iPhone: easy to save a file.

To save a file in folder 'Documents':

NSData *dat = [[NSData alloc] init]; // init data.
UIImage *image = [[UIImage alloc] initWithData:dat];

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docDir = [paths objectAtIndex: 0];
NSString * pathFile = [NSString stringWithFormat:@"image_%s.png", [[[NSDate date] descriptionWithCalendarFormat:@"%H_%M_%S" timeZone:nil locale: [NSUserDefaults standardUserDefaults] dictionaryRepresentation]] UTF8String]];
NSString *docFile = [docDir stringByAppendingPathComponent: pathFile];

NSLog(@"+++++++++++++++++++ %@", docFile);

NSData *imageData = UIImagePNGRepresentation(image);
[imageData writeToFile:docFile atomically:NO];