iPhone - archiving array of custom objects

Question

I've been trying for hours and I cannot solve this problem.

I'm making an app that saves unfinished Chess Games, so I'm trying to write an array to a file.

This is what the array is, if it makes sense:

-NSMutableArray savedGames
--GameSave a
---NSMutableArray board;
----Piece a, b, c, etc.
-----some ints
---NSString whitePlayer, blackPlayer;
---int playerOnTop, turn;
--GameSave b
---NSMutableArray board;
----Piece a, b, c, etc.
-----some ints
---NSString whitePlayer, blackPlayer;
---int playerOnTop, turn;

etc.

And these are my NSCoding methods:

GameSave.m

- (void)encodeWithCoder:(NSCoder *)coder {
    [coder encodeObject:whitePlayer forKey:@"whitePlayer"];
    [coder encodeObject:blackPlayer forKey:@"blackPlayer"];
    [coder encodeInt:playerOnTop forKey:@"playerOnTop"];
    [coder encodeInt:turn forKey:@"turn"];
    [coder encodeObject:board forKey:@"board"];
}

- (id)initWithCoder:(NSCoder *)coder {
    self = [[GameSave alloc] init];
    if (self != nil)
    {
        board = [coder decodeObjectForKey:@"board"];
        whitePlayer = [coder decodeObjectForKey:@"whitePlayer"];
        blackPlayer = [coder decodeObjectForKey:@"blackPlayer"];
        playerOnTop = [coder decodeIntForKey:@"playerOnTop"];
        turn = [coder decodeIntForKey:@"turn"];
    }   
    return self;
}

Piece.m

- (void)encodeWithCoder:(NSCoder *)coder {
    [coder encodeInt:color forKey:@"color"];
    [coder encodeInt:piece forKey:@"piece"];
    [coder encodeInt:row forKey:@"row"];
    [coder encodeInt:column forKey:@"column"];
}

- (id)initWithCoder:(NSCoder *)coder {
    self = [[Piece alloc] init];
    if (self != nil)
    {
        color = [coder decodeIntForKey:@"color"];
        piece = [coder decodeIntForKey:@"piece"];
        row = [coder decodeIntForKey:@"row"];
        column = [coder decodeIntForKey:@"column"];
    }   
    return self;
}

And this is the code that tries to archive and save to file:

- (void)saveGame {
    ChessSaverAppDelegate *delegate = (ChessSaverAppDelegate *) [[UIApplication sharedApplication] delegate];
    [[delegate gameSave] setBoard:board];

    NSMutableArray *savedGames = [NSKeyedUnarchiver unarchiveObjectWithFile:[self dataFilePath]];
    if (savedGames == nil) {
        [NSKeyedArchiver archiveRootObject:[delegate gameSave] toFile:[self dataFilePath]];
    } else {
        [savedGames addObject:[delegate gameSave]];
        [NSKeyedArchiver archiveRootObject:savedGames toFile:[self dataFilePath]];
    }
}

- (NSString *)dataFilePath {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    return [documentsDirectory stringByAppendingPathComponent:@"gameSaves.plist"];
}

Sorry, here's the problem:

After setting some breakpoints, an error is reached after this line from -saveGame:
[NSKeyedArchiver archiveRootObject:savedGames toFile:[self dataFilePath]];

And this is what shows up in the console:

2010-05-11 17:04:08.852 ChessSaver[62065:207] *** -[NSCFType encodeWithCoder:]: unrecognized selector sent to instance 0x3d3cd30
2010-05-11 17:04:08.891 ChessSaver[62065:207] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSCFType encodeWithCoder:]: unrecognized selector sent to instance 0x3d3cd30'
2010-05-11 17:04:08.908 ChessSaver[62065:207] Stack: (
    32339035,
    31077641,
    32720955,
    32290422,
    32143042,
    238843,
    25827,
    238843,
    564412,
    342037,
    238843,
    606848,
    17686,
    2733061,
    4646817,
    2733061,
    3140430,
    3149167,
    3144379,
    2837983,
    2746312,
    2773089,
    41684313,
    32123776,
    32119880,
    41678357,
    41678554,
    2777007,
    9884,
    9738
)

If it matters, -saveGame is called from a UIBarButton in a navigation controller.

Any help is appreciated, thanks.

Answer 1

This is wrong:

- (id)initWithCoder:(NSCoder *)coder {
    self = [[Piece alloc] init];
    if (self != nil)
    {

By the time initWithCoder: is called an instance of the class has already been allocated. In your code you are leaking that instance. You should be doing this:

- (id)initWithCoder:(NSCoder *)coder {
    self = [super init]; // or [self init] if needed
    if (self != nil)
    {

Also, when you decode an object, you don't own it, so you need to retain it.

        board = [[coder decodeObjectForKey:@"board"] retain];

If you don't "get" retain/release rules, learn them now, or you will be continually shooting yourself in the foot.

Also, your saveGame method has several problems.

1. If savedGames is nil, you are saving an object, not an array, to the file. That means on the next run, the unarchived object will not be an array as you expect.
2. Unarchived arrays are immutable. Declaring the type to NSMutableArray doesn't matter, if you archive a mutable array you will get an immutable one on unarchive. You need to call mutableCopy on the unarchived array if you want to modify it.

I'm not sure about the exception, exactly, but fix the above problems and either it will go away or the solution will become easier to find.

Answer 2

If you can't get your NSKeyedArchiver to work, perhaps another option would be writeToFile:

- (NSString *) saveFilePath:(NSString*)fileName {
 NSArray *saveFilePathArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
 NSString *filePath = [[NSString alloc] initWithFormat:@"%@.extension", fileName];
 return [[saveFilePathArray objectAtIndex:0] stringByAppendingPathComponent:filePath];
 [saveFilePathArray release];
 [filePath release];
}

 ...
 NSMutableArray *yourUnsavedGameObject = [[NSMutableArray alloc] initWithArray:yourUnsavedGame];
 // Write the dictionary to a file
 [yourUnsavedGameObject writeToFile:[self saveFilePath] atomically:YES];
 ...