您必须了解的是该backgroundColor属性不是有状态的。因此，您必须使用setBackgroundImage(_:for:barMetrics:)。

使用以下功能，我们可以轻松删除边框和分隔线。

对于Swift 3和4+：

extension UISegmentedControl {
    func removeBorders() {
        setBackgroundImage(imageWithColor(color: backgroundColor ?? .clear), for: .normal, barMetrics: .default)
        setBackgroundImage(imageWithColor(color: tintColor!), for: .selected, barMetrics: .default)
        setDividerImage(imageWithColor(color: UIColor.clear), forLeftSegmentState: .normal, rightSegmentState: .normal, barMetrics: .default)
    }

    // create a 1x1 image with this color
    private func imageWithColor(color: UIColor) -> UIImage {
        let rect = CGRect(x: 0.0, y: 0.0, width:  1.0, height: 1.0)
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()
        context!.setFillColor(color.cgColor);
        context!.fill(rect);
        let image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image!
    }
}

对于Swift 2.2：

extension UISegmentedControl {
    func removeBorders() {
        setBackgroundImage(imageWithColor(backgroundColor!), forState: .Normal, barMetrics: .Default)
        setBackgroundImage(imageWithColor(tintColor!), forState: .Selected, barMetrics: .Default)
        setDividerImage(imageWithColor(UIColor.clearColor()), forLeftSegmentState: .Normal, rightSegmentState: .Normal, barMetrics: .Default)
    }

    // create a 1x1 image with this color
    private func imageWithColor(color: UIColor) -> UIImage {
        let rect = CGRectMake(0.0, 0.0, 1.0, 1.0)
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()
        CGContextSetFillColorWithColor(context, color.CGColor);
        CGContextFillRect(context, rect);
        let image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        return image
    }
}

调用上面的函数。

segmentedControl.removeBorders()