In a class cluster, only an abstract superclass is public. Allocating an instance actually creates
an object of a private subclass. As such, subclassing always happens on an abstract class and
requires all methods to be implemented. See also the class cluster documentation.
"Class clusters are a design pattern that the Foundation framework makes extensive use of.
Class clusters group a number of private concrete subclasses under a public abstract superclass.
The grouping of classes in this way simplifies the publicly visible architecture of an object-oriented
framework without reducing its functional richness. Class clusters are based on the
Abstract Factory design pattern."
Q: 如何子类化(subclass)class cluster?
Q: primitive methods是指什么呢?
1. Class Clusters - Concepts in Objective-C Programming
https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaEncyclopedia/ClassClusters/ClassClusters.html
2. Abstract Factory (Todo)
原文：http://www.cnblogs.com/cwgk/p/4632547.html