I created a project in Swift. I added view controller file and one XIB using right click on the project - > Add Files to projectname -> Empty (xib) file.
After adding the xib file, i removed the view and added View Controller. Under the View controller , i have added few button. But i am not able set the constraints.But if i add the View object instead of View Controller, i am able to set the constraints. I don't understand why the xib constraints are disabled for View controller. Attached the screen shot

Layout is set to
Translates Mask Into Constraints, meaning that IB is gonna generate the constraints for you automatically. Set it toAutomaticand it'll work.