NSOperationqueue 背景，下载图片

Question

我创建了一个NSOperationQueue 来下载图片（来自 Twitter for Cell）：

NSOperationQueue *queue = [[NSOperationQueue alloc]init];
   [queue addOperationWithBlock:^{
    NSString *ImagesUrl = [[NSString alloc]initWithFormat:@"http://api.twitter.com/1/users/profile_image/%@",[[status objectForKey:@"user"]objectForKey:@"screen_name"]];;
        NSURL *imageurl = [NSURL URLWithString:ImagesUrl];
        UIImage *img = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageurl]];
        [[NSOperationQueue mainQueue]addOperationWithBlock:^{
            if (img.size.width == 0 || [ImagesUrl isEqualToString:@"<null>"]) {
                [statusCell.imageCellTL setFrame:CGRectZero];
                statusCell.imageCellTL.image = [UIImage imageNamed:@"Placeholder"] ;
            }else

            [statusCell.imageCellTL setImage:img];

这工作正常，但是当它似乎移动滚动并查看图像时仍在加载，并且它们会更改几次，直到您获得图片。

而且我不喜欢诊断时间档案，所以我想以某种方式在后台制作这个NSOperationQueue

还可以展示如何使“Imagecache”无需下载已下载的图像。

**（状态 = Twitter 时间线的 NSDictionary）。

编辑::(所有单元格)

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


    static NSString *CellIdentifier = @"Celulatime";
    UITableViewCell *Cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];


    if ( [Cell isKindOfClass:[TimeLineCell class]] ) {
        TimeLineCell *statusCell = (TimeLineCell *) Cell;
        status = [self.dataSource objectAtIndex:indexPath.row];


        statusCell.TextCellTL.text = [status objectForKey:@"text"];
        statusCell.NomeCellTL.text = [status valueForKeyPath:@"user.name"];
        statusCell.UserCellTL.text = [NSString stringWithFormat:@"@%@", [status valueForKeyPath:@"user.screen_name"]];


        NSDate *created_at = [status valueForKey:@"created_at"];
        if ( [created_at isKindOfClass:[NSDate class] ] ) {
            NSTimeInterval timeInterval = [created_at timeIntervalSinceNow];
            statusCell.timeCellTL.text = [self timeIntervalStringOf:timeInterval];
        } else if ( [created_at isKindOfClass:[NSString class]] ) {
            NSDate *date = [self.twitterDateFormatter dateFromString: (NSString *) created_at];
            NSTimeInterval timeInterval = [date timeIntervalSinceNow];
            statusCell.timeCellTL.text = [self timeIntervalStringOf:timeInterval];
        }

        NSString *imageUrlString = [[NSString alloc]initWithFormat:@"http://api.twitter.com/1/users/profile_image/%@",[[status objectForKey:@"user"]objectForKey:@"screen_name"]];;
        UIImage *imageFromCache = [self.imageCache objectForKey:imageUrlString];

        if (imageFromCache) {
            statusCell.imageCellTL.image = imageFromCache;
            [statusCell.imageCellTL setFrame:CGRectMake(9, 6, 40, 40)]; 
        }
        else
        {
            statusCell.imageCellTL.image = [UIImage imageNamed:@"TweHitLogo57"];
            [statusCell.imageCellTL setFrame:CGRectZero]; 

            [self.imageluckluck addOperationWithBlock:^{
                NSURL *imageurl = [NSURL URLWithString:imageUrlString];
                UIImage *img = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageurl]];

                if (img != nil) {


                    [self.imageCache setObject:img forKey:imageUrlString];

                    // now update UI in main queue
                    [[NSOperationQueue mainQueue] addOperationWithBlock:^{

                        TimeLineCell *updateCell = (TimeLineCell *)[tableView cellForRowAtIndexPath:indexPath];

                        if (updateCell) {
                            [updateCell.imageCellTL setFrame:CGRectMake(9, 6, 40, 40)]; 
                            [updateCell.imageCellTL setImage:img];
                        }
                    }];
                }
            }];
        }


        }
    return Cell;
    }

Answer 1

几个观察：

1. 您可能应该在您的类中定义一个NSOperationQueue 并在viewDidLoad（以及NSCache）中对其进行初始化，并向该队列添加操作，而不是为每个图像创建一个新的NSOperationQueue。此外，许多服务器限制了它们将支持来自每个客户端的并发请求数，因此请确保相应地设置 maxConcurrentOperationCount。

