Cocoa® Programming

Class Clusters

The Foundation framework and other Cocoa frameworks contain several class clusters. Class clusters consist of multiple hidden classes that are accessed through an abstract public superclass. In Objective-C, when an object is described as abstract it means that programs use instances of classes that inherit from the abstract class, but rarely use instances of the abstract class itself. Concrete subclasses of the abstract class are used. Chapter 6 introduced the class cluster design pattern. Apple provides excellent documentation about class clusters and the reasons for using them at http://developer.apple.com/techpubs/macosx/Cocoa/TasksAndConcepts/ProgrammingTopics/Foundation/Concepts/ClassClusters.html , and in the online documentation ...