What Is Cocoa?
Cocoa is an application environment for both the OS X operating system and iOS, the operating system used on Multi-Touch devices such as iPhone, iPad, and iPod touch. It consists of a suite of object-oriented software libraries, a runtime system, and an integrated development environment.
This chapter expands on this definition, describing the purpose, capabilities, and components of Cocoa on both platforms. Reading this functional description of Cocoa is an essential first step for a developer trying to understand Cocoa. The Cocoa Environment
Cocoa is a set of object-oriented frameworks that provides a runtime environment for applications running in OS X and iOS. Cocoa is the preeminent application environment for OS X and the only application environment for iOS. (Carbon is an alternative environment in OS X, but it is a compatibility framework with procedural programmatic interfaces intended to support existing OS X code bases.) Most of the applications you see in OS X and iOS, including Mail and Safari, are Cocoa applications. An integrated development environment called Xcode supports application development for both platforms. The combination of this development environment and Cocoa makes it easy to create a well-factored, full-featured application.
As with all application environments, Cocoa presents two faces; it has a runtime aspect and a development aspect. In its runtime aspect, Cocoa applications present the user interface and are tightly integrated with the other visible components of the operating system; in OS X, these include the Finder, the Dock, and other applications from all environments.
But it is the development aspect that is the more interesting one to programmers. Cocoa is an integrated suite of object-oriented software components—classes—that enables you to rapidly create robust, full-featured OS X and iOS applications. These classes are reusable and adaptable software building blocks; you can use them as-is or extend them for your specific requirements. Cocoa classes exist for just about every conceivable development necessity, from user-interface objects to data formatting. Where a development need hasn’t been anticipated, you can easily create a subclass of an existing class that answers that need.
Cocoa has one of the most distinguished pedigrees of any object-oriented development environment. From its introduction as NeXTSTEP in 1989 to the present day, it has been continually refined and tested (see A Bit of History). Its elegant and powerful design is ideally suited for the rapid development of software of all kinds, not only applications but command-line tools, plug-ins, and various types of bundles. Cocoa gives your application much of its behavior and appearance “for free,” freeing up more of your time to work on those features that are distinctive. (For details on what Cocoa offers, see Features of a Cocoa Application.)
You can use several programming languages when developing Cocoa software, but the essential, required language is Objective-C. Objective-C is a superset of ANSI C that has been extended with certain syntactical and semantic features (derived from Smalltalk) to support object-oriented programming. The few added conventions are easy to learn and use. Because Objective-C rests on a Foundation of ANSI C, you can freely intermix straight C code with Objective-C code. Moreover, your code can call functions defined in non-Cocoa programmatic interfaces, such as the BSD library interfaces in /usr/include. You can even mix C++ code with your Cocoa code and link the compiled code into the same executable.
The most important Cocoa class libraries come packaged in two core frameworks for each platform: Foundation and AppKit for OS X, and Foundation and UIKit for iOS. As with all frameworks, these contain not only a dynamically sharable library (or sometimes several versions of libraries required for backward compatibility), but header files, API documentation, and related resources. The pairing of Foundation with AppKit or UIKit reflects the division of the Cocoa programmatic interfaces into those classes that are not related to a graphical user interface and those that are. For each platform, its two core frameworks are essential to any Cocoa project whose end product is an application. Both platforms additionally support the Core Data framework, which is as important and useful as the core frameworks.
OS X also ships with several other frameworks that publish Cocoa programmatic interfaces, such as the WebKit and Address Book frameworks; more Cocoa frameworks will be added to the operating system over time. See The Cocoa Frameworks for further information.