为了在UITableView或UICollectionView中获得更好的性能，使用轻量级库智能惰性加载，如果你想从url异步加载图像，可以使用这种惰性加载方法

因此，在这个项目中，我们可以通过使用操作和操作队列并发优化应用程序的性能，在任何视图(UICollectionView或UITableView)中下载多个图像。以下是这个项目的重点智能延迟加载:创建图像下载服务。根据单元格的可见性优先下载。

ImageDownloadService类将创建一个单例实例，并有一个NSCache实例来缓存已下载的图像。我们已经将Operation类继承给TOperation，以便根据我们的需要修改功能。我认为就功能而言，operation子类的属性非常清楚。我们使用KVO监测操作状态的变化。

翠鸟是一个最好的库加载图像到URL。

Github URL - https://github.com/onevcat/Kingfisher

// If you want to use Activity Indicator.
imageview_pic.kf.indicatorType = .activity
imageview_pic.kf.setImage(with: URL(string: "Give your url string"))

// If you want to use custom placeholder image.
imageview_pic.kf.setImage(with: URL(string: "Give your url string"), placeholder: UIImage(named: "placeholder image name"), options: nil, progressBlock: nil, completionHandler: nil)

你需要做的是:

UIImage(data: data)

在Swift中，他们已经用规则构造函数取代了大多数Objective C工厂方法。

See:

https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithObjective-CAPIs.html#//apple_ref/doc/uid/TP40014216-CH4-XID_26

Swift 4.1我创建了一个函数，只是传递图像url，图像生成后缓存键设置为完成块。

   class NetworkManager: NSObject {
  
  private var imageQueue = OperationQueue()
  private var imageCache = NSCache<AnyObject, AnyObject>()
  
  func downloadImageWithUrl(imageUrl: String, cacheKey: String, completionBlock: @escaping (_ image: UIImage?)-> Void) {
    
    let downloadedImage = imageCache.object(forKey: cacheKey as AnyObject)
    if let  _ = downloadedImage as? UIImage {
      completionBlock(downloadedImage as? UIImage)
    } else {
      let blockOperation = BlockOperation()
      blockOperation.addExecutionBlock({
        let url = URL(string: imageUrl)
        do {
          let data = try Data(contentsOf: url!)
          let newImage = UIImage(data: data)
          if newImage != nil {
            self.imageCache.setObject(newImage!, forKey: cacheKey as AnyObject)
            self.runOnMainThread {
              completionBlock(newImage)
            }
          } else {
            completionBlock(nil)
          }
        } catch {
          completionBlock(nil)
        }
      })
      self.imageQueue.addOperation(blockOperation)
      blockOperation.completionBlock = {
        print("Image downloaded \(cacheKey)")
      }
    }
  }
}
extension NetworkManager {
  fileprivate func runOnMainThread(block:@escaping ()->Void) {
    if Thread.isMainThread {
      block()
    } else {
      let mainQueue = OperationQueue.main
      mainQueue.addOperation({
        block()
      })
    }
  }
}

let url = NSURL.URLWithString("http://live-wallpaper.net/iphone/img/app/i/p/iphone-4s-wallpapers-mobile-backgrounds-dark_2466f886de3472ef1fa968033f1da3e1_raw_1087fae1932cec8837695934b7eb1250_raw.jpg");
var err: NSError?
var imageData :NSData = NSData.dataWithContentsOfURL(url,options: NSDataReadingOptions.DataReadingMappedIfSafe, error: &err)
var bgImage = UIImage(data:imageData)

对于Swift-3及以上版本:

extension UIImageView {
  public func imageFromUrl(urlString: String) {
    if let url = URL(string: urlString) {
        let request = URLRequest(url: url)
        NSURLConnection.sendAsynchronousRequest(request as URLRequest, queue: .main, completionHandler: { (response, data, error) in
            if let imageData = data as NSData? {
                self.image = UIImage(data: imageData as Data)
            }
        })
    }
  }
}