Saving, Loading, and Application States - iOS Programming: The Big Nerd Ranch Guide - page 425

In BNRItem.m , implement encodeWithCoder: to add the instance variables to the
container.
- (void)encodeWithCoder:(NSCoder *)aCoder
{
[aCoder encodeObject:itemName forKey:@"itemName"];
[aCoder encodeObject:serialNumber forKey:@"serialNumber"];
[aCoder encodeObject:dateCreated forKey:@"dateCreated"];
[aCoder encodeObject:imageKey forKey:@"imageKey"];
[aCoder encodeInt:valueInDollars forKey:@"valueInDollars"];
}
Notice that pointers to objects are encoded with encodeObject:forKey: , but
valueInDollars is encoded with encodeInt:forKey: . Check the documentation
for NSCoder to see all of the types you can encode. Regardless of the type of the en-
coded value, there is always a key, which is a string that identifies which instance variable
is being encoded. By convention, this key is the name of the instance variable being en-
coded.
When an object is encoded, that object is sent encodeWithCoder: . When an object is
sent encodeWithCoder: , it encodes its instance variables in the same way - by send-
ing them encodeWithCoder: ( Figure 14.1 ). Thus, encoding an object is a recursive
process where objects encode other objects.
Figure 14.1 Encoding an object