Archiving(Chapter 10 of Cocoa Programming for Mac OS X)

One protocol is NSCoding. If your class implements NSCoding, it promises to implement the following methods:

     - (id)initWithCoder:(NSCoder *)coder;     

     - (void)encodeWithCoder:(NSCoder *)coder;

An NSCoder is an abstraction of a stream of bytes. You can write your data to a coder or read your data from a coder. The initWithCoder: method in your object will read data from the coder and save that data to its instance variables. The encodeWithCoder: method in your object will read its instance variables and write those values to the coder. In this chapter, you will implement both methods in your Person class.

    NSCoder is actually an abstract class. You won't ever create instances of an abstract class. Instead, an abstract class has some capabilities that are intended to be inherited by subclasses. You will create instances of the concrete subclasses. Namely, you will use NSKeyedUnarchiver to read objects from a stream of data, and you will use NSKeyedArchiver to write objects to the stream of data.