Saving, Loading, and Application States - iOS Programming: The Big Nerd Ranch Guide - page 433

The unarchiveObjectWithFile: method will create an instance of NSKeyedUn-
archiver and load the archive located at the itemArchivePath into that instance.
The NSKeyedUnarchiver will then inspect the type of the root object in the archive
and create an instance of that type. In this case, the type will be an NSMutableArray
because you created this archive with a root object of this type. (If the root object was a
BNRItem instead, unarchiveObjectWithFile: would return an instance of
BNRItem .)
The newly allocated NSMutableArray is then sent initWithCoder: and, as you
may have guessed, the NSKeyedUnarchiver is passed as the argument. The array
starts decoding its contents (instances of BNRItem ) from the NSKeyedUnarchiver
and sends each of these objects the message initWithCoder: passing the same
NSKeyedUnarchiver .
The clever part about NSKeyedUnarchiver is that it knows which object is currently
decoding its instance variables. Thus, you don't need to uniquely identify each object in
an archive. You only need to identify the relationship between an object and its instance
variables, and this relationship is already identified by the key used to encode and decode
the object in the NSCoding protocol methods.
You can now build and run the application. Any BNRItem s a user enters will be available
until the user explicitly deletes them. One thing to note about testing your saving and
loading code: If you kill Homepwner from Xcode , the BNRItem s will not be saved. You
must hit the home button first and then kill it from Xcode by clicking the Stop button.
Now that you can save and load BNRItem s, there is no reason to auto-populate each one
with random data. In BNRItemStore.m , modify the implementation of createItem
so that it creates an empty BNRItem instead of one with random data.
- (BNRItem *)createItem
{
BNRItem *p = [BNRItem randomItem];
BNRItem *p = [[BNRItem alloc] init];
[allItems addObject:p];
return p;
}