bevy_app/

propagate.rs

use alloc::vec::Vec;
use core::marker::PhantomData;

use crate::{App, Plugin};
#[cfg(feature = "bevy_reflect")]
use bevy_ecs::reflect::ReflectComponent;
use bevy_ecs::{
    change_detection::DetectChangesMut,
    component::Component,
    entity::Entity,
    hierarchy::ChildOf,
    intern::Interned,
    lifecycle::{Insert, Remove, RemovedComponents},
    observer::On,
    query::{Changed, Has, Or, QueryFilter, With, Without},
    relationship::{Relationship, RelationshipTarget},
    schedule::{IntoScheduleConfigs, ScheduleLabel, SystemSet},
    system::{Commands, Local, Query},
};
#[cfg(feature = "bevy_reflect")]
use bevy_reflect::Reflect;

/// Plugin to automatically propagate a component value to all direct and transient relationship
/// targets (e.g. [`bevy_ecs::hierarchy::Children`]) of entities with a [`Propagate`] component.
///
/// The plugin Will maintain the target component over hierarchy changes, adding or removing
/// `C` when a relationship `R` (e.g. [`ChildOf`]) is added to or removed from a
/// relationship tree with a [`Propagate<C>`] source, or if the [`Propagate<C>`] component
/// is added, changed or removed.
///
/// Optionally you can include a query filter `F` to restrict the entities that are updated.
/// Note that the filter is not rechecked dynamically: changes to the filter state will not be
/// picked up until the  [`Propagate`] component is touched, or the hierarchy is changed.
/// All members of the tree between source and target must match the filter for propagation
/// to reach a given target.
/// Individual entities can be skipped or terminate the propagation with the [`PropagateOver`]
/// and [`PropagateStop`] components.
///
/// The schedule can be configured via [`HierarchyPropagatePlugin::new`].
/// You should be sure to schedule your logic relative to this set: making changes
/// that modify component values before this logic, and reading the propagated
/// values after it.
pub struct HierarchyPropagatePlugin<
    C: Component + Clone + PartialEq,
    F: QueryFilter = (),
    R: Relationship = ChildOf,
> {
    schedule: Interned<dyn ScheduleLabel>,
    _marker: PhantomData<fn() -> (C, F, R)>,
}

impl<C: Component + Clone + PartialEq, F: QueryFilter, R: Relationship>
    HierarchyPropagatePlugin<C, F, R>
{
    /// Construct the plugin. The propagation systems will be placed in the specified schedule.
    pub fn new(schedule: impl ScheduleLabel) -> Self {
        Self {
            schedule: schedule.intern(),
            _marker: PhantomData,
        }
    }
}

/// Causes the inner component to be added to this entity and all direct and transient relationship
/// targets. A target with a [`Propagate<C>`] component of its own will override propagation from
/// that point in the tree.
#[derive(Component, Clone, PartialEq)]
#[cfg_attr(
    feature = "bevy_reflect",
    derive(Reflect),
    reflect(Component, Clone, PartialEq)
)]
pub struct Propagate<C: Component + Clone + PartialEq>(pub C);

/// Stops the output component being added to this entity.
/// Relationship targets will still inherit the component from this entity or its parents.
#[derive(Component, Clone)]
#[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Component))]
pub struct PropagateOver<C>(PhantomData<fn() -> C>);

/// Stops the propagation at this entity. Children will not inherit the component.
#[derive(Component, Clone)]
#[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Component))]
pub struct PropagateStop<C>(PhantomData<fn() -> C>);

/// The set in which propagation systems are added. You can schedule your logic relative to this set.
#[derive(SystemSet, Clone, PartialEq, PartialOrd, Ord)]
pub struct PropagateSet<C: Component + Clone + PartialEq> {
    _p: PhantomData<fn() -> C>,
}

