Question

Short Version:

I define a property with (nonatomic, retain) and assumed that the property would be retained. But unless I call retain when assigning a dictionary to the property, The app crashes with an EXEC BAD ACCESS error.

Long Version:

I have a singleton which has a dictionary. The header is defined like this

@interface BRManager : NSObject {
}

@property (nonatomic, retain) NSMutableDictionary *gameState;

+ (id)sharedManager;
- (void) saveGameState;

@end

In the implementation file, I have a method that’s called in the init. This method loads a plist form the bundle and makes a copy of it in the users documents folder on the device.

- (void) loadGameState
{
 
    NSFileManager *fileManger=[NSFileManager defaultManager];
    NSError *error;
    NSArray *pathsArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    NSString *doumentDirectoryPath=[pathsArray objectAtIndex:0];
    NSString *destinationPath= [doumentDirectoryPath stringByAppendingPathComponent:@"gameState.plist"];
    NSLog(@"plist path %@",destinationPath);    
    
    if (![fileManger fileExistsAtPath:destinationPath]){
        
        NSString *sourcePath=[[[NSBundle mainBundle] resourcePath]stringByAppendingPathComponent:@"gameStateTemplate.plist"];
        [fileManger copyItemAtPath:sourcePath toPath:destinationPath error:&error];
        
        gameState = [NSMutableDictionary dictionaryWithContentsOfFile:sourcePath];
                
    }else{
        gameState = [NSMutableDictionary dictionaryWithContentsOfFile:destinationPath];
    }
    
}

Now here’s how I thought this should work. In the header I define the gameState property with (nonatomic, retain). I assumed (probably incorrectly) that ‘retain’ meant that the gameState dictionary would be retained. However, I have another method in my singleton (saveGameState) that get’s called when the AppDelegate -> ‘applicationWillResignActive‘.

- (void) saveGameState
{
    NSArray *pathsArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    NSString *doumentDirectoryPath=[pathsArray objectAtIndex:0];
    NSString *plistPath = [doumentDirectoryPath stringByAppendingPathComponent:@"gameState.plist"];
    [gameState writeToFile:plistPath atomically:YES];
}

This throws an EXEC BAD ACCESS error on gameState. If I modify loadGameState to retain the gameState dictionary, everything works as it should. eg:

gameState = [[NSMutableDictionary dictionaryWithContentsOfFile:sourcePath] retain];

I’m guessing this is the correct behaviour, but why? Does (nonatomic, retain) not mean what I think it means, or is something else at play here?

I’ve not really grok’d memory management yet, so I stumble on this stuff all the time.

Answer 1

You must use the accessor:

self.gameState = [NSMutableDictionary dictionaryWithContentsOfFile:sourcePath];

or (is equivalent to):

[self setGameState:[NSMutableDictionary dictionaryWithContentsOfFile:sourcePath]];

instead of

gameState = [NSMutableDictionary dictionaryWithContentsOfFile:sourcePath];

which only sets the ivar without any notion of property.