1

我已经看到一些帖子建议在块内使用类似以下的内容异步下载图像,以便setNeedsDisplay在主线程上调用并让它更快地显示。

dispatch_async(main_queue, ^{
                [view setNeedsDisplay];
            });

我正在尝试这个,如下所示,但图像在下载后不会立即显示。通常有大约 4 - 5 秒的延迟。我知道这一点,因为如果我选择任何特定的行,图像就会出现,而其他的仍然没有显示。

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

    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    }

    UIImageView *iv = (UIImageView*)[cell viewWithTag:kCellSubViewWavImageView];;

        //async for scroll performance
        dispatch_queue_t queue = 
        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_async(queue, ^{

            NSURL *url = [[NSURL alloc] initWithString:[self.user.favWavformURLAr objectAtIndex:indexPath.row]];
            NSLog(@"background");
            NSData *imageData = [[NSData alloc] initWithContentsOfURL:url];
            UIImage *image = [[UIImage alloc] initWithData:imageData];

            iv.image = image;

            dispatch_queue_t main_queue = dispatch_get_main_queue();
            dispatch_async(main_queue, ^{
                NSLog(@"main thread");
                [iv setNeedsDisplay];
            });

        });

    }

    return cell;
}

顺便说一句,下面的NSLog(@"background");andNSLog(@"main thread");调用按以下顺序打印,用于调用最初的 6 个单元格,我认为这是我所期望的。但仍然不起作用:

2012-02-17 20:46:27.120 SoundcloudFavs[8836:1c03] background
2012-02-17 20:46:27.169 SoundcloudFavs[8836:1b03] background
2012-02-17 20:46:27.170 SoundcloudFavs[8836:6b07] background
2012-02-17 20:46:27.173 SoundcloudFavs[8836:7503] background
2012-02-17 20:46:27.174 SoundcloudFavs[8836:7103] background
2012-02-17 20:46:27.177 SoundcloudFavs[8836:8003] background
2012-02-17 20:46:27.219 SoundcloudFavs[8836:207] main thread
2012-02-17 20:46:27.270 SoundcloudFavs[8836:207] main thread
2012-02-17 20:46:27.282 SoundcloudFavs[8836:207] main thread
2012-02-17 20:46:27.285 SoundcloudFavs[8836:207] main thread
2012-02-17 20:46:27.296 SoundcloudFavs[8836:207] main thread
2012-02-17 20:46:27.300 SoundcloudFavs[8836:207] main thread

有任何想法吗?

4

1 回答 1

3

尝试在主线程中设置下载的图像。

dispatch_async(main_queue, ^{
            NSLog(@"main thread");
            iv.image = image;
        });

此外,最好将图像添加为子视图,而不是单元格而是cell.contentView.

于 2012-02-17T21:25:17.960 回答