dispatch_group_t readingGroup = dispatch_group_create();

    NSFileManager* manager = [NSFileManager defaultManager];

    NSString *docsDir =  [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"Data"];

    NSDirectoryEnumerator *dirEnumerator = [manager enumeratorAtURL:[NSURL fileURLWithPath:docsDir]
                                         includingPropertiesForKeys:[NSArray arrayWithObjects:NSURLNameKey,
                                                                     NSURLIsDirectoryKey,nil]
                                                            options:NSDirectoryEnumerationSkipsHiddenFiles
                                                       errorHandler:nil];


    // An array to store the all the enumerated file names in
    NSMutableArray *arrayFiles;

    // Enumerate the dirEnumerator results, each value is stored in allURLs
    for (NSURL *url in dirEnumerator) {

        // Retrieve the file name. From NSURLNameKey, cached during the enumeration.
        NSString *fileName;
        [url getResourceValue:&fileName forKey:NSURLNameKey error:NULL];

        // Retrieve whether a directory. From NSURLIsDirectoryKey, also cached during the enumeration.
        NSNumber *isDirectory;
        [url getResourceValue:&isDirectory forKey:NSURLIsDirectoryKey error:NULL];


        if (![isDirectory boolValue]) {

                dispatch_group_enter(readingGroup);
                TWReaderDocument* doc = [TWReaderDocument documentFileURL:url withCompletionBlock:^(BOOL success) {

                    dispatch_group_leave(readingGroup);

                }];

                [arrayFiles addObject:doc];

        }
        else if ([[[fileName componentsSeparatedByString:@"_" ] objectAtIndex:0] isEqualToString:@"ThalesData"]) {

            TreeItem* treeItem = [[TreeItem alloc] init];

            arrayFiles = [NSMutableArray arrayWithCapacity:10];

            treeItem.child = arrayFiles;
            treeItem.nodeName = [[fileName componentsSeparatedByString:@"_" ] lastObject];
            [self addItem:treeItem];


        }
    }

    dispatch_group_notify(readingGroup, dispatch_get_main_queue(), ^{ // 4

        NSLog(@"All concurrent tasks completed");

    });