Editing Cocoa (API) (section)

==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 <code>NSString</code> and <code>NSAttributedString</code>, which provide [[Unicode]] [[string (computer science)|strings]], and the <code>[[NSText]]</code> system in AppKit, which allows the programmer to place string objects in the GUI.

<code>NSText</code> 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]], [[Ligature (typography)|ligatures]], running text around arbitrary [[shape]]s, [[rotation]], full Unicode support, and [[Spatial anti-aliasing|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 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 "[https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocCategories.html#//apple_ref/doc/uid/TP30001163-CH20-SW1 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.