GCD UITableView异步加载图像，加载错误的单元格，直到新的图像下载

问题描述

我有一个UITableView与自定义单元格。我使用Grand Central Dispatch异步加载图像。一切工作正常，但当我向下滚动，以前加载的图像显示，直到新的图像下载。这是我的代码：

I have a UITableView with custom cells. I load images asynchronously using Grand Central Dispatch. Everything works fine, but when I scroll down, previously loaded images are shown till the new image is downloaded. Here is my code:

if (![[NSFileManager defaultManager] fileExistsAtPath:[path stringByAppendingPathComponent:@"image.png"]])
{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,  0ul);
    dispatch_async(queue, ^{
        NSString *url=[pat stringByAppendingPathComponent:@"comments.txt"];
        NSString *u=[NSString stringWithContentsOfFile:url encoding:NSUTF8StringEncoding error:nil];
        NSURL *imageURL=[NSURL URLWithString:u];
        NSData *image=[NSData dataWithContentsOfURL:imageURL];
        [image writeToFile:[pat stringByAppendingPathComponent:@"image.png"] atomically:YES];
        dispatch_sync(dispatch_get_main_queue(), ^{
            cell.imageView.image=[UIImage imageWithContentsOfFile:[pat stringByAppendingPathComponent:@"image.png"]];
            [cell setNeedsLayout];
            NSLog(@"Download");
        });
    });
}
else
{
    NSLog(@"cache");
    cell.imageView.image=[UIImage imageWithContentsOfFile:[pat stringByAppendingPathComponent:@"image.png"]];
}

任何建议。
P.S.我重复使用单元格

Any suggestions appreciated.P.S. I reuse the cells

推荐答案

而不是捕获您需要捕获索引路径的单元格， / p>

Rather than capturing the cell you need to capture the index path, then get the cell back using:

UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

这样，如果单元格现在离屏，你会得到零，

That way, if the cell is now off screen you'll get nil back and the image won't be set on the wrong cell.

在 dispatch_async（）之后，您需要添加的另一件事是 cell.imageView.image = somePlaceholderImage 。

The other thing you need to add after your dispatch_async() is a cell.imageView.image=somePlaceholderImage.

例如：

if (![[NSFileManager defaultManager] fileExistsAtPath:[path stringByAppendingPathComponent:@"image.png"]])
{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,  0ul);
    dispatch_async(queue, ^{
        NSString *url=[pat stringByAppendingPathComponent:@"comments.txt"];
        NSString *u=[NSString stringWithContentsOfFile:url encoding:NSUTF8StringEncoding error:nil];
        NSURL *imageURL=[NSURL URLWithString:u];
        NSData *image=[NSData dataWithContentsOfURL:imageURL];
        [image writeToFile:[pat stringByAppendingPathComponent:@"image.png"] atomically:YES];
        dispatch_sync(dispatch_get_main_queue(), ^{
            UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
            cell.imageView.image=[UIImage imageWithContentsOfFile:[pat stringByAppendingPathComponent:@"image.png"]];
            [cell setNeedsLayout];
            NSLog(@"Download");
        });
    });
    cell.imageView.image=[UIImage imageNamed:@"placeholder"];
}
else
{
    NSLog(@"cache");
    cell.imageView.image=[UIImage imageWithContentsOfFile:[pat stringByAppendingPathComponent:@"image.png"]];
}

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