Getting album id from spotify link

gistfile1.txt

extension String {
    func getAlbumId() -> String? {
        let url = self
        let base = Constants.BASE_URL_NEW_ALBUM
        let start = base.count

        if url.contains(base) && url.count > start {
            
            let startIndex = url.index(url.startIndex, offsetBy: start)
            
            let substring = url[startIndex...]
            var idString = ""
            for letter in substring {
                if letter == "?" {
                    return idString
                } else {
                    idString.append(letter)
                }
            }
            return nil
        }
        return nil
    }
}