# Objective
The `SceneInstanceReady` event would be more ergonomic (and potentially
efficient) if it could be delivered to listeners attached to the scene
entities becoming ready rather than into a World-global queue.
This is an evolution of @Shatur's work in #9313.
## Solution
The scene spawner is changed to trigger observers on the scene entity
when it is ready rather than enqueue an event with `EventWriter`.
This addresses the two outstanding feature requests mentioned on #2218,
that i) the events should be "scoped" in some way and ii) that the
`InstanceId` should be included in the event.
## Testing
Modified the `scene_spawner::tests::event` test to use the new
mechanism.
---
## Changelog
- Changed `SceneInstanceReady` to trigger an entity observer rather than
be written to an event queue.
- Changed `SceneInstanceReady` to carry the `InstanceId` of the scene.
## Migration Guide
If you have a system which read `SceneInstanceReady` events:
> ```fn ready_system(ready_events: EventReader<'_, '_,
SceneInstanceReady>) {```
It must be rewritten as an observer:
> ```commands.observe(|trigger: Trigger<SceneInstanceReady>| {```
Or, if you were expecting the event in relation to a specific entity or
entities, as an entity observer:
> ```commands.entity(entity).observe(|trigger:
Trigger<SceneInstanceReady>| {```
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