Check out example codes for "display image from url swift". It will help you in understanding the concepts better.

Code Example 1

let imageCache = NSCache<AnyObject, AnyObject>()

extension UIImageView {
    func setImage(from url: URL, contentMode mode: UIViewContentMode = .scaleAspectFit) {
        contentMode = mode
        
        if let imageFromCache = imageCache.object(forKey: url as AnyObject) {
            self.image = imageFromCache as? UIImage
            return
        }
        
        URLSession.shared.dataTask(with: url) { data, response, error in
            guard
                let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
                let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
                let data = data, error == nil,
                let imageToCache = UIImage(data: data)
                else { return }
            DispatchQueue.main.async() {
                imageCache.setObject(imageToCache, forKey: url as AnyObject)
                self.image = imageToCache
            }
        }.resume()
    }
    func setImage(from link: String, contentMode mode: UIViewContentMode = .scaleAspectFit) {
        guard let url = URL(string: link) else { return }
        setImage(from: url, contentMode: mode)
    }
}

Code Example 2

extension UIImageView {
    func downloaded(from url: URL, contentMode mode: UIViewContentMode = .scaleAspectFit) {  // for swift 4.2 syntax just use ===> mode: UIView.ContentMode
        contentMode = mode
        URLSession.shared.dataTask(with: url) { data, response, error in
            guard
                let httpURLResponse = response as? HTTPURLResponse, httpURLResponse.statusCode == 200,
                let mimeType = response?.mimeType, mimeType.hasPrefix("image"),
                let data = data, error == nil,
                let image = UIImage(data: data)
                else { return }
            DispatchQueue.main.async() {
                self.image = image
            }
        }.resume()
    }
    func downloaded(from link: String, contentMode mode: UIViewContentMode = .scaleAspectFit) {  // for swift 4.2 syntax just use ===> mode: UIView.ContentMode
        guard let url = URL(string: link) else { return }
        downloaded(from: url, contentMode: mode)
    }
}


// Usage:
imageView.downloaded(from: "https://cdn.arstechnica.net/wp-content/uploads/2018/06/macOS-Mojave-Dynamic-Wallpaper-transition.jpg")

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