Question

我看过一些帖子建议在块中使用类似下面的内容来异步下载图像，以便在主线程上调用setNeedsDisplay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，但下载后图像不会显示。通常大约有4到5秒的延迟。我知道这一点，因为如果我选择任何特定行，则会显示图像，而其他行仍未显示。

-(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");和NSLog(@"main thread");调用按照以下顺序打印，用于调用最初的6个单元格，这是我所期望的。但仍然无效：

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

有什么想法吗？

Answer 1

尝试在主线程中设置下载的图像。

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

最好将图像添加为不属于单元格的子视图，而是cell.contentView。

cellForRowAtIndexPath：setNeedsDisplay不适用于在后台下载的图像，即使在主线程上调用它也是如此

1 个答案: