Cocoa (API)

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Image:Cocoa dev screenshot.png Cocoa is Apple Computer's native object-oriented application programming environment for the Mac OS X operating system. It is one of five major APIs available for Mac OS X; the others are Carbon, Toolbox (for the Classic environment), POSIX (for the BSD environment), and Java. (Environments such as Perl and Python are considered minor environments because they are not generally used for full-fledged application programming).

Cocoa applications are typically developed using the development tools provided by Apple, specifically Xcode (formerly Project Builder) and Interface Builder, using the Objective-C language. However, the Cocoa programming environment can be accessed using other tools, such as Python, with the aid of a bridging mechanism such as PyObjC. It is also possible to write Objective-C Cocoa programs in a simple text editor and build it manually with GCC or even GNUstep's makefile scripts.

For end-users, Cocoa applications are considered to be those written using the Cocoa programming environment. Such applications usually have a distinctive feel, since the Cocoa programming environment automates many aspects of an application to comply with Apple's Human Interface Guidelines. As such, Cocoa applications are generally characterised by sleek, clean interfaces and good performance.

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Cocoa history

Cocoa is derived from the NeXTSTEP and OPENSTEP programming environments developed by NeXT in the late 1980s. Apple acquired NeXT in December 1996, and subsequently went to work on the Rhapsody operating system that was supposed to be the direct successor of OPENSTEP and use OPENSTEP technology proper. It was to have an emulation base for Mac OS applications, called 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.

Much of the work that went into developing OPENSTEP was applied to the development of Mac OS X. Cocoa is the most visible part of that synergy. There are, however, some important fundamental differences, the most visible of which is that NeXTSTEP and OPENSTEP used Display PostScript for on-screen display of text and graphics, while Cocoa depends on Apple's Quartz (which uses PDF). Cocoa also has a level of internet support, including the NSURL and WebKit HTML classes, and others, while under OPENSTEP there was only rudimentary support for managed network connections through NSFileHandle classes and Berkeley sockets.

Prior to its current use, the "Cocoa" trademark was the name of an application that allowed children to create multimedia projects. It was originally known as KidSim, and is now licensed to a third party and marketed as Stagecast Creator. The program was discontinued in one of the rationalizations that followed Steve Jobs' return to the company. The name was re-used to avoid the delay while registering a new trademark, with Stagecast agreeing to market the older Cocoa under a new name.

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Memory management

One feature of the Cocoa environment that is certainly unusual, if not unique, is its facility for managing dynamically allocated memory. Cocoa's NSObject class, from which most classes, both vendor and user, are derived, implements a reference counting scheme for memory management. Objects derived from the NSObject root class respond to a retain and a release message and keep a retain count which can be queried by sending a retainCount message. 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, its memory freed. (Deallocation is to Objective-C objects as destruction is to C++ objects. The dealloc method is functionally equivalent to a C++ destructor.)

In addition to manual reference counting, application programmers may choose to make use of autorelease pools. Sending an object an autorelease message registers a future release with that thread's nearest autorelease pool. When the autorelease pool is itself released, it sends a corresponding release message for every registered autorelease. Autorelease pools are generally created and released at the beginning and end of the event loop, guaranteeing program flow has passed out of the block where objects were autoreleased. This means the application has predictable performance and memory collection is generally invisible to the user, whereas under most fully automated schemes the application will sometimes suddenly stop responding when the garbage collection system is started.

Automatic garbage collection for Cocoa is currently under development by Apple. When it is completed, the programmer will have the choice of whether to manually manage memory of objects or not. Opinions on this are divided. Some say that reference counting is superior because it allows the programmer to have precise control over when objects are deallocated, but does not add the burden of doing so for every object a program allocates, nor incur the performance penalty usually associated with automatic garbage collection. Others say the entire scheme is unnecessary, that Java-style automatic garbage collection is superior, because it removes the possibility of programmer error in memory management. When garbage collection is introduced in Cocoa, it must be able to interoperate with both approaches, as all the existing Cocoa code uses reference counting.

A combination of the two features is also possible. Modern garbage collectors often include features to be started and stopped mid-task, allowing the application to control how much time will be allotted to system calls. Combining such a system with the AppKit's autorelease/event loop method appears to offer a 'best of both worlds' solution. Such a system was successfully implemented under GNUStep, GNU's open source analog of OpenStep.

