问SetNeedsDisplay无作用

EN

class Canvas: UIView {

  override func draw(_ rect: CGRect) {

      super.draw(rect)
      guard let context = UIGraphicsGetCurrentContext() else { return }

      context.setStrokeColor(login_theme_color.cgColor)
      context.setLineWidth(10.0)
      context.setLineCap(.butt)

      lines.forEach { (line) in
      for(i, p) in line.enumerated(){
          //context.move(to: p)
          //context.addLine(to: p)
          if i == 0 {
              context.move(to: p)
              print("First point of new line is \(p)")
          }else{
              context.addLine(to: p)
              print("point is \(p)")
          }
      }
  }
       context.strokePath()
}

  var lines = [[CGPoint]]()

  override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
      lines.append([CGPoint]())
      //setNeedsDisplay()
  }
  override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
      guard let point = touches.first?.location(in: nil) else { return }
      guard var lastLine = lines.popLast() else { return }
      lastLine.append(point)
      lines.append(lastLine)
      setNeedsDisplay()
  }

}

    let canvas = Canvas()

    @objc fileprivate func setupCanvas(){
        //canvas.setNeedsDisplay()
        containerView.addSubview(canvas)
        canvas.translatesAutoresizingMaskIntoConstraints = false
        canvas.topAnchor.constraint(equalTo: rectangle2.topAnchor, constant: 74).isActive = true
        canvas.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor, constant: (view.frame.width/2) + 8).isActive = true
        canvas.rightAnchor.constraint(equalTo: rectangle2.rightAnchor, constant: -4).isActive = true
        canvas.bottomAnchor.constraint(equalTo: howitWorksText.bottomAnchor, constant: 0).isActive = true
        canvas.layer.cornerRadius = 30
        canvas.layer.shadowRadius = 0.5
        canvas.layer.shadowColor = UIColor.white.cgColor
        //canvas.backgroundColor = UIColor.clear
        //canvas.layer.borderWidth = 2.0
        //canvas.layer.borderColor = UIColor.black.cgColor
        canvas.layer.masksToBounds = true
        canvas.backgroundColor = .white
        //canvas.setNeedsDisplay()
    }