Using Constraints to align a button in the center of your app.

Select your button and position in the center of your app

Select the align icon and check the check-boxes for Horizontal Center in Container and Vertical Center in Container and then
click Add 2 Constraints.

Your 2 new constraints will appear in the View Controller Scene.

Run the iOS simulator and your button will appear in the center of the app

Rotate your app to the right and your button will still appear in the center of the app.