Make SceneSpawner::spawn_dynamic return the spawned scene's InstanceId

[Carnagion]

What problem does this solve or what need does it fill?

Currently, SceneSpawner::spawn_dynamic returns unit (()) rather than the spawned scene's InstanceId.

It is difficult to work with any DynamicScene that is spawned this way, because the lack of an InstanceId makes it impossible to do things such as iterating over the scene's entities using SceneSpawner::iter_instance_entities or checking if it's ready using SceneSpawner::is_instance_ready:

scene_spawner.spawn_dynamic(dynamic_scene_handle); // no instance id returned
// cannot use with SceneSpawner::iter_instance_entities or SceneSpawner::is_instance_ready

It also makes the API inconsistent, since every other method in SceneSpawner returns an InstanceId in some form, including other DynamicScene-spawning methods such as SceneSpawner::spawn_dynamic_as_child:

// this works
let id = scene_spawner.spawn_dynamic_as_child(dynamic_scene_handle);
for entity in scene_spawner.iter_instance_entities(id) {
}

// so does this - the Scene counterpart to SceneSpawner::spawn_dynamic
let id = scene_spawner.spawn(scene_handle);
for entity in scene_spawner.iter_instance_entities(id) {
}

What solution would you like?

Change spawn_dynamic's return type to InstanceId and return the spawned scene's InstanceId:

let id = scene_spawner.spawn_dynamic(dynamic_scene_handle);
for entity in scene_spawner.iter_instance_entities(id) {
}

What alternative(s) have you considered?

A possible workaround would be to go through multiple steps to transform a Handle<DynamicScene> to a Handle<Scene>, with the help of Res<Assets<DynamicScene>>, Scene::from_dynamic_scene, and ResMut<Assets<Scene>>.

This adds unnecessary complexity and is not very elegant, but is unfortunately the only solution for those who do not want to spawn the DynamicScene as a child of another entity, and want to do something with the scene's InstanceId.