Hi there, I do not understand how data are passed to a func with parameters when it is used as selector sign.

Example: if I'd like to add an argument "okMessage: String" to be printed on saving then adding at the very end of the func something like "print(okMessage)" (no matter usage, just for sake of understanding), then pass "hello, all done here" , how could pass that string?

the code from the lesson

  @IBAction func save(_ sender: Any) {
        guard let image = imageView.image else { return }
        UIImageWriteToSavedPhotosAlbum(image, self, #selector(image(_:didFinishSavingError:contentIinfo:)), nil)
    }

@objc func image(_ image: UIImage, didFinishSavingError error: Error?, contentIinfo: UnsafeRawPointer) {

        if let error = error {
            let ac = UIAlertController(title: "Save error", message: error.localizedDescription, preferredStyle: .alert)
            ac.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
            present(ac, animated: true, completion: nil)
        } else {
            let ac = UIAlertController(title: "Saved!r", message: "modified image saved", preferredStyle: .alert)
            ac.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
            present(ac, animated: true, completion: nil)
        }

    }

You don't. When you create a selector, you are passing in a signature of a function. If that function doesn't include a parameter for an extra String, you can't pass in an extra String. Most of the time, your selector will be called by the system and it expects the functions it calls to have particular signatures.

If you want your didFinishSavingError function to have extra info, you will need to do something else, like set a property in your class that you can read from within the function.