Question

I have a custom object that simply inherits from NSObject. It has 3 members – two floats and an NSDate.

My app is going to have an array with a number of these objects kicking around, and I need to persist it between runs. How can I accomplish this?

I’ve thought about using a SQLite db, but I’m thinking that it’s a bit overkill since the only query I’d ever do would be select *.

In an ideal world I’d like to use an xml plist file. I’m not sure if I can do this with my custom object though. I know there’s a set of Property List objects, and that NSArray comes under that, but writeToFile:atomically: only works with property list objects.

Any thoughts would be appreciated, thanks!

Answer 1

NSCoding will do exactly what you want. I recommend you read up on it in the Apple docs, but I thought it was pretty straightforward to use. Your class (and any child classes) will need to implement the NSCoding protocol and you’ll need to add -encodeWithCoder: and -initWithCoder: methods to your objects. Most of the common framework classes implement NSCoding already.

The code for your class will look something like this:

-(void) encodeWithCoder: (NSCoder*) coder {   [coder encodeInteger: versionValue forKey: versionKey];   [coder encodeObject: myStuff forKey: myStuffKey]; }  -(id) initWithCoder: (NSCoder*) coder {   self = [super init];   if ( ! self) return nil;   myStuff = [[coder decodeObjectForKey: myStuffKey] retain];   return self; } 

It’s recommended you add a version number when encoding to give you flexibility to manage changes to your archive format in future versions.

In my class, I added a convenience method to archive my object:

-(void) archiveToFile: (NSString*) path {   NSMutableData *data = [[NSMutableData alloc] init];   NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData: data];   [archiver encodeObject: self forKey: myArchiveKey];   [archiver finishEncoding];   [archiver release];   [data writeToFile: path atomically: YES];   [data release]; } 

and another one to unarchive or create a new object:

+(MyArchive*) newFromFile: (NSString*) path             orWithMyStuff: (MyStuff*) myStuff {   NSData *data = [[NSData alloc] initWithContentsOfFile: path];   NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData: data];   MyArchive *myArchive = [unarchiver decodeObjectForKey: myArchiveKey];   [unarchiver finishDecoding];    if (myArchive) {     [myArchive retain];   } else {     myArchive = [[MyArchive alloc] initWithStuff: myStuff;   }   [unarchiver release];   [data release];   return myArchive; } 

Since your top level object is an NSArray, you’ll need to modify the last two methods for your case, but most of the boilerplate code will be the same.