使用ARKit/RealityKit中提供的API计算相机与目标之间的距离并基于该距离改变模型的颜色。

示例代码如下：

// 先实例化ARAnchor（或自己的模型），并添加到ARView中
let anchor = AnchorEntity(plane: .horizontal, minimumBounds: [0.1, 0.1])
arView.scene.addAnchor(anchor)

// 然后创建一个ShapeEntity，指定其颜色并添加到ARAnchor
let box = MeshResource.generateBox(size: 0.2)
let color = MaterialColorParameter.color(UIColor.red)
let material = SimpleMaterial(color: color, isMetallic: false)
let entity = ModelEntity(mesh: box, materials: [material])
anchor.addChild(entity)

// 在ARSessionDelegate的session(_:didUpdate:)回调方法中计算相机与目标之间的距离
extension ViewController: ARSessionDelegate {
    func session(_ session: ARSession, didUpdate anchors: [ARAnchor]) {
        for anchor in anchors {
            guard let anchorEntity = arView.scene.findEntity(anchor: anchor) else { continue }
            let cameraTransform = arView.cameraTransform
            let distance = cameraTransform.translation.distance(to: anchorEntity.transform.translation)
            // 基于距离更新模型的颜色（示例）
            if distance <= 0.5 {
                entity.components[ModelComponent].materials = [SimpleMaterial(color: .green, isMetallic: false)]
            } else if distance <= 1.0 {
                entity.components[ModelComponent].materials = [SimpleMaterial(color: .yellow, isMetallic: false)]
            } else {
                entity.components[ModelComponent].materials = [SimpleMaterial(color: .red, isMetallic: false)]
            }
        }
    }
}