gpuDrawLayer3.swift

func updateFlattenedLayer() {
    // 1
    guard let drawingLayer = drawingLayer,
        // 2
        let optionalDrawing = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(
            NSKeyedArchiver.archivedData(withRootObject: drawingLayer, requiringSecureCoding: false))
            as? CAShapeLayer,
        // 3
        let newDrawing = optionalDrawing else { return }
    // 4
    self.layer.addSublayer(newDrawing)
}