ios - Copied SCNGeometry does not work immediately in current runloop - Stack Overflow

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I observed a very strange behavior where a copied SCNGeometry doesn't work immediately. I had to wait one run-loop (DispatchQueue.main.async) in order to make it work. Is it a bug or am I doing something wrong?

Here's my code:

  func helper_cloneGeo(geo: SCNGeometry) -> SCNGeometry {
    let copiedGeo = geo.copy() as! SCNGeometry
    copiedGeo.materials = geo.materials.map { $0.copy() as! SCNMaterial }
    return copiedGeo
  }
  
  override init() {
    super.init()

    let geo = readGeo(fn: "alcohol.usdz", texture: "texture.png").geometry!
    
    // HERE!!!
    // SCNGeometry is only clone-able after async!!
    // DispatchQueue.main.async {
      let copiedGeo = self.helper_cloneGeo(geo: geo)
      let newNode = SCNNode(geometry: copiedGeo)
      self.rootNode.addChildNode(newNode)
    // }
    
  }

The readGeo is just a helper function that reads a 3d model file, which should be correct, because the original (not cloned) node works.

You can download my sample project here:

In this project, when you run it, it shows the model. Then if you comment out the DispatchQueue.main.async (like in the code above), the model won't be shown.

I observed a very strange behavior where a copied SCNGeometry doesn't work immediately. I had to wait one run-loop (DispatchQueue.main.async) in order to make it work. Is it a bug or am I doing something wrong?

Here's my code:

  func helper_cloneGeo(geo: SCNGeometry) -> SCNGeometry {
    let copiedGeo = geo.copy() as! SCNGeometry
    copiedGeo.materials = geo.materials.map { $0.copy() as! SCNMaterial }
    return copiedGeo
  }
  
  override init() {
    super.init()

    let geo = readGeo(fn: "alcohol.usdz", texture: "texture.png").geometry!
    
    // HERE!!!
    // SCNGeometry is only clone-able after async!!
    // DispatchQueue.main.async {
      let copiedGeo = self.helper_cloneGeo(geo: geo)
      let newNode = SCNNode(geometry: copiedGeo)
      self.rootNode.addChildNode(newNode)
    // }
    
  }

The readGeo is just a helper function that reads a 3d model file, which should be correct, because the original (not cloned) node works.

You can download my sample project here: https://drive.google.com/file/d/1kYyqCAJXnXqpZc6vaaXORe_UVmLH6wHL/view?usp=sharing

In this project, when you run it, it shows the model. Then if you comment out the DispatchQueue.main.async (like in the code above), the model won't be shown.

• ios
• swift
• scenekit

Share Improve this question Follow edited Jan 7 at 18:22 OMGPOP asked Jan 7 at 18:03 OMGPOPOMGPOP 1,07288 gold badges5252 silver badges101101 bronze badges 4

• By doing some basic debugging, you can see that geo.elements and geo.sources are all empty, and this is why nothing got copied. I suspect that flattenedClone somehow made a node with a "lazy" geometry. If you don't flattenedClone and just clone node.childNodes.first?.childNodes.first?.geometry. It works as expected. – Sweeper Commented Jan 7 at 19:00
• Hmmm, i have to flattenedClone it in my real project, so that a node only has 1 geometry which is easier to deal with, regardless of usdz file setup (which i cant control since i buy them in batch from different studios) – OMGPOP Commented Jan 7 at 20:18
• My point wasn't to offer an alternative in the first place... You are not asking for an alternative, right? Since DispatchQueue.main.async already works. – Sweeper Commented Jan 7 at 20:43
• Sorry for the confusion, i can't use DispatchQueue.main.async, since i have to add the models in the init of my container node. i could add the child node async'ly, but i have bunch of infras relying on the container being setup properly. I could load the assets on app launch, but that's also quite a huge change due to the scale of my infra – OMGPOP Commented Jan 7 at 23:37

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2 Answers 2

Sorted by: Reset to default 1

By doing some basic debugging, you can see that geo.elements and geo.sources are all empty, and this is why nothing got copied. I suspect that SCNNode.flattenedClone (which you used in readGeo) somehow made a node with a "lazy" geometry that doesn't get immediately created.

If you don't want to do this asynchronously, you can always just run the run loop yourself.

let geo = readGeo(fn: "alcohol.usdz", texture: "texture.png").geometry!

RunLoop.main.run(until: .now) // run the run loop for just one iteration

let copiedGeo = self.helper_cloneGeo(geo: geo)
let newNode = SCNNode(geometry: copiedGeo)

Not sure why but this works. Likely related to SceneKit internal implementation, as Sweeper suggested.

func helper_cloneGeo(geo: SCNGeometry) -> SCNGeometry {
  return SCNNode(geometry: geo).clone().geometry!
}

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