如何调整UISlider的thumbImage的size？

ProgressSlider.swift

class ProgressSlider: UISlider {
    override func thumbRect(forBounds bounds: CGRect, trackRect rect: CGRect, value: Float) -> CGRect {
        let max = maximumValue == 0 ? 1.0 : maximumValue
        let side: CGFloat = 8
        let centerX: CGFloat = rect.width * CGFloat(value / max)
        let r = CGRect(x: ceil(centerX) - side / 2, y: rect.midY - side / 2, width: side, height: side)
        return r
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        // SDK里的原图像素 57 * 44，中间圆圈 28 * 28
        if let innerThumbView = self.value(forKey: "innerThumbView") as? UIImageView,
            let thumbViewNeue = self.value(forKey: "thumbViewNeue") as? UIImageView,
            let sv = innerThumbView.superview as? UIImageView, sv == thumbViewNeue {
            innerThumbView.contentMode = .scaleAspectFit
            if let image = innerThumbView.image {
                innerThumbView.frame.size = CGSize(width: image.size.width * 8 / 14, height: image.size.height * 8 / 14)
                innerThumbView.center = CGPoint(x: sv.bounds.width / 2, y: sv.bounds.height / 2)
            }
        }
        // 这个方法仍不成功，原因是，image是个tiff图，而且是所谓.9图，所以scale这种事就变成拼尸块了，而不是按意愿缩小。
        // 最终的解决办法，还得是提供thumbImage，图片大小就是滑块大小。
    }
}