class ImageStore: NSObject { 
    static let imageCache = NSCache<NSString, UIImage>()
}

extension UIImageView {
    func url(_ url: String?) {
        DispatchQueue.global().async { [weak self] in
            guard let stringURL = url, let url = URL(string: stringURL) else {
                return
            }
            func setImage(image:UIImage?) {
                DispatchQueue.main.async {
                    self?.image = image
                }
            }
            let urlToString = url.absoluteString as NSString
            if let cachedImage = ImageStore.imageCache.object(forKey: urlToString) {
                setImage(image: cachedImage)
            } else if let data = try? Data(contentsOf: url), let image = UIImage(data: data) {
                DispatchQueue.main.async {
                    ImageStore.imageCache.setObject(image, forKey: urlToString)
                    setImage(image: image)
                }
            }else {
                setImage(image: nil)
            }
        }
    }
}

用法:

let imageView = UIImageView()
imageView.url("image url")

(Swift 4更新) 为了直接回答最初的问题，下面是发布的Objective-C代码片段的快速等效版本。

let url = URL(string: image.url)
let data = try? Data(contentsOf: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check / try-catch
imageView.image = UIImage(data: data!)

免责声明:

需要注意的是，Data(contentsOf:)方法将在执行代码的同一线程中同步下载url的内容，因此不要在应用程序的主线程中调用此方法。

让相同的代码异步运行，而不阻塞UI的简单方法是使用GCD:

let url = URL(string: image.url)

DispatchQueue.global().async {
    let data = try? Data(contentsOf: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check / try-catch
    DispatchQueue.main.async {
        imageView.image = UIImage(data: data!)
    }
}

也就是说，在现实生活中的应用程序中，如果您希望获得最佳的用户体验并避免同一映像的多次下载，您可能还希望不仅下载它们，而且缓存它们。已经有相当多的库可以无缝地做到这一点，而且它们都非常易于使用。我个人推荐翠鸟:

import Kingfisher

let url = URL(string: "url_of_your_image")
// this downloads the image asynchronously if it's not cached yet
imageView.kf.setImage(with: url) 

就是这样

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)

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")

干净的方式:

extension URL {
    var favIcon16: UIImage? { getFav(ofSize: .s) }
    var favIcon32: UIImage? { getFav(ofSize: .m) }
    var favIcon64: UIImage? { getFav(ofSize: .l) }
    var favIcon128: UIImage? { getFav(ofSize: .xl) }
    var favIcon256: UIImage? { getFav(ofSize: .xxl) }
    var favIcon512: UIImage? { getFav(ofSize: .xxxl) }

    private func getFav(ofSize s: FavSize) -> UIImage? {
        guard UIApplication.shared.canOpenURL(self),
              let favUrl = URL(string: "https://www.google.com/s2/favicons?sz=\(s.rawValue)&domain=\(self.absoluteURL)"),
              let data = try? Data(contentsOf: favUrl)
        else { return nil }
        return UIImage(data: data)
 }
    private enum FavSize: Int, CaseIterable { case s = 16, m = 32, l = 64, xl = 128, xxl = 256, xxxl = 512 }
}

和用法:

let myUrl = URL(string: "http://facebook.com")
myImgView.image = myUrl.favIcon16

斯威夫特2。x答案下载图像到文件(与Leo Dabus的答案相反，它将图像存储在内存中)。根据Leo Dabus的回答和Rob的回答，从完成处理程序的NSURLSession DownloadTaskWithRequest中获取数据:

    // Set download vars
    let downloadURL = NSURL() // URL to download from
    let localFilename = "foobar.png" // Filename for storing locally 

    // Create download request
    let task = NSURLSession.sharedSession().downloadTaskWithURL(downloadURL) { location, response, error in
        guard location != nil && error == nil else {
            print("Error downloading message: \(error)")
            return
        }

        // If here, no errors so save message to permanent location
        let fileManager = NSFileManager.defaultManager()
        do {
            let documents = try fileManager.URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false)
            let fileURL = documents.URLByAppendingPathComponent(localFilename)
            try fileManager.moveItemAtURL(location!, toURL: fileURL)
            self.doFileDownloaded(fileURL, localFilename: localFilename)
            print("Downloaded message @ \(localFilename)")
        } catch {
            print("Error downloading message: \(error)")
        }
    }

    // Start download
    print("Starting download @ \(downloadURL)")
    task.resume()


// Helper function called after file successfully downloaded
private func doFileDownloaded(fileURL: NSURL, localFilename: String) {

    // Do stuff with downloaded image

}