A simple marching ants CALayer implementation

MarchingAntsSelectionLayer.swift

class MarchingAntsSelectionLayer: CAShapeLayer {
   func setup() {
      let dashPattern: [NSNumber] = [2.0, 2.0]
      let dashPhase = dashPattern.map { $0.floatValue }.reduce(0) { $0 + $1 }

      self.lineDashPattern = dashPattern

      let lineDashPhaseAnimation = CABasicAnimation(keyPath: "lineDashPhase")
      lineDashPhaseAnimation.byValue = dashPhase
      lineDashPhaseAnimation.duration = 0.5
      lineDashPhaseAnimation.timingFunction = CAMediaTimingFunction(name: CAMediaTimingFunctionName.linear)
      lineDashPhaseAnimation.repeatCount = .greatestFiniteMagnitude

      self.add(lineDashPhaseAnimation, forKey: "lineDashPhaseAnimation")

      self.path = CGPath(rect: self.bounds, transform: nil)
      self.lineWidth = 1.0
      self.strokeColor = NSColor.white.cgColor
      self.fillColor = NSColor.selectedContentBackgroundColor.cgColor.copy(alpha: 0.2)
   }

   func setSelectionRect(rect: CGRect) {
      CATransaction.begin()
      CATransaction.setDisableActions(true)
      self.frame = rect
      self.path = CGPath(rect: self.bounds, transform: nil)
      CATransaction.commit()
   }
}