如何在Swift中使用URLSession缓存图像

Question

我想增强下面的代码来缓存图像,只有在以前没有缓存它们时才下载它们.我似乎无法找到如何使用URLSession对象执行此操作的任何好例子.

extension UIImageView {
    func loadImageWithURL(_ url: URL) -> URLSessionDownloadTask {
        let session = URLSession.shared

        let downloadTask = session.downloadTask(with: url, completionHandler: { [weak self] url, response, error in

            if error == nil, let url = url,
                let data = try? Data(contentsOf: url), let image = UIImage(data: data) {

                    DispatchQueue.main.async {
                        if let strongSelf = self {
                            strongSelf.image = image
                        }
                    }
            }
        })
        downloadTask.resume()
        return downloadTask
    }
}

Answer 1

针对Swift 3进行了更新

import UIKit

let imageCache = NSCache<AnyObject, AnyObject>()

class ImageLoader: UIImageView {

    var imageURL: URL?

    let activityIndicator = UIActivityIndicatorView()

    func loadImageWithUrl(_ url: URL) {

        // setup activityIndicator...
        activityIndicator.color = .darkGray

        addSubview(activityIndicator)
        activityIndicator.translatesAutoresizingMaskIntoConstraints = false
        activityIndicator.centerXAnchor.constraint(equalTo: centerXAnchor).isActive = true
        activityIndicator.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true

        imageURL = url

        image = nil
        activityIndicator.startAnimating()

        // retrieves image if already available in cache
        if let imageFromCache = imageCache.object(forKey: url as AnyObject) as? UIImage {

            self.image = imageFromCache
            activityIndicator.stopAnimating()
            return
        }

        // image does not available in cache.. so retrieving it from url...
        URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in

            if error != nil {
                print(error as Any)
                DispatchQueue.main.async(execute: {
                    self.activityIndicator.stopAnimating()
                })
                return
            }

            DispatchQueue.main.async(execute: {

                if let unwrappedData = data, let imageToCache = UIImage(data: unwrappedData) {

                    if self.imageURL == url {
                        self.image = imageToCache
                    }

                    imageCache.setObject(imageToCache, forKey: url as AnyObject)
                }
                self.activityIndicator.stopAnimating()
            })
        }).resume()
    }
}

用法:

// assign ImageLoader class to your imageView class
let yourImageView: ImageLoader = {

    let iv = ImageLoader()
    iv.frame = CGRect(x: 10, y: 100, width: 300, height: 300)
    iv.backgroundColor = UIColor(red: 0.94, green: 0.94, blue: 0.96, alpha: 1.0)
    iv.contentMode = .scaleAspectFill
    iv.clipsToBounds = true
    return iv
}()


// unwrapped url safely...
   if let strUrl = "https://picsum.photos/300/300".addingPercentEncoding(withAllowedCharacters: .urlFragmentAllowed),
      let imgUrl = URL(string: strUrl) {

      yourImageView.loadImageWithUrl(imgUrl) // call this line for getting image to yourImageView
}

Answer 2

对此的一种潜在解决方案是利用NSCache来处理缓存。本质上，您要做的是检查您是否已经在本地加载了图像，而不是在您实际发出请求之前每次都获取。

然而，这是我的一个实现 - 它是一个子类而不是扩展：

class CustomImageView: UIImageView {

    // MARK: - Constants

    let imageCache = NSCache<NSString, AnyObject>()

    // MARK: - Properties

    var imageURLString: String?

    func downloadImageFrom(urlString: String, imageMode: UIViewContentMode) {
        guard let url = URL(string: urlString) else { return }
        downloadImageFrom(url: url, imageMode: imageMode)
    }

    func downloadImageFrom(url: URL, imageMode: UIViewContentMode) {
        contentMode = imageMode
        if let cachedImage = imageCache.object(forKey: url.absoluteString as NSString) as? UIImage {
            self.image = cachedImage
        } else {
            URLSession.shared.dataTask(with: url) { data, response, error in
                guard let data = data, error == nil else { return }
                DispatchQueue.main.async {
                    let imageToCache = UIImage(data: data)
                    self.imageCache.setObject(imageToCache!, forKey: url.absoluteString as NSString)
                    self.image = imageToCache
                }
            }.resume()
        }
    }
}

此外，这里有一个有用的资源：https : //www.hackingwithswift.com/example-code/system/how-to-cache-data-using-nscache