如何使用dispatch_async按照UITableView中的单元格顺序下载照片？ [英] How can i download photos by order of cells in UITableView with dispatch_async?

问题描述

我有一个有照片的UITableView，我从URL获取这些照片，我使用该块异步下载这些照片，我希望这些照片按照UITableView中的单元格顺序下载？

I have a UITableView that has photos, i get these photos from URLs and i use the block for downloading these photo asynchronously, and i want these photos be downloaded by order of the cells in the 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;
    });
});

推荐答案

你需要这样做：

1）首先创建并且可变数组充满NSNull。

1) First create and mutable array full of NSNull.

2）在CellForRowAtIndexPath上，询问该indexPath处的数组.row等于NSNull。

2) On CellForRowAtIndexPath, ask if the array at that indexPath.row is equal to NSNull.

2.1）如果相等，请下载如下图像：

2.1) If is equal, download the image like this:

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）如果不相等（有图像），只需在图像中设置图像。

2.2) If is not equal(have an Image) just set the image in the array.

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

通过执行此操作，您的图像可以保持单元格顺序，并提高滚动的性能。

By Doing this your images keep cell order and you increase the performance of the scrolling.

PD-1：您应该使用一个库来处理图像的下载和缓存，如HTTPRequest

PD-1: You should use a library that handles the download and caching of the images like HTTPRequest

PD-2：在你的ReReceiveMemoryWarning你应该这样做：

PD-2: In your Did reReceiveMemoryWarning you should do something like this:

- (void)didReceiveMemoryWarning
{
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    [imagesArray removeAllObjects];
    //Fill the array with NSNull's again;

}

进一步改善：

在数组中你可以有3种状态：

In the Array you could have 3 states:

1. NSNull：无图像


2. NewState（EG。some string）：正在下载图片。


3. UIImage：图片已下载。

使用此功能，如果在下载图像时连续向上和向下滚动，可以防止多次下载相同的图像。

With this you could prevent for downloading several times the same image if you scroll up and down continuously while the images are being downloaded.

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