如何使用 SwiftUI 更新 SceneKit 场景？

Question

I have a cylinder as SCNCylinder in a SCNScene in SceneKit and want to display it in a frame in SwiftUI. My goal is to rotate the cylinder by a angle of 180° or 90° (as the user chooses). To take the input (of the angle of rotation) i have used Text() and onTapGesture{ .... } property in SwiftUI. After I tap the text, the cylinder rotates but now I have two cylinders, one at the original position and one rotating at an desired angle. I am not sure why this happens. I want the same cylinder to rotate, not a identical copy of that doing it. I have connected the SwiftUI view and SceneKit view by using @State and @Binding .

Here is my code :

struct ContentView: View {
 @State var rotationAngle = 0

var body: some View {

    VStack{

        Text("180°").onTapGesture {
            self.rotationAngle = 180
        }

        Spacer()

        Text("90°").onTapGesture {
            self.rotationAngle = 90
        }

        SceneKitView(angle: $rotationAngle)
            .position(x: 225.0, y: 200)
            .frame(width: 300, height: 300, alignment: .center)

    }
  }
}

struct SceneKitView: UIViewRepresentable {

 @Binding var angle: Int

func degreesToRadians(_ degrees: Float) -> CGFloat {
    return CGFloat(degrees * .pi / 180)
}

func makeUIView(context: UIViewRepresentableContext<SceneKitView>) -> SCNView {

    let sceneView = SCNView()
    sceneView.scene = SCNScene()
    sceneView.allowsCameraControl = true
    sceneView.autoenablesDefaultLighting = true
    sceneView.backgroundColor = UIColor.white
    sceneView.frame = CGRect(x: 0, y: 10, width: 0, height: 1)

    return sceneView
}

func updateUIView(_ sceneView: SCNView, context: UIViewRepresentableContext<SceneKitView>) {

        let cylinder = SCNCylinder(radius: 0.02, height: 2.0)
        let cylindernode = SCNNode(geometry: cylinder)
        cylindernode.position = SCNVector3(x: 0, y: 0, z: 0)
        cylinder.firstMaterial?.diffuse.contents = UIColor.green

        cylindernode.pivot = SCNMatrix4MakeTranslation(0, -1, 0)

        let inttofloat = Float(self.angle)

         let rotation = SCNAction.rotate(by: self.degreesToRadians(inttofloat), around: SCNVector3(1, 0, 0), duration: 5)

         cylindernode.runAction(rotation)

         sceneView.scene?.rootNode.addChildNode(cylindernode)

}
typealias UIViewType = SCNView

}

I want to have a single cylinder rotation at a given angle.

Answer 1

The problem is, that updateUIView will be called several times. You can check this by adding a debug point there. Because of that your cylinder will be added several times. So you can solve this by many ways...one way would be to delete all nodes in your sceneview before starting your animation like so:

func updateUIView(_ sceneView: SCNView, context: UIViewRepresentableContext<SceneKitView>) {

    sceneView.scene?.rootNode.enumerateChildNodes { (node, stop) in
        node.removeFromParentNode()
    }