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

}

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")

Xcode 12•Swift 5

Leo Dabus的答案非常棒!我只是想提供一个一体化的功能解决方案:

if let url = URL(string: "http://www.apple.com/euro/ios/ios8/a/generic/images/og.png") {
    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data, error == nil else { return }
        
        DispatchQueue.main.async { /// execute on main thread
            self.imageView.image = UIImage(data: data)
        }
    }
    
    task.resume()
}

这是从URL加载/下载图像的工作代码。NSCache自动和显示占位符图像下载和加载实际图像(Swift 4 | Swift 5代码)。

func NKPlaceholderImage(image:UIImage?, imageView:UIImageView?,imgUrl:String,compate:@escaping (UIImage?) -> Void){
    
    if image != nil && imageView != nil {
        imageView!.image = image!
    }
    
    var urlcatch = imgUrl.replacingOccurrences(of: "/", with: "#")
    let documentpath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
    urlcatch = documentpath + "/" + "\(urlcatch)"
    
    let image = UIImage(contentsOfFile:urlcatch)
    if image != nil && imageView != nil
    {
        imageView!.image = image!
        compate(image)
        
    }else{
        
        if let url = URL(string: imgUrl){
            
            DispatchQueue.global(qos: .background).async {
                () -> Void in
                let imgdata = NSData(contentsOf: url)
                DispatchQueue.main.async {
                    () -> Void in
                    imgdata?.write(toFile: urlcatch, atomically: true)
                    let image = UIImage(contentsOfFile:urlcatch)
                    compate(image)
                    if image != nil  {
                        if imageView != nil  {
                            imageView!.image = image!
                        }
                    }
                }
            }
        }
    }
}

像这样使用:

// Here imgPicture = your imageView
// UIImage(named: "placeholder") is Display image brfore download and load actual image. 

NKPlaceholderImage(image: UIImage(named: "placeholder"), imageView: imgPicture, imgUrl: "Put Here your server image Url Sting") { (image) in }

使用Ascyimageview，你可以很容易地加载imageurl在imageview。

let image1Url：URL = URL（string： “（imageurl）” as String）！ imageview.imageURL=image1Url

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。