[Updated to Swift 4] - UITabBarController slide animation. Slide left and right when tabs are selected. Based on code from StackOverflow

SlideAnimationTabBarController.swift

func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
		
		if let fromView = tabBarController.selectedViewController?.view,
			let toView = viewController.view, fromView != toView,
			let controllerIndex = self.viewControllers?.index(of: viewController) {
			
			let viewSize = fromView.frame
			let scrollRight = controllerIndex > tabBarController.selectedIndex
			
			// Avoid UI issues when switching tabs fast
			if fromView.superview?.subviews.contains(toView) == true { return false }
			
			fromView.superview?.addSubview(toView)
			
			let screenWidth = UIScreen.main.bounds.size.width
			toView.frame = CGRect(x: (scrollRight ? screenWidth : -screenWidth), y: viewSize.origin.y, width: screenWidth, height: viewSize.size.height)

			UIView.animate(withDuration: 0.25, delay: TimeInterval(0.0), options: [.curveEaseOut, .preferredFramesPerSecond60], animations: {
				fromView.frame = CGRect(x: (scrollRight ? -screenWidth : screenWidth), y: viewSize.origin.y, width: screenWidth, height: viewSize.size.height)
				toView.frame = CGRect(x: 0, y: viewSize.origin.y, width: screenWidth, height: viewSize.size.height)
			}, completion: { finished in
				if finished {
					fromView.removeFromSuperview()
					tabBarController.selectedIndex = controllerIndex
				}
			})
			return true
		}
		return false
	}