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。

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")

Swift 4.2和AlamofireImage

如果使用库不是问题，您可以通过AlamofireImage的帮助来完成。 我的样本来自它的Github

占位符图片

let imageView = UIImageView(frame: frame)
let url = URL(string: "https://httpbin.org/image/png")!
let placeholderImage = UIImage(named: "placeholder")!
imageView.af_setImage(withURL: url, placeholderImage: placeholderImage)

它有许多方便的功能和扩展工作与图像。从缓存到缩放和调整大小，甚至在图像上应用过滤器。如果图像在你的应用中很重要，我建议使用这个框架来节省你的时间。

从服务器加载图像:-

func downloadImage(from url: URL , success:@escaping((_ image:UIImage)->()),failure:@escaping ((_ msg:String)->())){
    print("Download Started")
    getData(from: url) { data, response, error in
        guard let data = data, error == nil else {
            failure("Image cant download from G+ or fb server")
            return
        }

        print(response?.suggestedFilename ?? url.lastPathComponent)
        print("Download Finished")
        DispatchQueue.main.async() {
             if let _img = UIImage(data: data){
                  success(_img)
            }
        }
    }
}
func getData(from url: URL, completion: @escaping (Data?, URLResponse?, Error?) -> ()) {
    URLSession.shared.dataTask(with: url, completionHandler: completion).resume()
}

用法:-

  if let url = URL(string: "http://www.apple.com/euro/ios/ios8/a/generic/images/og.png") {
                        self.downloadImage(from:url , success: { (image) in
                            print(image)

                        }, failure: { (failureReason) in
                            print(failureReason)
                        })
                    }

一个快速的黑客，如果你想快速检查图像从url

 let imageURL = NSURL(string: "https://farm2.staticflickr.com/1591/26078338233_d1466b7da2_m.jpg")
 let imagedData = NSData(contentsOfURL: imageURL!)!
 imageView?.image = UIImage(data: imagedData)

我在一个tableview中实现了一个只有图像的自定义单元格

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell{
        
        let cell = tableView.dequeueReusableCellWithIdentifier("theCell", forIndexPath: indexPath) as! customTableViewCell

        let imageURL = NSURL(string: "https://farm2.staticflickr.com/1591/26078338233_d1466b7da2_m.jpg")
        
        let imagedData = NSData(contentsOfURL: imageURL!)!
            
        cell.imageView?.image = UIImage(data: imagedData)
        
        return cell
        
    }

我把这个问题的最佳答案的代码包装成一个单一的，可重用的类扩展UIImageView，所以你可以直接在你的故事板中使用异步加载UIImageViews(或从代码中创建它们)。

这是我的班级:

import Foundation
import UIKit

class UIImageViewAsync :UIImageView
{

    override init()
    {
        super.init(frame: CGRect())
    }

    override init(frame:CGRect)
    {
        super.init(frame:frame)
    }

    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    func getDataFromUrl(url:String, completion: ((data: NSData?) -> Void)) {
        NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: url)!) { (data, response, error) in
            completion(data: NSData(data: data))
        }.resume()
    }

    func downloadImage(url:String){
        getDataFromUrl(url) { data in
            dispatch_async(dispatch_get_main_queue()) {
                self.contentMode = UIViewContentMode.ScaleAspectFill
                self.image = UIImage(data: data!)
            }
        }
    }
}

下面是如何使用它:

imageView.downloadImage("http://www.image-server.com/myImage.jpg")