Make HierarchyPlugin optional (#423)

# Objective

Avian 0.1.0 introduced a regression where `HierarchyPlugin` and `ColliderHierarchyPlugin` are needed for colliders to function, as `ColliderParent` was only inserted by `ColliderHierarchyPlugin`. Hierarchies shouldn't be necessary.

## Solution

Update `ColliderParent` for colliders that are on the `RigidBody` entity in the `ColliderBackendPlugin`, and change panics to warnings when using `ColliderHierarchyPlugin` without `HierarchyPlugin`.

src/collision/collider/backend.rs

@@ -211,6 +211,14 @@ impl<C: ScalableCollider> Plugin for ColliderBackendPlugin<C> {
             ),
         );
 
+        // Update collider parents for colliders that are on the same entity as the rigid body.
+        app.add_systems(
+            self.schedule,
+            update_root_collider_parents::<C>
+                .after(PrepareSet::InitColliders)
+                .before(PrepareSet::Finalize),
+        );
+
         let physics_schedule = app
             .get_schedule_mut(PhysicsSchedule)
             .expect("add PhysicsSchedule first");
@@ -274,6 +282,31 @@ pub(crate) fn init_colliders<C: AnyCollider>(
     }
 }
 
+/// Updates [`ColliderParent`] for colliders that are on the same entity as the [`RigidBody`].
+///
+/// The [`ColliderHierarchyPlugin`] should be used to handle hierarchies.
+#[allow(clippy::type_complexity)]
+fn update_root_collider_parents<C: AnyCollider>(
+    mut commands: Commands,
+    mut bodies: Query<
+        (Entity, Option<&mut ColliderParent>),
+        (With<RigidBody>, With<C>, Or<(Added<RigidBody>, Added<C>)>),
+    >,
+) {
+    for (entity, collider_parent) in &mut bodies {
+        if let Some(mut collider_parent) = collider_parent {
+            collider_parent.0 = entity;
+        } else {
+            commands.entity(entity).try_insert((
+                ColliderParent(entity),
+                // TODO: This probably causes a one frame delay. Compute real value?
+                ColliderTransform::default(),
+                PreviousColliderTransform::default(),
+            ));
+        }
+    }
+}
+
 /// Generates [`Collider`]s based on [`ColliderConstructor`]s.
 ///
 /// If a [`ColliderConstructor`] requires a mesh, the system keeps running

src/collision/collider/hierarchy.rs

@@ -117,35 +117,33 @@ impl Plugin for ColliderHierarchyPlugin {
                 .in_set(NarrowPhaseSet::First),
         );
     }
+
+    fn finish(&self, app: &mut App) {
+        if !app.is_plugin_added::<HierarchyPlugin>() {
+            warn!("`ColliderHierarchyPlugin` requires Bevy's `HierarchyPlugin` to function. If you don't need collider hierarchies, consider disabling this plugin.",);
+        }
+    }
 }
 
 #[derive(Reflect, Clone, Copy, Component, Debug, Default, Deref, DerefMut, PartialEq)]
 #[reflect(Component)]
 pub(crate) struct PreviousColliderTransform(pub ColliderTransform);
 
+/// Updates [`ColliderParent`] for descendant colliders of [`RigidBody`] entities.
+///
+/// The [`ColliderBackendPlugin`] handles collider parents for colliders that are
+/// on the same entity as the rigid body.
 #[allow(clippy::type_complexity)]
 fn update_collider_parents(
     mut commands: Commands,
-    mut bodies: Query<(Entity, Option<&mut ColliderParent>, Has<ColliderMarker>), With<RigidBody>>,
+    mut bodies: Query<Entity, (With<RigidBody>, With<AncestorMarker<ColliderMarker>>)>,
     children: Query<&Children>,
     mut child_colliders: Query<
         Option<&mut ColliderParent>,
         (With<ColliderMarker>, Without<RigidBody>),
     >,
 ) {
-    for (entity, collider_parent, has_collider) in &mut bodies {
-        if has_collider {
-            if let Some(mut collider_parent) = collider_parent {
-                collider_parent.0 = entity;
-            } else {
-                commands.entity(entity).try_insert((
-                    ColliderParent(entity),
-                    // TODO: This probably causes a one frame delay. Compute real value?
-                    ColliderTransform::default(),
-                    PreviousColliderTransform::default(),
-                ));
-            }
-        }
+    for entity in &mut bodies {
         for child in children.iter_descendants(entity) {
             if let Ok(collider_parent) = child_colliders.get_mut(child) {
                 if let Some(mut collider_parent) = collider_parent {

src/sync/ancestor_marker.rs

@@ -78,6 +78,9 @@ impl<C: Component> Plugin for AncestorMarkerPlugin<C> {
             },
         );
 
+        // Initialize `HierarchyEvent` in case `HierarchyPlugin` is not added.
+        app.add_event::<HierarchyEvent>();
+
         // Update markers when changes are made to the hierarchy.
         // TODO: This should be an observer. It'd remove the need for this scheduling nonsense
         //       and make the implementation more robust.
@@ -90,13 +93,6 @@ impl<C: Component> Plugin for AncestorMarkerPlugin<C> {
             app.add_systems(self.schedule, update_markers_on_hierarchy_changes::<C>);
         }
     }
-
-    fn finish(&self, app: &mut App) {
-        assert!(
-            app.is_plugin_added::<HierarchyPlugin>(),
-            "`AncestorMarkerPlugin` requires Bevy's `HierarchyPlugin` to function.",
-        );
-    }
 }
 
 /// A marker component that marks an entity as an ancestor of an entity with the given component `C`.

src/tests.rs

@@ -35,8 +35,9 @@ fn create_app() -> App {
     app.add_plugins((
         MinimalPlugins,
         TransformPlugin,
-        HierarchyPlugin,
-        PhysicsPlugins::default(),
+        PhysicsPlugins::default()
+            .build()
+            .disable::<ColliderHierarchyPlugin>(),
         bevy::asset::AssetPlugin::default(),
         #[cfg(feature = "bevy_scene")]
         bevy::scene::ScenePlugin,
@@ -256,8 +257,9 @@ fn no_ambiguity_errors() {
     App::new()
         .add_plugins((
             MinimalPlugins,
-            HierarchyPlugin,
-            PhysicsPlugins::new(DeterministicSchedule),
+            PhysicsPlugins::new(DeterministicSchedule)
+                .build()
+                .disable::<ColliderHierarchyPlugin>(),
             bevy::asset::AssetPlugin::default(),
             #[cfg(feature = "bevy_scene")]
             bevy::scene::ScenePlugin,