shows a simple pan gesture handler for use in part 2 of a literalpie medium post

SceneKit-simple-pan-gesture-handler.swift

@objc func handlePan(panGesture: UIPanGestureRecognizer) {
  guard let view = view as? SCNView else { return }
  let location = panGesture.location(in: self.view)
  switch panGesture.state {
  case .began:
    guard let hitNodeResult = view.hitTest(location, options: nil).first else { return }
    // panStartZ and draggingNode should be defined in the containing class
    panStartZ = CGFloat(view.projectPoint(lastPanLocation!).z)
    draggingNode = hitNodeResult.node
  case .changed:
    let location = panGesture.location(in: view)
    let worldTouchPosition = view.unprojectPoint(SCNVector3(location.x, location.y, panStartZ!))
    draggingNode?.worldPosition = worldTouchPosition
  default:
    break
  }
}

How to define: lastPanLocation variable?

You should be able to add it to the viewController, outside the function shown in this gist:

var lastPanLocation: SCNVector3?

@objc func handlePan(panGesture: UIPanGestureRecognizer) {
...
}

here is the appropriate code.