我如何下载照片的单元格的顺序在UITableView与dispatch_async

Question

我有一个UITableView有照片，我从url获取这些照片我使用块异步下载这些照片，我希望这些照片下载的顺序单元格在UITableView?

   // Download the images from the review of the businesses and put them into the "imageArray"
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://feature.site.com%@",entry.avatarUrl]]];
    UIImage *imageField = [[UIImage alloc] initWithData:imageData];
    dispatch_async(dispatch_get_main_queue(), ^{
        [imageArray addObject:imageField];
        photo.image = imageField;
    });
});

Answer 1

您需要这样做:

1)首先创建一个充满NSNull的可变数组

2)在CellForRowAtIndexPath上，询问该indexPath上的数组。row = NSNull

2.1)如果相等，则像这样下载图像:

dispatch_queue_t bgqueue = dispatch_queue_create("com.yourApp.yyourclass.bgqueue", NULL);
dispatch_async(bgqueue, ^{
    NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://feature.site.com%@",entry.avatarUrl]]];
    UIImage *imageField = [[UIImage alloc] initWithData:imageData];
    dispatch_async(dispatch_get_main_queue(), ^{
        [imageArray addObject:imageField atIndex:indexPath.row];
        tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone]
    });
});
dispatch_release(bgqueue);

2.2)如果不等于(有一个图像)，就在数组中设置图像。

photo.image = [imageArray objectAtIndex:indexPath.row];

通过这样做，您的图像保持单元格的顺序，并提高滚动的性能。

PD-1:您应该使用一个库来处理图像的下载和缓存，如HTTPRequest

PD-2:在你的Did recreceivememywarning中，你应该这样做:

- (void)didReceiveMemoryWarning
{
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];
    [imagesArray removeAllObjects];
    //Fill the array with NSNull's again;
}

进一步改进:

在数组中你可以有三种状态:

1. NSNull: No Image
2. NewState(如。
3. UIImage: Image download .

如果你在下载图片的时候连续上下滚动，你可以防止下载几次相同的图片