java web application server and a server-based web application framework originally developed by NeXT Software, Inc.
The System Object Model (SOM) is an object-oriented shared library technology developed by IBM that supports defining an interface to an object so that its interface is separate from its implementation.
RubyCocoa is a macOS framework that provides a bridge between the Ruby and the Objective-C programming languages, allowing the user to manipulate Objective-C objects from Ruby, and vice versa. It makes it possible to write a Cocoa application completely in Ruby as well as to write an application that mixes Ruby and Objective-C code. An Apple project called MacRuby was under development to replace RubyCocoa in 2008. A proprietary spin-off called RubyMotion was subsequently released in 2012, available for iOS, macOS and Android.
Rhapsody is an operating system that was developed by Apple Computer after its purchase of NeXT in the late 1990s. It is the fifth major release of the Mach-based operating system that was developed at NeXT in the late 1980s, previously called OPENSTEP and NEXTSTEP. Rhapsody was targeted to developers for a transition period between the Classic Mac OS and Mac OS X. Rhapsody represented a new and exploratory strategy for Apple, more than an operating system, and runs on x86-based PCs and on Power Macintosh.
AppKit is a graphical user interface toolkit. It initially served as the UI framework for NeXTSTEP. Along with Foundation and Display PostScript, it became one of the core parts of the OpenStep specification of APIs. Later, AppKit and Foundation became part of Cocoa, the Objective-C API framework of macOS. GNUstep, GNU's implementation of the OpenStep/Cocoa API, also contains an implementation of the AppKit API.
Core Data is an object graph and persistence framework provided by Apple in the macOS and iOS operating systems. It was introduced in Mac OS X 10.4 Tiger and iOS with iPhone SDK 3.0. It allows data organized by the relational entity–attribute model to be serialized into XML, binary, or SQLite stores. The data can be manipulated using higher level objects representing entities and their relationships. Core Data manages the serialized version, providing object lifecycle and object graph management, including persistence. Core Data interfaces directly with SQLite, insulating the developer from the underlying SQL.
In the macOS, iOS, NeXTSTEP, and GNUstep programming frameworks, property list files are files that store serialized objects. Property list files use the filename extension .plist, and thus are often referred to as p-list files.
In NeXTSTEP, OPENSTEP, and their lineal descendants macOS, iOS, iPadOS, tvOS, watchOS, and visionOS, and in GNUstep, a bundle is a file directory with a defined structure and file extension, allowing related files to be grouped together as a conceptually single item.
The architecture of macOS describes the layers of the operating system that is the culmination of Apple Inc.'s decade-long research and development process to replace the classic Mac OS.
The term target–action design paradigm refers to a kind of software architecture, where a computer program is divided into objects which dynamically establish relationships by telling each other which object they should target and what action or message to send to that target when an event occurs. This is especially useful when implementing graphical user interfaces, which are by nature event-driven.
Mono is a free and open-source software framework that aims to run software made for the .NET Framework on Linux and other OSes. Originally by Ximian which was acquired by Novell, it was later developed by Xamarin which was acquired by Microsoft. In August 2024, Microsoft transferred ownership of Mono to WineHQ.
Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTSTEP operating system. Due to Apple macOS’s direct lineage from NeXTSTEP, Objective-C was the standard language used, supported, and promoted by Apple for developing macOS and iOS applications from 1997, when Apple purchased NeXT until the introduction of the Swift language in 2014.
In computer science, bridging describes systems that map the runtime behaviour of different programming languages so they can share common resources. They are often used to allow "foreign" languages to operate a host platform's native object libraries, translating data and state across the two sides of the bridge. Bridging contrasts with "embedding" systems that allow limited interaction through a black box mechanism, where state sharing is limited or non-existent.
↑ "Using the Java Bridge"(PDF). Apple Inc.Because Java is a strongly typed language, it requires more information about the classes and interfaces it manipulates at compile time. Therefore, before using Objective-C classes as Java ones, a description of them has to be written and compiled.
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