SCNNode rotation around a generic axis with animation - swift

Question

I need to rotate the gray bar around the red dot (see picture below).
I managed to do that translating the center point of the bar, rotating the bar and then translating the bar back to the original position.
Down here my code:
struct TestView2: View {
var scene: SCNScene
var arm: SCNNode
var length = 16.0
var with = 1.0
init() {
scene = SCNScene()
scene.setAttribute(400000, forKey: SCNScene.Attribute.endTime.rawValue)
let rootNode = scene.rootNode
let armShape = SCNBox(width: with, height: 0.5, length: length, chamferRadius: 0)
armShape.firstMaterial?.diffuse.contents = NSColor.gray
arm = SCNNode()
arm.geometry = armShape
arm.position = SCNVector3(0, 0, 0)
rootNode.addChildNode(arm)
let axisShape = SCNCylinder(radius: with/2, height: 0.51)
axisShape.firstMaterial?.diffuse.contents = NSColor.red
let axis = SCNNode()
axis.geometry = axisShape
axis.position = SCNVector3(0, 0, length/2-with/2)
rootNode.addChildNode(axis)
// Camera
let camera = SCNNode()
camera.camera = SCNCamera()
camera.camera?.usesOrthographicProjection = true
camera.camera?.orthographicScale = 15
camera.camera?.zNear = 0
camera.camera?.zFar = 100
camera.position = SCNVector3(0, 0, 50)
let cameraOrbit = SCNNode()
cameraOrbit.addChildNode(camera)
rootNode.addChildNode(cameraOrbit)
var eulerAngles = cameraOrbit.eulerAngles
eulerAngles.x = CGFloat(Float.pi / 2.0)
eulerAngles.y = 0
eulerAngles.z = 0
cameraOrbit.eulerAngles = eulerAngles
}
var body: some View {
VStack {
SceneView(scene: scene, options: [.allowsCameraControl, .autoenablesDefaultLighting])
Button("Rotate") {
let currentTransform = arm.transform
let translationTransform = SCNMatrix4MakeTranslation(0, 0, length/2-with/2)
let rotationTrasform = SCNMatrix4MakeRotation(90 * .pi / 180.0, 0, 1, 0)
let backTranslationTransform = SCNMatrix4MakeTranslation(0, 0, -(length/2-with/2))
let newTransform = SCNMatrix4Mult(backTranslationTransform,
SCNMatrix4Mult(rotationTrasform,
SCNMatrix4Mult(translationTransform, currentTransform)))
arm.transform = newTransform
}
}
}
}
The bar rotate as expected. I then added an animation:
...
let animation = CABasicAnimation(keyPath: "transform")
let currentTransform = arm.transform
let translationTransform = SCNMatrix4MakeTranslation(0, 0, length/2-with/2)
let rotationTrasform = SCNMatrix4MakeRotation(90 * .pi / 180.0, 0, 1, 0)
let backTranslationTransform = SCNMatrix4MakeTranslation(0, 0, -(length/2-with/2))
let newTransform = SCNMatrix4Mult(backTranslationTransform,
SCNMatrix4Mult(rotationTrasform,
SCNMatrix4Mult(translationTransform, currentTransform)))
animation.fromValue = currentTransform
animation.toValue = newTransform
animation.duration = 1.0
arm.addAnimation(animation, forKey: nil)
arm.transform = newTransform
...
The result is very clumsy though because the translations before and after the rotation are visible:
Since I am new to SceneKit, I think my approach to rotate the bar is completely wrong.
How can I improve the code to have both the rotation and animation at the same time?