How to add SCNNodes without blocking main thread?

Question

I'm creating and adding a large number of SCNNodes to a SceneKit scene, which causes the app to freeze for a second or two.

I thought I could fix this by putting all the action in a background thread using DispatchQueue.global(qos: .background).async(), but no dice. It behaves exactly the same.

I saw this answer and put the nodes through SCNView.prepare() before adding them, hoping it would slow down the background thread and prevent blocking. It didn't.

Here's a test function that reproduces the problem:

func spawnNodesInBackground() {
    // put all the action in a background thread
    DispatchQueue.global(qos: .background).async {
        var nodes = [SCNNode]()
        for i in 0...5000 {
            // create a simple SCNNode
            let node = SCNNode()
            node.position = SCNVector3(i, i, i)
            let geometry = SCNSphere(radius: 1)
            geometry.firstMaterial?.diffuse.contents = UIColor.white.cgColor
            node.geometry = geometry
            nodes.append(node)
        }
        // run the nodes through prepare()
        self.mySCNView.prepare(nodes, completionHandler: { (Bool) in
            // nodes are prepared, add them to scene
            for node in nodes {
                self.myRootNode.addChildNode(node)
            }
        })
    }
}

When I call spawnNodesInBackground() I expect the scene to continue rendering normally (perhaps at a reduced frame rate) while new nodes are added at whatever pace the CPU is comfortable with. Instead the app freezes completely for a second or two, then all the new nodes appear at once.

Why is this happening, and how can I add a large number of nodes without blocking the main thread?

Answer 1

I don't think this problem is solvable using the DispatchQueue. If I substitute some other task instead of creating SCNNodes it works as expected, so I think the problem is related to SceneKit.

The answers to this question suggest that SceneKit has its own private background thread that it batches all changes to. So regardless of what thread I use to create my SCNNodes, they all end up in the same queue in the same thread as the render loop.

The ugly workaround I'm using is to add the nodes a few at a time in SceneKit's delegated renderer(_:updateAtTime:) method until they're all done.