Save own Class with NSCoder

Question

I'm trying to store some custom class/data to a file in my iPhone/iPad app.

I have a Class RSHighscoreList

@interface RSHighscoreList : NSObject {
    NSMutableArray *list;
}

which contains objects of RSHighscore in the list

@interface RSHighscore : NSObject {
    NSString *playerName;
    NSInteger points;
}

When I try to store all to file

- (void)writeDataStore {
    RSDataStore *tmpStore = [[RSDataStore alloc] init];
    _tmpStore.highscorelist = self.highscorelist.list;
    NSMutableData *data = [[NSMutableData alloc] init];
    NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];

    [archiver encodeObject:tmpStore forKey:kDataKey];
    [archiver finishEncoding];
    [data writeToFile:[self dataFilePath] atomically:YES];

    [archiver release];
    [data release];
}

@interface RSDataStore : NSObject <NSCoding, NSCopying> {
    NSMutableArray *highscorelist; 
}

- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:highscorelist forKey:@"Highscorelist"];
}

The app will crash with an error message

-[RSHighscore encodeWithCoder:]: unrecognized selector sent to instance 0x573cc20
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[RSHighscore encodeWithCoder:]: unrecognized selector sent to instance 0x573cc20'

I wonder why the error tells about RSHighscore, even if that is 'wrapped'. Does anyone have a good idea?

Answer 1

The class you're going to encode or initWithCoder should conform to <NSCoding> protocol So you just should add this in your interface, otherwise indeed the runtime will not recognize the selector as it's the part of <NSCoding> protocol

Answer 2

RSDataStore has an -encodeWithCoder: method, but (according to the error message) RSHighscore doesn't. You need to implement the NSCoding protocol for every class you're serializing.

@implementation RSHighscore
static NSString *const kPlayerName = @"PlayerName";
static NSString *const kPoints = @"Points";

-(id)initWithCoder:(NSCoder *)decoder {
    if ((self=[super init])) {
        playerName = [[decoder decodeObjectForKey:kPlayerName] retain];
        points = [decoder decodeIntegerForKey:kPoints];
    }
    return self;
}
-(void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:playerName forKey:kPlayerName];
    [encoder encodeInt:points forKey:kPoints];
}
...

If the base class of RSHighscore is ever changed to something other than NSObject, the -initWithCoder: method might need to be changed to call [super initWithCoder:decoder] rather than [super init]. Alternatively, add <NSCoding> to NSObject and change RSHighscore's -initWithCoder: now.

@interface NSObject (NSCoding)
-(id)initWithCoder:(NSCoder*)decoder;
-(void)encodeWithCoder:(NSCoder*)encoder;
@end

@implementation NSObject (NSCoding)
-(id)initWithCoder:(NSCoder*)decoder {
    return [self init];
}
-(void)encodeWithCoder:(NSCoder*)encoder {}
@end

@implementation RSHighscore
-(id)initWithCoder:(NSCoder *)decoder {
    if ((self=[super initWithCoder:decoder])) {
        playerName = [[decoder decodeObjectForKey:kPlayerName] retain];
        points = [decoder decodeIntegerForKey:kPoints];
    }
    return self;
}
...