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。

在Swift中使用此代码

imageView.image=UIImage(data: NSData(contentsOfURL: NSURL(string: "http://myURL/ios8.png")!)!

斯威夫特4

这个方法将从一个网站异步下载图像并缓存它:

    func getImageFromWeb(_ urlString: String, closure: @escaping (UIImage?) -> ()) {
        guard let url = URL(string: urlString) else {
return closure(nil)
        }
        let task = URLSession(configuration: .default).dataTask(with: url) { (data, response, error) in
            guard error == nil else {
                print("error: \(String(describing: error))")
                return closure(nil)
            }
            guard response != nil else {
                print("no response")
                return closure(nil)
            }
            guard data != nil else {
                print("no data")
                return closure(nil)
            }
            DispatchQueue.main.async {
                closure(UIImage(data: data!))
            }
        }; task.resume()
    }

在使用:

    getImageFromWeb("http://www.apple.com/euro/ios/ios8/a/generic/images/og.png") { (image) in
        if let image = image {
            let imageView = UIImageView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
            imageView.image = image
            self.view.addSubview(imageView)
        } // if you use an Else statement, it will be in background
    }

class Downloader {
    
    class func downloadImageWithURL(url: String) -> UIImage! {
        
        let date = NSData(contentsOf: URL(string: url)!)
        return UIImage(data: date! as Data)
    }
}

用法:

let img = Downloader.downloadImageWithURL(url: imageURL)
        self.imageView1.image = img

Swift 4:一个简单的小图像加载器(例如:缩略图)，使用NSCache，总是运行在主线程上:

class ImageLoader {

  private static let cache = NSCache<NSString, NSData>()

  class func image(for url: URL, completionHandler: @escaping(_ image: UIImage?) -> ()) {

    DispatchQueue.global(qos: DispatchQoS.QoSClass.background).async {

      if let data = self.cache.object(forKey: url.absoluteString as NSString) {
        DispatchQueue.main.async { completionHandler(UIImage(data: data as Data)) }
        return
      }

      guard let data = NSData(contentsOf: url) else {
        DispatchQueue.main.async { completionHandler(nil) }
        return
      }

      self.cache.setObject(data, forKey: url.absoluteString as NSString)
      DispatchQueue.main.async { completionHandler(UIImage(data: data as Data)) }
    }
  }

}

用法:

ImageLoader.image(for: imageURL) { image in
  self.imageView.image = image
}

Xcode 8或更高版本•Swift 3或更高版本

同步:

if let filePath = Bundle.main.path(forResource: "imageName", ofType: "jpg"), let image = UIImage(contentsOfFile: filePath) {
    imageView.contentMode = .scaleAspectFit
    imageView.image = image
}

异步:

创建一个带有完成处理程序的方法，从您的url获取图像数据

func getData(from url: URL, completion: @escaping (Data?, URLResponse?, Error?) -> ()) {
    URLSession.shared.dataTask(with: url, completionHandler: completion).resume()
}

创建一个方法来下载映像(启动任务)

func downloadImage(from url: URL) {
    print("Download Started")
    getData(from: url) { data, response, error in
        guard let data = data, error == nil else { return }
        print(response?.suggestedFilename ?? url.lastPathComponent)
        print("Download Finished")
        // always update the UI from the main thread
        DispatchQueue.main.async() { [weak self] in
            self?.imageView.image = UIImage(data: data)
        }
    }
}

用法:

override func viewDidLoad() {
    super.viewDidLoad()
    print("Begin of code")
    let url = URL(string: "https://cdn.arstechnica.net/wp-content/uploads/2018/06/macOS-Mojave-Dynamic-Wallpaper-transition.jpg")! 
    downloadImage(from: url)
    print("End of code. The image will continue downloading in the background and it will be loaded when it ends.")
}

扩展:

extension UIImageView {
    func downloaded(from url: URL, contentMode mode: ContentMode = .scaleAspectFit) {
        contentMode = mode
        URLSession.shared.dataTask(with: url) { data, response, error in
            guard
                let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
                let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
                let data = data, error == nil,
                let image = UIImage(data: data)
                else { return }
            DispatchQueue.main.async() { [weak self] in
                self?.image = image
            }
        }.resume()
    }
    func downloaded(from link: String, contentMode mode: ContentMode = .scaleAspectFit) { 
        guard let url = URL(string: link) else { return }
        downloaded(from: url, contentMode: mode)
    }
}

用法:

imageView.downloaded(from: "https://cdn.arstechnica.net/wp-content/uploads/2018/06/macOS-Mojave-Dynamic-Wallpaper-transition.jpg")