Helper Objects (Chapter 6 of Cocoa Programming for Mac OS X)

　　Many objects in the Cocoa framework are extended in much the same way. That is, there is an existing object that needs to be extended for your purpose. Instead of subclassing the table view, you simply supply it with a helper object. For example, when a table view is about to display itself, it will turn to the helper object to ask "How many rows of data am I displaying?" "What should be displayed in the first column, second row?"

　　Thus, to extend an existing Cocoa class, you will frequently write a helper object. This chapter focuses on creating helper objects and connecting them to the standard Cocoa objects.