Bubble and trouble: release notes for bubbling observers (#1823)

Co-authored-by: Benjamin Brienen <[email protected]>

release-content/0.15/release-notes/13991_Minimal_Bubbling_Observers.md

@@ -1,4 +1,38 @@
 <!-- Minimal Bubbling Observers -->
 <!-- https://github.com/bevyengine/bevy/pull/13991 -->
 
-<!-- TODO -->
+Virtually every pointer interaction (like mouse click) is rare (humans are slow!), and often requires a complex response.
+To respond to these events, `@aevyrie` created [`bevy_event_listener`],
+which runs systems on-demand and bubbles unhandled events up the entity hierarchy, to their parent.
+
+This pattern is particularly useful in UI, where unhandled interactions are often intended for the pane that *contains* the entity that's on top, but is also valuable for in-game interactions: clicking on a unit's sword should select the unit!
+
+Rather than upstreaming `@aevyrie`'s crate as `bevy_event_hearer`, we quickly realized that there was a huge overlap with the recently added [observers].
+Developed independently long after that crate's inception, observers watch for and then respond to entity-targeted events.
+However, observers have no way to pass unhandled events up the hierarchy to parents who might care about them.
+
+To support this, we've extended the [`Event`] trait with an associated type `Traversal` and an associated constant `AUTO_PROPAGATE`.
+This behavior is opt-in: when you derive the `Event` type, these are set to `()` and `false` respectively.
+
+For the [`Pointer<E>`] event type, we've chosen to implement this as:
+
+```rust
+impl <E> Event for Pointer<E>{
+    type Traversal = &Parent;
+    const AUTO_PROPAGATE: bool = true;
+}
+```
+
+This means that, unless you call [`Trigger::propagate(false)`], pointer events will be replicated up the hierarchy (accessing the `Entity` stored in the [`Parent`] component) until it reaches the entity root.
+
+Any type that implements the [`Traversal`] trait can be used as the associated type and can access arbitrary read-only query data from the world.
+While using the standard entity hierarchy is a sensible choice for *many* applications, bubbling can be used for arbitrary event propagation using your own [proto-relations].
+Let us know what you cook up: user feedback is indispensable for building a better Bevy!
+
+[Observers]: https://bevyengine.org/news/bevy-0-14/#ecs-hooks-and-observers
+[`bevy_event_listener`]: (https://github.com/aevyrie/bevy_eventlistener)
+[`Event`]: https://docs.rs/bevy/0.15.0/bevy/ecs/event/trait.Event.html
+[`Pointer<E>`]: https://docs.rs/bevy/0.15.0/bevy/picking/events/struct.Pointer.html
+[`Trigger::propagate(false)`]: https://docs.rs/bevy/0.15.0/bevy/ecs/prelude/struct.Trigger.html#method.propagate
+[`Parent`]: https://docs.rs/bevy/0.15.0/bevy/hierarchy/struct.Parent.html
+[proto-relations]: https://github.com/bevyengine/bevy/issues/3742

release-content/0.15/release-notes/_release-notes.toml

@@ -34,10 +34,16 @@ prs = [14269]
 file_name = "14269_Basic_isometry_types.md"
 
 [[release_notes]]
-title = "Minimal Bubbling Observers"
+contributors = [
+    "@aevyrie",
+    "@cart",
+    "@alice-i-cecile",
+    "@torsteingrindvik",
+    "@BenjaminBrienen",
+]
+title = "Bubbling Observers"
 authors = ["@NthTensor"]
-contributors = ["@cart", "@alice-i-cecile", "@torsteingrindvik"]
-prs = [13991]
+prs = [13991, 15385]
 file_name = "13991_Minimal_Bubbling_Observers.md"
 
 [[release_notes]]