Question

I’m having trouble in writing mutable dictionary to a file. Here’s the code that I’m writing.
I’m reading the file like below: (first time when app is ran, it won’t have any data)

NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString* documentsDirectory = [paths objectAtIndex:0];
self.favSaveFilePath = [[NSString alloc] initWithString:[documentsDirectory stringByAppendingPathComponent:@"Favorites.plist"]];
if([fm fileExistsAtPath:favSaveFilePath])
{
    favoriteJokesDictionary = [[NSMutableDictionary alloc] initWithContentsOfFile:self.favSaveFilePath];
    favoriteJokes = [[NSMutableArray alloc] initWithArray:[self.favoriteJokesDictionary objectForKey:@"FavoriteJokes"]];
    return;
}

I’m adding new dictionary to array as below:

-(BOOL)addJokeToFavorite:(NSDictionary*)joke
{
    BOOL success = NO;
    [self.favoriteJokes addObject:joke];
    success = [self.favoriteJokesDictionary writeToFile:self.favSaveFilePath atomically:YES];
    return success;
}

I don’t know why its not wring the dictionary to file. Can any one point me the mistake that I’m doing?

Answer 1

The Cocoa API is very specific about what kind of values can legally be in a dictionary when it is written out to file. One particular limitation that has been known to cause problems and which is not thoroughly discussed in the official API documentation is that all of the keys in your dictionary must be of type NSString (and your values must be one of NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary), even though the dictionary itself supports keys and values of any object type.

The reason for this restriction has to do with the fact that the dictionary gets stored as an XML document (or “property-list”, in Apple-speak) by default. So I’d recommend verifying that your dictionary meets the requirements to be saved to file, and restructuring it if it doesn’t.

Or, if restructuring your dictionary is not feasible, you might want to take a look at the NSKeyedArchiver class instead.