iOS 无卡顿同时使用圆角、阴影和边框的实现

override init(frame: CGRect) {
  super.init(frame: frame)

  imageView = UIImageView(image: UIImage(named: "img"))
  imageView.layer.cornerRadius = 14
  imageView.layer.masksToBounds = true
  backgroundView = imageView

  shadowView = ShadowView()
  shadowView.layer.cornerRadius = 20
  shadowView.applyShadow(.black, CGSize(width: 0, height: 15), 0.2, 40)
  insertSubview(shadowView, belowSubview: imageView)

  contentView.layer.cornerRadius = 14
  contentView.layer.borderWidth = 1
  contentView.layer.borderColor = UIColor.orange.cgColor
  contentView.layer.masksToBounds = true
}

class ShadowView: UIView {
  private var shadowColor: UIColor?
  private var shadowOpacity: CGFloat = 1
  private var shadowOffset: CGSize = CGSize(width: 0, height: 3)
  private var shadowBlur: CGFloat = 6

  override func layoutSubviews() {
    super.layoutSubviews()

    updateShadow()
  }

  func applyShadow(_ color: UIColor?, _ offset: CGSize, _ opacity: CGFloat, _ blur: CGFloat) {
    shadowColor = color
    shadowOffset = offset
    shadowOpacity = opacity
    shadowBlur = blur

    updateShadow()
  }

  private func updateShadow() {
    layer.shadowColor = shadowColor?.cgColor
    layer.shadowOffset = shadowOffset
    layer.shadowOpacity = Float(shadowOpacity)
    layer.shadowRadius = shadowBlur * 0.5
    layer.shadowPath = UIBezierPath(roundedRect: self.bounds, cornerRadius: layer.cornerRadius).cgPath
  }
}