addNodeRef method
Counts references to shared node / Object3D resources. These resources can be reused, or "instantiated", at multiple nodes in the scene hierarchy. Mesh, Camera, and Light instances are instantiated and must be marked. Non-scenegraph resources (like Materials, Geometries, and Textures) can be reused directly and are not marked here.
Example: CesiumMilkTruck sample model reuses "Wheel" meshes.
Implementation
void addNodeRef(Map<String,dynamic> cache, int? index) {
if (index == null) return;
if (cache["refs"][index] == null) {
cache["refs"][index] = cache["uses"][index] = 0;
}
cache["refs"][index]++;
}