How to let Xcode know which element to use for the target iPhone?


I am designing an app for iOS in SWIFT. I have decided that the auto layout will be compact and will be only for iPhone in portrait mode. I wanted to clear up a few hiccups on designing buttons for the 2 different screen ratios: the first being the iPhone 4s and the second ratio being the iPhone 5 and above. Because the new iPhones have a taller height, does that mean I will need to design 2 different ratios of buttons, and if so, how do I use them in Xcode? For example here's the stock apple calculator;

link to picture:

As you can see the the buttons on the taller iPhone have a square shape. The buttons on the 4s are more stubby. What do I need to do in Xcode to let the app know which asset to use for the respective phone and how would I implement that in Auto Layout? thanks

In order to do this you setup autolayout constraints. This will make the buttons change shape/size depending on the device. You do not need to have multiple assets because all the phones you are talking about have the same retina (2x) display.