/// Interface that fetches image from a source.
/// Image can be fetched from a web service or even from a local source.
internal protocol ImageFetcher {
    func fetch(then completion: @escaping (UIImage?, Error?) -> Void)
}

/// ImageFetcher implementation to fetch image from web service
internal class ImageDownloader: ImageFetcher {
    let url: URL
    init(url: URL) {
        self.url = url
    }

    func fetch(then completion: @escaping (UIImage?, Error?) -> Void) {
        // URL Operation
        // Call completion(image, error)
    }
}

/// ImageFetcher implementation to fetch image from the documents directory
internal class ImageFileFetcher: ImageFetcher {
    let imageName: String
    init(imageName: String){
        self.imageName = imageName
    }

    func fetch(then completion: @escaping (UIImage?, Error?) -> Void) {
        // FileManager operation to get image from the saved document directory.
        // Call completion(image, error)
    }
}


/////////////////////////////////////////////
/// Similarly you can have ImageFetcher to mock all these use cases to test our User class implementation.
/////////////////////////////////////////////

/// User Model
public class User {
    // Stores the
    public var name: String

    /// Private image holder for the user.
    /// Nil either when image is not there or not fetched.
    var iProfileImage: UIImage?

    /// Display profile image for the user.
    /// Returns either the actual user profile image or the default display image.
    public var profileImage: UIImage {
        return iProfileImage ?? .defaultProfileImage
    }

    init(name:String) {
        self.name = name
    }

    func fetchProfileImage(fetcher: ImageFetcher, then completion: @escaping (Bool)-> Void) {
        fetcher.fetch { (image, error) in
            if let fetchError = error {
                print("Failed to fetch Image with error - \(fetchError)")
                completion(false)
            } else if let fetchedImage = image {
                print("Image fetched")
                self.iProfileImage = fetchedImage
                completion(true)
            } else {
                print("Failed to fetch Image with unknown error")
                completion(false)
            }
        }
    }
}