...With Sizes we'll avoid launching all the supported devices simulator, running our app and navigating through each screen manually which not only takes a lot of time but its also highly error-prone since we sometimes forget to test one particular combination and we ship a screen with a layout bug to the App Store. Xcode 11 introduced Previews which is a fantastic way to quickly validate your UI in multiple scenarios just like Sizes did. If you'd like to know more about Xcode Previews I recommend this WWDC session. You can even use Previews with UIKit so there's no reason to stick to this library.