   @interface ViewController ()
   @property (nonatomic, strong) NSOperationQueue *imageOperationQueue;
   @property (nonatomic, strong) NSCache *imageCache;
   @end
   
   @implementation ViewController
   
   - (void)viewDidLoad
   {
        [super viewDidLoad];
   
       self.imageOperationQueue = [[NSOperationQueue alloc]init];
       self.imageOperationQueue.maxConcurrentOperationCount = 4;
   
       self.imageCache = [[NSCache alloc] init];
   }
   
   // the rest of your implementation
   
   @end
   

2. 您的tableView:cellForRowAtIndexPath: 应该 (a) 在开始异步图像加载之前初始化image（这样您就不会从那里看到重用单元格中的旧图像）； (b) 在更新之前确保单元格仍然可见：

   NSString *imageUrlString = [[NSString alloc]initWithFormat:@"http://api.twitter.com/1/users/profile_image/%@",[[status objectForKey:@"user"]objectForKey:@"screen_name"]];;
   UIImage *imageFromCache = [self.imageCache objectForKey:imageUrlString];
   
   if (imageFromCache) {
       statusCell.imageCellTL.image = imageFromCache;
       [statusCell.imageCellTL setFrame: ...]; // set your frame accordingly
   }
   else
   {
       statusCell.imageCellTL.image = [UIImage imageNamed:@"Placeholder"];
       [statusCell.imageCellTL setFrame:CGRectZero]; // not sure if you need this line, but you had it in your original code snippet, so I include it here
   
       [self.imageOperationQueue addOperationWithBlock:^{
           NSURL *imageurl = [NSURL URLWithString:imageUrlString];
           UIImage *img = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageurl]];
   
           if (img != nil) {
   
               // update cache
               [self.imageCache setObject:img forKey:imageUrlString];
   
               // now update UI in main queue
               [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                   // see if the cell is still visible ... it's possible the user has scrolled the cell so it's no longer visible, but the cell has been reused for another indexPath
                   TimeLineCell *updateCell = (TimeLineCell *)[tableView cellForRowAtIndexPath:indexPath];
   
                   // if so, update the image
                   if (updateCell) {
                       [updateCell.imageCellTL setFrame:...]; // I don't know what you want to set this to, but make sure to set it appropriately for your cell; usually I don't mess with the frame.
                       [updateCell.imageCellTL setImage:img];
                   }
               }];
           }
       }];
   }
   

3. 不需要对UIApplicationDidReceiveMemoryWarningNotification 进行特殊处理，因为尽管NSCache 不会响应此内存警告，但它会在内存变低时自动驱逐其对象。

我没有测试过上面的代码，但希望你能明白。这是典型的模式。你的原始代码检查了[ImagesUrl isEqualToString:@"<null>"]，我不知道怎么会这样，但如果你需要一些额外的逻辑，除了我的if (img != nil) ...，然后相应地调整那行。

Answer 2

Ray Wenderlich 的一个很好的例子：http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues

它还具有cancel功能，可以在用户按下取消按钮时取消操作。

Answer 3

使用 swift 3 在 tableview 中下载异步图像

class ViewController: UIViewController {
    var queue = OperationQueue()

    let imageURLs = ["https://amazingslider.com/wp-content/uploads/2012/12/dandelion.jpg", "https://media.treehugger.com/assets/images/2011/10/tiger-running-snow.jpg.600x315_q90_crop-smart.jpg", "https://www.w3schools.com/css/trolltunga.jpg", "https://www.w3schools.com/w3css/img_lights.jpg", "https://cdn.pixabay.com/photo/2015/04/28/20/55/aurora-borealis-744351_960_720.jpg", "https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS2L0pETnybA5sld783iz1mgtOFS8vxBTjB4tYXeRtQWDxig3dc"]

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }


}

extension ViewController: UITableViewDelegate, UITableViewDataSource {
    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return imageURLs.count
    }
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell: ImagesTableViewCell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath) as! ImagesTableViewCell
        var img: UIImage!
        let operation = BlockOperation(block: {
            img  = Downloader.downloadImageWithURl(self.imageURLs[indexPath.row])
        })

        operation.completionBlock = {
            DispatchQueue.main.async {
                cell.imgView?.image = img
            }

        }
        queue.addOperation(operation)

        return cell
    }
}

class Downloader {
    class func downloadImageWithURl(_ url: String) -> UIImage! {
        if let data = try? Data(contentsOf: URL(string: url)!) {
            return UIImage(data: data)!
        }
        return nil
    }

}