在单元格中获取图像（PFFile）是阻止滚动

Question

我有一个与 Parse 相关的应用程序，我在使用自定义单元格显示表格视图时出现问题UIImageView.首次尝试下载照片时， 用户界面停止响应约1-2秒。

问题似乎是UIImage *image = [UIImage imageWithData: data];，它不在主要队列中，但仍然阻止 UI 。

有什么问题？

我使用的代码是：

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

        MyEvent *currentEvent = self.tableViewElements[indexPath.section][indexPath.row]; //class that stores event details without UIImages. UIImages are stored in a NSMutableDictionary

        NSDate *currentDate = currentEvent.dateStart;
        NSCalendar* calendar = [NSCalendar currentCalendar];
        NSDateComponents* components = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit fromDate:currentDate]; 

InvitationTableViewCell *cell = [tableView       dequeueReusableCellWithIdentifier:@"InvitationTableViewCell"];

        cell.eventImageView.tag = indexPath.row;

        cell.eventImageView.image = nil;

        cell.eventNameLabel.text = currentEvent.name;
        cell.eventDayLabel.text = [NSString stringWithFormat:@"%d", [components day]];
        cell.eventMonthLabel.text = monthsNameArray[[components month] -1];

        if ([[AppModel sharedModel].myEventsImagesDictionary objectForKey:currentEvent.eventId])
        {
            UIImage *image = [[AppModel sharedModel].myEventsImagesDictionary objectForKey:currentEvent.eventId];
            cell.eventImageView.image = image;
        }
        else
            if (currentEvent.pictureFile)
            {
                dispatch_queue_t downloadQueue = dispatch_queue_create("Image Downloader", NULL);

                dispatch_async(downloadQueue, ^{
                    //currentEvent.pictureFile is a PFFile
                    [currentEvent.pictureFile getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
                        if (!error) {
                            if (cell.eventImageView.tag == indexPath.row)
                            {
                                UIImage *image = [UIImage imageWithData: data];
                                UIImage *croppedImage = [Utility cropImage:image toFitInSize:CGSizeMake(320.0, 90.0)];
                                image = nil;
                                [[AppModel sharedModel].myEventsImagesDictionary setObject:croppedImage forKey: currentEvent.eventId];
                                dispatch_async(dispatch_get_main_queue(), ^{
                                    if (croppedImage)
                                    {
                                        cell.eventImageView.image = croppedImage;
                                    }
                                });
                            }
                        }
                    }];
                });
            }
        return cell;
}