You use Sketch's export features to design assets. You use AutoLayout to design the screens at 1x for the different sizes (320, 375, 414). Here you set the constraints and you share those with your developers.
Why is this useful? They have the same options in Xcode so they can use your logic to create a pixel perfect design. If instead they would work on a 3X exported design it will never be right.
Misha, well that's precisely what you can do with Auto Layout. Here you go: https://widgetic.com/assets/widgets/demo/Sketch/Grid%20Auto%20Layout.sketch.
You use Sketch's export features to design assets. You use AutoLayout to design the screens at 1x for the different sizes (320, 375, 414). Here you set the constraints and you share those with your developers.
Why is this useful? They have the same options in Xcode so they can use your logic to create a pixel perfect design. If instead they would work on a 3X exported design it will never be right.