Add Top border to selected TabBar

You can use the function to create borders on any side -

enum Side: String {
    case top, left, right, bottom
}

extension UIImage {
    func createSelectionIndicator(color: UIColor, size: CGSize, lineThickness: CGFloat, side: Side) -> UIImage {
        var xPosition = 0.0
        var yPosition = 0.0
        var imgWidth = 2.0
        var imgHeight = 2.0
        switch side {
            case .top:
                xPosition = 0.0
                yPosition = 0.0
                imgWidth = size.width
                imgHeight = lineThickness
            case .bottom:
                xPosition = 0.0
                yPosition = size.height - lineThickness
                imgWidth = size.width
                imgHeight = lineThickness
            case .left:
                xPosition = 0.0
                yPosition = 0.0
                imgWidth = lineThickness
                imgHeight = size.height
            case .right:
                xPosition = size.width - lineThickness
                yPosition = 0.0
                imgWidth = lineThickness
                imgHeight = size.height
        }
        UIGraphicsBeginImageContextWithOptions(size, false, 0)
        color.setFill()
        UIRectFill(CGRect(x: xPosition, y: yPosition, width: imgWidth, height: imgHeight))
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image!
    }
}

tabBar.selectionIndicatorImage = UIImage().createSelectionIndicator(color: BLUE, size: CGSize(width: tabBar.frame.width/CGFloat(tabBar.items!.count), height:  tabBar.frame.height), lineThickness: 2.0, side: .top)

Let me know if this was useful or you need any other help. Happy Coding :)

import UIKit

class MyViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        tabBarController?.tabBar.addBorder(.top, color: .red, thickness: 2.0)
    }
   
}
extension UITabBar {
    func addBorder(_ edge: UIRectEdge, color: UIColor, thickness: CGFloat) {
        let subview = UIView()
        subview.translatesAutoresizingMaskIntoConstraints = false
        subview.backgroundColor = color
        self.addSubview(subview)
        switch edge {
        case .top, .bottom:
            subview.leftAnchor.constraint(equalTo: self.leftAnchor, constant: 0).isActive = true
            subview.rightAnchor.constraint(equalTo: self.rightAnchor, constant: 0).isActive = true
            subview.heightAnchor.constraint(equalToConstant: thickness).isActive = true
            if edge == .top {
                subview.topAnchor.constraint(equalTo: self.topAnchor, constant: 0).isActive = true
            } else {
                subview.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: 0).isActive = true
            }
        case .left, .right:
            subview.topAnchor.constraint(equalTo: self.topAnchor, constant: 0).isActive = true
            subview.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: 0).isActive = true
            subview.widthAnchor.constraint(equalToConstant: thickness).isActive = true
            if edge == .left {
                subview.leftAnchor.constraint(equalTo: self.leftAnchor, constant: 0).isActive = true
            } else {
                subview.rightAnchor.constraint(equalTo: self.rightAnchor, constant: 0).isActive = true
            }
        default:
            break
        }
    }
}

tabBarController?.tabBar.addBorder(.top, color: .orange, thickness: 5.0)