IIC中的UICollectionView didSelectItemAtIndexPath崩溃

Question

请指导一下路径。

 NSIndexPath *index = [[self.collectionView indexPathsForSelectedItems]objectAtIndex:0];
    _assetVersion = assetVer;
    _bookPosition = [[bookList objectAtIndex:index.row]bookPosition];
    _bookId = [[bookList objectAtIndex:index.row]bookID];
    bookPath = [[bookList objectAtIndex:index.row]bookPath];
    _downloadURLbook = [[bookList objectAtIndex:index.row]downloadURL];
    NSLog(@"%@",_bookPosition);

    cell.bookId = _bookId;
      NSLog(@"%@",bookPath);

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{     
            NSString *stringURL = [NSString stringWithFormat:@"http://powershelf-dev.3viewsolution.com/dhl/webservice/book_extract.php?main_menu=%@&language=%@&id=%@",_checkType,_checkLang,(_bookPosition)];
            NSURLResponse *response = nil;
            NSError *error = nil;
            NSURL *url = [NSURL URLWithString:stringURL];
            NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
            NSURLConnection *urlConnection = [[NSURLConnection alloc]init];
            NSData *data = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:&response error:&error];
            [self connection:urlConnection didReceiveResponse:response];
            [self connection:urlConnection didReceiveData:data];
            dispatch_async(dispatch_get_main_queue(), ^{

            //code for check data

            });
    });

我不知道这个为什么总是像这样崩溃

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[DownloadedBookDetail bookPosition]: unrecognized selector sent to instance 0x146cf4a0'

但是，当我使用来自网络服务的图像时，它是有效的。如果我使用bookIndex = indexPath;没有代码

NSIndexPath *index = [[self.collectionView indexPathsForSelectedItems]objectAtIndex:0];
        _assetVersion = assetVer;
        _bookPosition = [[bookList objectAtIndex:index.row]bookPosition];
        _bookId = [[bookList objectAtIndex:index.row]bookID];
        bookPath = [[bookList objectAtIndex:index.row]bookPath];
        _downloadURLbook = [[bookList objectAtIndex:index.row]downloadURL];

它也会起作用。但我需要使用来自数据库检查数据的数据和Web服务。 我需要将bookIndex = indexPath;与Web服务一起使用 我去看看这个

if([db isDownloaded:bookName bookVersion:[bookVersion floatValue]]){    
                        [self performSegueWithIdentifier:@"reader" sender:self];
                    }
                    else{
                        [self startDownload];
                    }   

感谢任何想法。