A common pattern for UI layout is to have a bar at the bottom of the screen like UITabBar. It has a fixed height in most cases, but with the iPhone X, you want it to stretch to the bottom of the screen, outside of the safe area while keeping it's contents (in UITabBar's case, the tab buttons) glued to the top of the bar.

Auto Layout anchors provide a very simple way to wire this up. In your view controller:

let buttonsHeight = 44
footerBar.topAnchor.constraint(equalTo: view.layoutGuide.bottomAnchor, constant: -buttonsHeight),
footerBar.bottomAnchor.constraint(equalTo: view.bottomAnchor),

The trick is to anchor the top and bottom of footerBar.

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