Built an app that takes a username and password from user. It checks username and password and then sign-up to app. I practiced on Segue, Optionals, Alerts, Functions and Lifecycle.
-> General view of app's main page.
-> if the user types wrong password or username (it does matter. There are different situations for each.)
-> if the user types his/her password and username correctly, he/she sign up to app successfully. (There is no saving process to Database, it's just symbolic app.)
