Minimal Bubbling Observers

Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["game", "engine", "gamedev", "graphics", "bevy"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/bevyengine/bevy"
 documentation = "https://docs.rs/bevy"
-rust-version = "1.78.0"
+rust-version = "1.79.0"
 
 [workspace]
 exclude = [
@@ -2549,6 +2549,17 @@ description = "Demonstrates observers that react to events (both built-in life-c
 category = "ECS (Entity Component System)"
 wasm = true
 
+[[example]]
+name = "observer_propagation"
+path = "examples/ecs/observer_propagation.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.observer_propagation]
+name = "Observer Propagation"
+description = "Demonstrates event propagation with observers"
+category = "ECS (Entity Component System)"
+wasm = true
+
 [[example]]
 name = "3d_rotation"
 path = "examples/transforms/3d_rotation.rs"

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 AUTO_PROPAGATE: 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 +31,19 @@ use std::{
     label = "invalid `Event`",
     note = "consider annotating `{Self}` with `#[derive(Event)]`"
 )]
-pub trait Event: Component {}
+pub trait Event: Component {
+    /// The component that describes which Entity to propagate this event to next, when [propagation] is enabled.
+    ///
+    /// [propagation]: crate::observers::Trigger::enable_propagation
+    type Traverse: Traversal;
+
+    /// When true, this event will always attempt to propagate when [triggered], without requiring a call
+    /// to [`Trigger::enable_propagation`].
+    ///
+    /// [triggered]: crate::commands::Commands::trigger_targets
+    /// [`Trigger::enable_propagation`]: crate::observers::Trigger::enable_propagation
+    const AUTO_PROPAGATE: bool = false;
+}
 
 /// 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;