Cannot get Camera position in RealityKit

Question

I am trying to load a 3d model "cube.usdz" onto the world position of the camera.

I've already looked at this solution: https://stackoverflow.com/a/45089244/13246089, but I have no idea how to implement it and where do I even call the function "session"? Sorry I am very new to this and don't know what I am doing.

Why does my code always go to "why does nothing work"? Why can't I get the position of my camera? Any assistance or suggestions would be greatly appreciated.

import UIKit
import RealityKit
import ARKit

class Load3DModelViewController: UIViewController, ARSessionDelegate {

    @IBOutlet weak var arView: ARView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // set up AR configuration
        let configuration = ARWorldTrackingConfiguration()
        configuration.planeDetection = .horizontal
        arView.session.run(configuration)
        
        load3DModel()
        arView.session.delegate = self
    }

    func load3DModel() {
        let modelFileName = "cube.usdz"

        guard let modelEntity = try? Entity.load(named: modelFileName) else {
            print("Failed to load the 3D model")
            return
        }
        
        if let pos = arView.session.currentFrame?.camera.transform {
            let xpos = pos.columns.3.x
            let ypos = pos.columns.3.y
            let zpos = pos.columns.3.z
            let modelTranslation = SIMD3<Float>(xpos,ypos,zpos - 1)
            modelEntity.setPosition(modelTranslation, relativeTo: nil)
            arView.scene.addAnchor(modelEntity as! HasAnchoring)
        } else {
            print("\nwhy does nothing work\n")
        }
    }
}

Answer 1

Model that always follows the ARCamera

The first scenario represents how RealityKit automatically tracks the target (i.e. ARCamera's anchor), so the sphere always follows the ARCamera (with 1 meter offset along Z axis).

import UIKit
import RealityKit

class ViewController: UIViewController {
    
    @IBOutlet var arView: ARView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let sphere = ModelEntity(mesh: .generateSphere(radius: 0.05))
        sphere.position.z = -1.0            

        let cameraAnchor = AnchorEntity(.camera) 
        cameraAnchor.addChild(sphere)
        arView.scene.addAnchor(cameraAnchor)
    }
}

---

Anchor copying ARCamera's Transform when session started

This example shows how to create a sphere that will be tethered at the anchor's position with the coordinates of the camera location at the moment when session started + 2 seconds delay.

Take a step back to see the sphere.

import UIKit
import RealityKit

class ViewController: UIViewController {
    
    @IBOutlet var arView: ARView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let sphere = ModelEntity(mesh: .generateSphere(radius: 0.05))

        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {

            let worldAnchor = AnchorEntity() 
            worldAnchor.transform = self.arView.cameraTransform

            worldAnchor.addChild(sphere)
            self.arView.scene.addAnchor(worldAnchor)
        }
    }
}

More info you can find here.