Slide animation in tab bar controller

tab_controller_slide.swift

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
        animateSliding(fromController: selectedViewController, toController: viewController)
        return false
    }

func animateSliding(fromController: UIViewController?, toController: UIViewController?) {
        
        guard let fromController = fromController, let toController = toController  else { return }
        guard let fromIndex = viewControllers?.index(of: fromController),
            let toIndex = viewControllers?.index(of: toController) else { return }
        guard fromIndex != toIndex else { return }
        
        let fromView = fromController.view!
        let toView = toController.view!
        let viewSize = fromView.frame
        let scrollRight = fromIndex < toIndex
        fromView.superview?.addSubview(toView)
        toView.frame = CGRect(x: scrollRight ? screenWidth : -screenWidth,
                              y: viewSize.origin.y,
                              width: screenWidth,
                              height: viewSize.height)

        func animate() {
            fromView.frame = CGRect(x: scrollRight ? -screenWidth : screenWidth,
                                    y: viewSize.origin.y,
                                    width: screenWidth,
                                    height: viewSize.height)
            toView.frame = CGRect(x: 0,
                                    y: viewSize.origin.y,
                                    width: screenWidth,
                                    height: viewSize.height)
        }
        
        func finished(_ completed: Bool) {
            fromView.removeFromSuperview()
            selectedIndex = toIndex
        }
        
        UIView.animate(withDuration: 0.35, delay: 0, options: .curveEaseInOut,
                       animations: animate, completion: finished)
    }