override func viewDidLoad() {
    super.viewDidLoad()
    view.addSubview(sceneView)
    view.addSubview(infoLabel)
    //1
    let tapRecognizer = UITapGestureRecognizer(target: self, action: #selector(handleTap))
    tapRecognizer.numberOfTapsRequired = 1
    sceneView.addGestureRecognizer(tapRecognizer)
}

// MARK: Gesture handlers
@objc func handleTap(sender: UITapGestureRecognizer) {
    //2
    let tapLocation = sender.location(in: sceneView)
    //3
    let hitTestResults = sceneView.hitTest(tapLocation, types: .featurePoint)
    if let result = hitTestResults.first {
        //4
        let position = SCNVector3.positionFrom(matrix: result.worldTransform)
        //5
        let sphere = SphereNode(position: position)
        //6
        sceneView.scene.rootNode.addChildNode(sphere)
        let lastNode = nodes.last
        nodes.append(sphere)
        if lastNode != nil {
            //7
            let distance = lastNode!.position.distance(to: sphere.position)
            infoLabel.text = String(format: "Distance: %.2f meters", distance)
        }
    }
}