Minimal Bubbling Observers

crates/bevy_ecs/macros/src/component.rs

@@ -17,6 +17,8 @@ pub fn derive_event(input: TokenStream) -> TokenStream {
 
     TokenStream::from(quote! {
         impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {
+            type Traverse = #bevy_ecs_path::traversal::TraverseNone;
+            const SHOULD_BUBBLE: bool = false;
         }
 
         impl #impl_generics #bevy_ecs_path::component::Component for #struct_name #type_generics #where_clause {

crates/bevy_ecs/src/event/base.rs

@@ -1,4 +1,4 @@
-use crate::component::Component;
+use crate::{component::Component, traversal::Traversal};
 #[cfg(feature = "bevy_reflect")]
 use bevy_reflect::Reflect;
 use std::{
@@ -31,7 +31,13 @@ use std::{
     label = "invalid `Event`",
     note = "consider annotating `{Self}` with `#[derive(Event)]`"
 )]
-pub trait Event: Component {}
+pub trait Event: Component {
+    /// A system param that describes a traversal through the ECS.
+    type Traverse: Traversal;
+
+    /// Sets the default bubling state of the entity when used with observers.
+    const SHOULD_BUBBLE: bool;
+}
 
 /// An `EventId` uniquely identifies an event stored in a specific [`World`].
 ///

crates/bevy_ecs/src/lib.rs

@@ -29,6 +29,7 @@ pub mod removal_detection;
 pub mod schedule;
 pub mod storage;
 pub mod system;
+pub mod traversal;
 pub mod world;
 
 pub use bevy_ptr as ptr;