gpuDrawLayer2.swift

  override func draw(_ layer: CALayer, in ctx: CGContext) {
      // 1
      let drawingLayer = self.drawingLayer ?? CAShapeLayer()
      // 2
      drawingLayer.contentsScale = UIScreen.main.scale
      // 3
      let linePath = UIBezierPath()
      // 4
      for (index, point) in line.enumerated() {
          if index == 0 {
              linePath.move(to: point)
          } else {
              linePath.addLine(to: point)
          }
      }

      drawingLayer.path = linePath.cgPath
      drawingLayer.opacity = 1
      drawingLayer.lineWidth = lineWidth
      drawingLayer.lineCap = .round
      drawingLayer.fillColor = UIColor.clear.cgColor
      drawingLayer.strokeColor = lineColor.cgColor
      
      // 5
      if self.drawingLayer == nil {
          self.drawingLayer = drawingLayer
          layer.addSublayer(drawingLayer)
      }
  }