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Main frameworks

Cocoa consists primarily of two 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. Cocoa also includes a powerful versioning system to prevent the sort of problems that occur under Microsoft Windows, DLL Hell.

  • Foundation Kit, or more commonly simply Foundation, first appeared in OpenStep. On Mac OS X, it is based on Core Foundation. Foundation is a generic object-oriented library providing string and value manipulation, containers and iteration, distributed computing, 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 NeXTSTEP heritage.
  • Application Kit or AppKit is directly descended from the original NeXTSTEP Application Kit. It contains code with which programs can create and interact with graphical user interfaces. AppKit is built on top of Foundation, and uses the same "NS" prefix.

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 PDF drawing model provided by Quartz. This allows creation of custom drawing content using PostScript-like drawing commands, which also allow 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 only on the unique aspects of an application's content.

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Model-view-controller

The Smalltalk teams at Xerox PARC eventually settled on a design philosophy that led to easy development and high code reuse. Known as model-view-controller or MVC, the concept breaks an application into three sets of interacting object classes. Cocoa's design is a strict application of MVC principles.

In more recent versions of Cocoa, those shipping with OS X 10.3 and later, Apple have started providing pre-rolled controller objects. Apple use the term 'bindings' to refer to these controllers, since they 'bind' data in the model to controls and other elements in the user interface.

In Mac OS 10.4, Cocoa provides automatic support for the data model too, called Core Data. By providing framework support for all three MVC layers, developers are freed from writing much boilerplate-type code, and can spend time on more worthwhile areas, such as the features that make their application unique.

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Late binding

In most object-oriented languages, calls to methods are represented physically by a pointer to the code in memory. This constrains the design of an application since specific "command handling" classes are required, usually organised according to the chain of command design pattern. While Cocoa retains this approach for the most part, the "late binding" of Objective-C opens up more flexibility.

Under Objective-C, methods are represented by a selector, a string describing the method to be called. When a message is sent, the selector is sent into the ObjC runtime, matched against a list of available methods, and the method's implementation is called. Since the selector is text data, this allows it to 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 performance penalty for this, but this is small, and late binding allows the same selector to reference different implementations.

By a similar token, Cocoa provides a pervasive data manipulation technique called Key-Value Coding (KVC). This permits 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 itself. In traditional languages, this late binding is not possible, and it leads to great design flexibility — an object's type does not need to be known, yet any property of that object can be discovered using KVC. In addition, by extending this system using something Cocoa called Key-Value Observing (KVO), automatic support for Undo/Redo is provided.

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Rich objects

One of the most useful features of Cocoa are the powerful "base objects" the system supplies. As an example, consider the Foundation classes NSString and NSAttributedString, which provide Unicode strings, 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-aliased glyph 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 single dictionary used by all applications that uses the "squiggly underlining" introduced by Microsoft (actually a dashed red underline in Cocoa). Unlimited Undo/Redo support is built in. Using only the built-in features, one can write a text editor application in as few as 13 lines of code. With new controller objects, this may fall to zero. This is in contrast to the TextEdit APIs found in the earlier Mac OS.

When extensions are needed, Cocoa's use of Objective-C makes this a straightforward task. Objective-C includes the concept of "categories" which allows for modifications to an 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. Under more common frameworks this same task would require the programmer to make a new subclass supporting the additional features, and then change all instances of the classes to this new class.

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Implementations

The Cocoa frameworks are written in Objective-C, and hence Objective-C is the preferred language for development of Cocoa applications. Java bindings for the Cocoa frameworks (known as the "Java bridge") are also available but have not proven popular amongst Cocoa developers and are no longer updated. Further, the need for runtime binding means many of Cocoa's key features are not available with Java. AppleScript Studio, part of Apple's Xcode Tools makes it possible to write (less complex) Cocoa applications using AppleScript. There is also a third-party scripting language specifically designed for Cocoa, called F-Script, that allows for native access to Cocoa objects and provides a suite of graphical introspection tools.

Third party bindings are also available for other languages:

A more extensive list of implementations is available.

There is also an open source implementation of major parts of the Cocoa framework that allows cross-platform (including MS Windows) Cocoa application development. It is called GNUstep.

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References

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External links

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