UISegmentControl with underline

SegmentControl.swift

extension UISegmentedControl {
    
    private var underlineWidth: CGFloat { 20 }
    
    private var underlineHeight: CGFloat { 2 }
    
    private var segmentWidth: CGFloat {
         bounds.width / CGFloat(numberOfSegments)
    }
    
    private var segmentMinX: CGFloat {
        segmentWidth * CGFloat(selectedSegmentIndex)
    }
    
    private var underlineMinX: CGFloat {
        segmentMinX + (segmentWidth/2) - (underlineWidth/2)
    }
    
    private var underlineMinY: CGFloat {
        bounds.height - 1.0
    }
    
    func setup() {
        style()
        transparentBackground()
        addUnderline()
    }
    
    func style() {
        clipsToBounds = false
        tintColor = .clear
        backgroundColor = .clear
        if #available(iOS 13.0, *) {
            selectedSegmentTintColor = .clear
        }
        selectedSegmentIndex = 0
        setTitleTextAttributes([.font: UIFont.boldSystemFont(ofSize: 15), .foregroundColor: UIColor.black], for: .selected)
        setTitleTextAttributes([.font: UIFont.systemFont(ofSize: 14), .foregroundColor: UIColor.darkGray], for: .normal)
        sizeToFit()
    }

    func transparentBackground() {
        let backgroundImage = UIImage.coloredRectangleImageWith(color: UIColor.clear.cgColor, andSize: self.bounds.size)
        let dividerImage = UIImage.coloredRectangleImageWith(color: UIColor.clear.cgColor, andSize: CGSize(width: 1, height: self.bounds.height))
        setBackgroundImage(backgroundImage, for: .normal, barMetrics: .default)
        setBackgroundImage(backgroundImage, for: .selected, barMetrics: .default)
        setBackgroundImage(backgroundImage, for: .highlighted, barMetrics: .default)
        setDividerImage(dividerImage, forLeftSegmentState: .normal, rightSegmentState: .normal, barMetrics: .default)
    }
    
    func addUnderline() {     
        let underlineFrame = CGRect(x: underlineMinX, y: underlineMinY, width: underlineWidth, height: underlineHeight)
        let underline = UIView(frame: underlineFrame)
        underline.backgroundColor = UIColor.black
        underline.tag = 1
        self.addSubview(underline)
    }
    
    func setSelectedSegmentUnderline() {
        guard let underline = self.viewWithTag(1) else {return}
        UIView.animate(withDuration: 0.1, animations: {
            underline.frame.origin.x = self.underlineMinX
        })
    }
}

// MARK: - UIImage extension
extension UIImage {
    class func coloredRectangleImageWith(color: CGColor, andSize size: CGSize) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
        let graphicsContext = UIGraphicsGetCurrentContext()
        graphicsContext?.setFillColor(color)
        let rectangle = CGRect(x: 0.0, y: 0.0, width: size.width, height: size.height)
        graphicsContext?.fill(rectangle)
        let rectangleImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return rectangleImage!
    }

}

Usage:

class ViewController: UIViewController {

    @IBOutlet weak var segment: UISegmentedControl!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        segment.setup()
    }
    
    @IBAction func segmentValueChanged(_ sender: UISegmentedControl) {
        sender.setSelectedSegmentUnderline()
    }
    
}