iOS13でUISegmentedControlの背景を透明にするためには、透明画像を背景にセットする必要がある。

実装方法

UIImage+.swift

import UIKit

extension UIImage {

    public convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
        let rect = CGRect(origin: .zero, size: size)
        UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0)
        color.setFill()
        UIRectFill(rect)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        guard let cgImage = image?.cgImage else { return nil }
        self.init(cgImage: cgImage)
    }
}

HogeViewController.swift

final class HogeViewController: UIViewController {
    @IBOutlet weak var segmentedControl: UISegmentedControl!

    override func viewDidLoad() {
        super.viewDidLoad()
        setupSegmentedControl()
    }

    private func setupSegmentedControl() {
        let clearColorImage = UIImage(color: .clear, size: CGSize(width: 1, height: 1))
        segmentedControl.setBackgroundImage(clearColorImage, for: .normal, barMetrics: .default)
        segmentedControl.setDividerImage(clearColorImage, forLeftSegmentState: .normal, rightSegmentState: .normal, barMetrics: .default)
    }
}