iOS Subclassing UIDocument

#import "PTKDocument.h"
#import "PTKData.h"
#import "PTKMetadata.h"
#import "UIImageExtras.h"
 
#define METADATA_FILENAME   @"photo.metadata"
#define DATA_FILENAME       @"photo.data"
 
@interface PTKDocument ()
@property (nonatomic, strong) PTKData * data;
@property (nonatomic, strong) NSFileWrapper * fileWrapper;
@end
 
@implementation PTKDocument 
@synthesize data = _data;
@synthesize fileWrapper = _fileWrapper;
@synthesize metadata = _metadata;

- (void)encodeObject:(id<NSCoding>)object toWrappers:(NSMutableDictionary *)wrappers preferredFilename:(NSString *)preferredFilename {
    @autoreleasepool {            
        NSMutableData * data = [NSMutableData data];
        NSKeyedArchiver * archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
        [archiver encodeObject:object forKey:@"data"];
        [archiver finishEncoding];
        NSFileWrapper * wrapper = [[NSFileWrapper alloc] initRegularFileWithContents:data];
        [wrappers setObject:wrapper forKey:preferredFilename];
    }
}
 
- (id)contentsForType:(NSString *)typeName error:(NSError *__autoreleasing *)outError {
 
    if (self.metadata == nil || self.data == nil) {        
        return nil;    
    }
 
    NSMutableDictionary * wrappers = [NSMutableDictionary dictionary];
    [self encodeObject:self.metadata toWrappers:wrappers preferredFilename:METADATA_FILENAME];
    [self encodeObject:self.data toWrappers:wrappers preferredFilename:DATA_FILENAME];   
    NSFileWrapper * fileWrapper = [[NSFileWrapper alloc] initDirectoryWithFileWrappers:wrappers];
 
    return fileWrapper;
 
}