Extension on UITabBarController for hiding/showing the tab bar.

UITabBarController+ToggleTabBar.swift

extension UITabBarController {

    /**
     Show or hide the tab bar.

     - Parameter hidden: `true` if the bar should be hidden.
     - Parameter animated: `true` if the action should be animated.
     - Parameter transitionCoordinator: An optional `UIViewControllerTransitionCoordinator` to perform the animation
        along side with. For example during a push on a `UINavigationController`.
     */
    func setTabBar(
        hidden: Bool,
        animated: Bool = true,
        along transitionCoordinator: UIViewControllerTransitionCoordinator? = nil
    ) {
        guard isTabBarHidden != hidden else { return }

        let offsetY = hidden ? tabBar.frame.height : -tabBar.frame.height
        let endFrame = tabBar.frame.offsetBy(dx: 0, dy: offsetY)
        let vc: UIViewController? = viewControllers?[selectedIndex]
        var newInsets: UIEdgeInsets? = vc?.additionalSafeAreaInsets
        let originalInsets = newInsets
        newInsets?.bottom -= offsetY

        /// Helper method for updating child view controller's safe area insets.
        func set(childViewController cvc: UIViewController?, additionalSafeArea: UIEdgeInsets) {
            cvc?.additionalSafeAreaInsets = additionalSafeArea
            cvc?.view.setNeedsLayout()
        }

        // Update safe area insets for the current view controller before the animation takes place when hiding the bar.
        if hidden, let insets = newInsets { set(childViewController: vc, additionalSafeArea: insets) }

        guard animated else {
            tabBar.frame = endFrame
            return
        }

        // Perform animation with coordinato if one is given. Update safe area insets _after_ the animation is complete,
        // if we're showing the tab bar.
        weak var tabBarRef = self.tabBar
        if let tc = transitionCoordinator {
            tc.animateAlongsideTransition(in: self.view, animation: { _ in tabBarRef?.frame = endFrame }) { context in
                if !hidden, let insets = context.isCancelled ? originalInsets : newInsets {
                    set(childViewController: vc, additionalSafeArea: insets)
                }
            }
        } else {
            UIView.animate(withDuration: 0.3, animations: { tabBarRef?.frame = endFrame }) { didFinish in
                if !hidden, didFinish, let insets = newInsets {
                    set(childViewController: vc, additionalSafeArea: insets)
                }
            }
        }
    }

    /// `true` if the tab bar is currently hidden.
    var isTabBarHidden: Bool {
        return !tabBar.frame.intersects(view.frame)
    }

}

This does not work properly on iPhone X...

extension UITabBarController {

func setTabBar(
    hidden: Bool
) {
    guard isTabBarHidden != hidden else { return }
    
    let offsetY = hidden ? tabBar.frame.height : -tabBar.frame.height
    let endFrame = tabBar.frame.offsetBy(dx: 0, dy: offsetY)
    let cvc: UIViewController? = viewControllers?[selectedIndex]
    var newInsets: UIEdgeInsets?
    if #available(iOS 11.0, *) {
        newInsets = cvc?.additionalSafeAreaInsets
    }
    
    weak var tabBarRef = self.tabBar
    UIView.animate(withDuration: 0.1, animations: {
        
    }, completion: { isFinished in
        var height: CGFloat = 0
        if #available(iOS 11.0, *) {
            height = self.hasBottomNotch ? offsetY + (UIApplication.shared.keyWindow?.safeAreaInsets.bottom ?? 0.0) : offsetY
        } else {
            height = offsetY
        }
        newInsets?.bottom -= height
        if #available(iOS 11.0, *) {
            cvc?.additionalSafeAreaInsets = newInsets!
        } else {
            cvc?.view.frame = CGRect(x: 0, y: 0,
                                     width: cvc?.view.frame.width ?? 0.00,
                                     height: (cvc?.view.frame.height ?? 0.00) + height)
        }
        if isFinished {
            UIView.animate(withDuration: 0.2, animations: {
                tabBarRef?.frame = endFrame
            }, completion: nil)
        }
    })
}

var isTabBarHidden: Bool {
    return !tabBar.frame.intersects(view.frame)
}

}

Refactored

Sweet! I'm not using this myself anymore, but maybe others have use for it :)