ImageCache

class ImageCache {
    
    static let shared = ImageCache()
    
    let cache: NSCache<NSString,UIImage> = NSCache()
    
    func addImage(key:String, image:UIImage) {
        self.cache.setObject(image, forKey: key)
    }
    func getImage(key:String) -> UIImage? {
        return self.cache.object(forKey: key)
    }
    
    func fetchImage(url:NSURL, callback:(image:UIImage?)->()) {
        NSURLSession.shared().dataTask(with: url) { (data:NSData?, response:NSURLResponse?, error:NSError?) in
            guard
                let httpURLResponse = response as? NSHTTPURLResponse where httpURLResponse.statusCode == 200,
                let mimeType = response?.mimeType where mimeType.hasPrefix("image"),
                let data = data where error == nil,
                let image = UIImage(data: data)
            else {
                callback(image: nil)
                return
            }
            self.addImage(key: url.absoluteString, image: image)
            callback(image: image)
        }.resume()
    }
}