deregister events (#14083)

# Objective

Add ability to de-register events from the EventRegistry (and the
associated World).

The initial reasoning relates to retaining support for Event hot
reloading in `dexterous_developer`.

## Solution

Add a `deregister_events<T: Event>(&mut world)` method to the
`EventRegistry` struct.

## Testing

Added an automated test that verifies the event registry adds and
removes `Events<T>` from the world.

---------

Co-authored-by: BD103 <[email protected]>

Author: lee-orr

crates/bevy_ecs/src/event/mod.rs

@@ -280,6 +280,24 @@ mod tests {
         assert!(is_empty, "EventReader should be empty");
     }
 
+    #[test]
+    fn test_event_registry_can_add_and_remove_events_to_world() {
+        use bevy_ecs::prelude::*;
+
+        let mut world = World::new();
+        EventRegistry::register_event::<TestEvent>(&mut world);
+
+        let has_events = world.get_resource::<Events<TestEvent>>().is_some();
+
+        assert!(has_events, "Should have the events resource");
+
+        EventRegistry::deregister_events::<TestEvent>(&mut world);
+
+        let has_events = world.get_resource::<Events<TestEvent>>().is_some();
+
+        assert!(!has_events, "Should not have the events resource");
+    }
+
     #[test]
     fn test_update_drain() {
         let mut events = Events::<TestEvent>::default();

crates/bevy_ecs/src/event/registry.rs

@@ -41,7 +41,10 @@ pub enum ShouldUpdateEvents {
 }
 
 impl EventRegistry {
-    /// Registers an event type to be updated.
+    /// Registers an event type to be updated in a given [`World`]
+    ///
+    /// If no instance of the [`EventRegistry`] exists in the world, this will add one - otherwise it will use
+    /// the existing instance.
     pub fn register_event<T: Event>(world: &mut World) {
         // By initializing the resource here, we can be sure that it is present,
         // and receive the correct, up-to-date `ComponentId` even if it was previously removed.
@@ -77,4 +80,14 @@ impl EventRegistry {
             }
         }
     }
+
+    /// Removes an event from the world and it's associated [`EventRegistry`].
+    pub fn deregister_events<T: Event>(world: &mut World) {
+        let component_id = world.init_resource::<Events<T>>();
+        let mut registry = world.get_resource_or_insert_with(Self::default);
+        registry
+            .event_updates
+            .retain(|e| e.component_id != component_id);
+        world.remove_resource::<Events<T>>();
+    }
 }