can reflect observers (#23797)

# Objective

- Reflect observers *so that I can observe my reflection*

## Solution

- Reflect observers

## Testing

- CI (and followup PR)

Author: mockersf

crates/bevy_ecs/src/reflect/event.rs

@@ -5,7 +5,14 @@
 //!
 //! Same as [`component`](`super::component`), but for events.
 
-use crate::{event::Event, reflect::from_reflect_with_fallback, world::World};
+use alloc::boxed::Box;
+
+use crate::{
+    event::Event,
+    observer::{Observer, On},
+    reflect::from_reflect_with_fallback,
+    world::{DeferredWorld, World},
+};
 use bevy_reflect::{FromReflect, FromType, PartialReflect, Reflect, TypePath, TypeRegistry};
 
 /// A struct used to operate on reflected [`Event`] trait of a type.
@@ -37,6 +44,8 @@ pub struct ReflectEvent(ReflectEventFns);
 pub struct ReflectEventFns {
     /// Function pointer implementing [`ReflectEvent::trigger`].
     pub trigger: fn(&mut World, &dyn PartialReflect, &TypeRegistry),
+    /// Function pointer implementing [`ReflectEvent::create_observer`].
+    pub create_observer: fn(Box<dyn Fn(&dyn Reflect, DeferredWorld) + Send + Sync>) -> Observer,
 }
 
 impl ReflectEventFns {
@@ -59,6 +68,14 @@ impl ReflectEvent {
         (self.0.trigger)(world, event, registry);
     }
 
+    /// Creates an [`Observer`] for this [`Event`] that calls the given callback.
+    pub fn create_observer(
+        &self,
+        callback: Box<dyn Fn(&dyn Reflect, DeferredWorld) + Send + Sync>,
+    ) -> Observer {
+        (self.0.create_observer)(callback)
+    }
+
     /// Create a custom implementation of [`ReflectEvent`].
     ///
     /// This is an advanced feature,
@@ -105,6 +122,11 @@ where
                 let event = from_reflect_with_fallback::<E>(reflected_event, world, registry);
                 world.trigger(event);
             },
+            create_observer: |callback| {
+                Observer::new(move |event: On<E>, world: DeferredWorld| {
+                    callback((*event).as_reflect(), world);
+                })
+            },
         })
     }
 }