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我有一个带UIImageView字段的 tableView。我在从settersetupCell调用的方法中设置它。set_bookSetter 从 调用tableView:willDisplayCell:forRowAtIndexPath:。这是 setupCell 的一部分：

dispatch_queue_t downloadImage = dispatch_queue_create("Download Image", NULL);
    dispatch_async(downloadImage, ^{
        NSString* key = [_book.internalID stringByAppendingPathExtension:@"thumb"];
        UIImage* thumb = [[[LTImagesCacheController shared] cache] objectForKey:key];
        if (thumb == nil)
        {
            dispatch_async(dispatch_get_main_queue(), ^{
                _bookImageView.layer.borderWidth = 0.0;
                _bookImageView.image = [[LTImageProvider sharedImagesProvider] imageForName:@"DefaultBook.png" cache:YES];
            });

            NSString* imageName = [[LTImagesCacheController cacheDir] stringByAppendingPathComponent:key];
            if([[NSFileManager defaultManager] fileExistsAtPath:imageName])
            {
                NSDictionary *attr = [NSDictionary dictionaryWithObjectsAndKeys:[NSDate date], NSFileModificationDate, nil];
                [[NSFileManager defaultManager] setAttributes:attr ofItemAtPath:imageName error:nil];
                thumb = [UIImage imageWithContentsOfFile:imageName];
                [[[LTImagesCacheController shared] cache] setObject:thumb forKey:key];
            }
            else
            {
                NSString* url = _book.coverURL;
                thumb = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:url]]];
                if (thumb != nil)
                    [[[LTImagesCacheController shared] cache] setObject:thumb forKey:key];
            }
        }

        if (thumb != nil)
        {
            dispatch_async(dispatch_get_main_queue(), ^{
                _bookImageView.layer.borderWidth = 1.0;
                _bookImageView.image = thumb;
            });
        }
    });
dispatch_release(downloadImage);

当我运行我的应用程序时，有时我会在短时间内冻结 UI，不到 1 秒。我做错了什么？我想将 DefaultBook.png 设置为默认值，如果存在 _book.coverURL 中的图像，请设置它。我有大约 100-200 个单元格，拇指图像大小为 15-30 KB。

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