Object does not respond to selector

I'm trying to add functionality to save a picture from a URL to the user's photo library. here's the code:

import Foundation
import UIKit

class ImageDownloader: NSObject {
    static func writeToPhotoAlbum(imageUrl: String) {
        let imageURL = URL(string: imageUrl)!

        if let image = try? Data(contentsOf: imageURL) {
            UIImageWriteToSavedPhotosAlbum(UIImage(data: image)!, self, nil, nil)
        }
    }

    @objc func saveCompleted(
        _ image: UIImage,
        didFinishSavingWithError error: Error?,
        contextInfo: UnsafeRawPointer)
    {
        print("Save finished!")
    }

}

When i call the method like this:

ImageDownloader.writeToPhotoAlbum(imageUrl: "image_url")

I get this error:

ImageDownloader does not respond to selector

Any explanation and solution?