iOS 6 Programming Cookbook

12.7. Saving Objects to Files

Problem

You have added a new class to your project and you would like to be able to save this object to disk as a file and then read it back from disk whenever required.

Solution

Make sure that your class conforms to the NSCoding protocol and implement all the required methods of this method. Don’t worry; I will walk you through this in the Discussion section of this recipe.

Discussion

There are two really handy classes in iOS SDK for this specific purpose, which in the programming world is known as marshalling. They are called:

NSKeyedArchiver

A class that can archive or save the contents of an object or object tree by keys. Each value in the class, let’s say each property, can be saved to the archive, using a key that the programmer chooses. You will be given an archive file (we will talk more about this) and you will just save your values using keys that you choose. Just like a dictionary!

NSKeyedUnarchiver

This class does the reverse of the archiver class. It simply gives you the unarchived dictionary and asks you to read the values into your object’s properties.

In order for the archiver and the unarchiver to work, you need to make sure that the objects you are asking them to archive or unarchive conform to the NSCoding protocol. Let’s start with a simple Person class. Here is the header file of our class:

@interface Person : NSObject <NSCoding>

@property (nonatomic, copy) NSString *firstName;
@property (nonatomic, copy) NSString *lastName;

@end

Now if you ...