How do I serialize a simple object in iPhone sdk?

Question

I have a dictionary of objects; they are all POCO objects that should be serializable. What technique should I look at for writing these to disk. I'm looking for the simplest option to write a few lists to save state.

I think I have 3 options.

1. plist files. However this seems to be limited to only storing predefined objects (strings, numbers etc) not objects (like a person with a name and age).

2. CoreData. (New in 3.0) This would work well; however my data model would need to change to make this work. This would be a massive rework and I'm not sure if it is worth the effort.

3. SQLLite. Implement a simple SQL database to read to and from. I have done the least amount of reserch into this one, but I don't want to have to 'rewrite' some of the core data ORM functions.

Answer 1

You do this in the same way you'd do it on Mac OS X: your POCOs must conform to the NSCoding protocol. See here for a conceptual reference, and here for the NSCoding reference.

If the data isn't that crazy extensive and you don't have ridiculously complicated relationships between your objects, writing everything out as a plist is probably your best option; it's very fast to execute and simple to implement in your code. Like you said, CoreData will probably be a lot of extra work to adapt your code to, and sqlite really is only good for storing data that's perfect to be stored in a relational database. Keep in mind that sqlite is also slower and uses more resources than working with binary plists.

Answer 2

To serialize custom object you just need to conform to the NSCoding protocol. If your object extends NSObject all you need to do (I believe) is to implement these (example for person object):

// Encode an object for an archive
- (void)encodeWithCoder:(NSCoder *)coder
{
    [super encodeWithCoder:coder];
    [coder encodeObject:name forKey:@“Name”];
    [coder encodeInteger:age forKey:@“Age”];
}
// Decode an object from an archive
- (id)initWithCoder:(NSCoder *)coder
{
    self = [super initWithCoder:coder];
    name = [[coder decodeObjectForKey:@“Name”] retain];
    age  = [coder decodeIntegerForKey:@“Age”];
}

NSArray and NSDictionary already implement methods for serialization. They will serialize all the objects that they hold (if objects implement NSCoder interface - they do if they extend NSObject). NSObject's encodeWithCoder and initWithCoder do nothing by default so unless you implement your own code in your classes nothing gets serialized.

If you have NSArray or NSDictionary of objects you can synchronize them using:

// Writing
- (BOOL)writeToFile:(NSString *)aPath atomically:(BOOL)flag;
- (BOOL)writeToURL:(NSURL *)aURL atomically:(BOOL)flag;
// Reading
- (id)initWithContentsOfFile:(NSString *)aPath;
- (id)initWithContentsOfURL:(NSURL *)aURL;

Answer 3

NSCoder is the standard Cocoa idiom for serialization.