Swift 4-如何使用缓存加载图像？

Question

我有UIImageView，如果存在，我想在缓存中下载图像，我使用了扩展名func。

我有此代码，但无法正常工作：

extension UIImageView {
    func loadImageUsingCache (_ urlString : String) {
        let imageCache = NSCache<AnyObject, AnyObject>()
        if let cachedImage = imageCache.object(forKey: urlString as AnyObject) {
            self.image = cachedImage as? UIImage
            return
        }
        let url = URL(string: urlString)
        URLSession.shared.dataTask(with: url!) { (data, response, error) in
            if data != nil {
                if let image = UIImage(data: data!) {
                    imageCache.setObject(image, forKey: urlString as AnyObject)
                    DispatchQueue.main.async(execute: {
                        self.image = image
                    })
                }
            }
        }.resume()
    }
}

Answer 1

您正在为每个图像创建新的NSCache对象，但不保留它。您应该创建对象变量而不是本地变量。在这种情况下，它将无法扩展。您也可以尝试使用URLCache.shared代替。

Answer 2

//这是完美的解决方案//

var imageCache =字符串：UIImage

Class CustomImageView：UIImageView {

var lastImgUrlUsedToLoadImage: String?

func loadImage(with urlString: String) {

    // set image to nil
    self.image = nil

    // set lastImgUrlUsedToLoadImage
    lastImgUrlUsedToLoadImage = urlString

    // check if image exists in cache
    if let cachedImage = imageCache[urlString] {
        self.image = cachedImage
        return
    }

    // url for image location
    guard let url = URL(string: urlString) else { return }

    // fetch contents of URL
    URLSession.shared.dataTask(with: url) { (data, response, error) in

        // handle error
        if let error = error {
            print("Failed to load image with error", error.localizedDescription)
        }

        if self.lastImgUrlUsedToLoadImage != url.absoluteString {
            return
        }

        // image data
        guard let imageData = data else { return }

        // create image using image data
        let photoImage = UIImage(data: imageData)

        // set key and value for image cache
        imageCache[url.absoluteString] = photoImage

        // set image
        DispatchQueue.main.async {
            self.image = photoImage
        }
        }.resume()
}

}