/// Internal struct for managing propagation
#[derive(Component, Clone, PartialEq, Debug)]
#[cfg_attr(
    feature = "bevy_reflect",
    derive(Reflect),
    reflect(Component, Clone, PartialEq)
)]
pub struct Inherited<C: Component + Clone + PartialEq>(pub C);

impl<C> Default for PropagateOver<C> {
    fn default() -> Self {
        Self(Default::default())
    }
}

impl<C> Default for PropagateStop<C> {
    fn default() -> Self {
        Self(Default::default())
    }
}

impl<C: Component + Clone + PartialEq> core::fmt::Debug for PropagateSet<C> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("PropagateSet")
            .field("_p", &self._p)
            .finish()
    }
}

impl<C: Component + Clone + PartialEq> Eq for PropagateSet<C> {}

impl<C: Component + Clone + PartialEq> core::hash::Hash for PropagateSet<C> {
    fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
        self._p.hash(state);
    }
}

impl<C: Component + Clone + PartialEq> Default for PropagateSet<C> {
    fn default() -> Self {
        Self {
            _p: Default::default(),
        }
    }
}

impl<C: Component + Clone + PartialEq, F: QueryFilter + 'static, R: Relationship> Plugin
    for HierarchyPropagatePlugin<C, F, R>
{
    fn build(&self, app: &mut App) {
        app.add_systems(
            self.schedule,
            (
                update_source::<C, F, R>,
                update_removed_limit::<C, F, R>,
                propagate_inherited::<C, F, R>,
                propagate_output::<C, F>,
            )
                .chain()
                .in_set(PropagateSet::<C>::default()),
        );
        app.add_observer(on_r_inserted::<C, F, R>);
        app.add_observer(on_r_removed::<C, F, R>);
    }
}

/// add/remove `Inherited::<C>` for entities with a direct `Propagate::<C>`
pub fn update_source<C: Component + Clone + PartialEq, F: QueryFilter, R: Relationship>(
    mut commands: Commands,
    changed: Query<(Entity, &Propagate<C>), (Or<(Changed<Propagate<C>>, Without<Inherited<C>>)>,)>,
    mut removed: RemovedComponents<Propagate<C>>,
    relationship: Query<&R>,
    relations: Query<&Inherited<C>, Without<PropagateStop<C>>>,
    sources: Query<(), With<Propagate<C>>>,
) {
    for (entity, source) in &changed {
        commands
            .entity(entity)
            .try_insert(Inherited(source.0.clone()));
    }

    // set `Inherited::<C>` based on ancestry when `Propagate::<C>` is removed
    for removed in removed.read() {
        if !sources.contains(removed)
            && let Ok(mut commands) = commands.get_entity(removed)
        {
            if let Some(inherited) = relationship
                .get(removed)
                .ok()
                .and_then(|r| relations.get(r.get()).ok())
            {
                commands.insert(inherited.clone());
            } else {
                commands.try_remove::<Inherited<C>>();
            }
        }
    }
}

/// Add/remove [`Inherited::<C>`] when an entity gains or changes its `R` relationship
pub fn on_r_inserted<
    C: Component + Clone + PartialEq,
    F: QueryFilter + 'static,
    R: Relationship,
>(
    event: On<Insert, R>,
    mut commands: Commands,
    query: Query<(&R, Has<Inherited<C>>), (Without<Propagate<C>>, F)>,
    relations: Query<&Inherited<C>, Without<PropagateStop<C>>>,
) {
    let Ok((relation, has_inherited)) = query.get(event.entity) else {
        return;
    };
    if let Ok(inherited) = relations.get(relation.get()) {
        commands.entity(event.entity).try_insert(inherited.clone());
    } else if has_inherited {
        commands.entity(event.entity).try_remove::<Inherited<C>>();
    }
}

/// Remove [`Inherited::<C>`] when an entity loses its `R` relationship
pub fn on_r_removed<C: Component + Clone + PartialEq, F: QueryFilter + 'static, R: Relationship>(
    event: On<Remove, R>,
    mut commands: Commands,
    query: Query<(), (With<Inherited<C>>, Without<Propagate<C>>, F)>,
) {
    if query.contains(event.entity) {
        commands.entity(event.entity).try_remove::<Inherited<C>>();
    }
}

/// When `PropagateOver` or `PropagateStop` is removed, update the `Inherited::<C>` to trigger propagation
pub fn update_removed_limit<C: Component + Clone + PartialEq, F: QueryFilter, R: Relationship>(
    mut inherited: Query<&mut Inherited<C>>,
    mut removed_skip: RemovedComponents<PropagateOver<C>>,
    mut removed_stop: RemovedComponents<PropagateStop<C>>,
) {
    for entity in removed_skip.read() {
        if let Ok(mut inherited) = inherited.get_mut(entity) {
            inherited.set_changed();
        }
    }
    for entity in removed_stop.read() {
        if let Ok(mut inherited) = inherited.get_mut(entity) {
            inherited.set_changed();
        }
    }
}

/// add/remove `Inherited::<C>` for targets of entities with modified `Inherited::<C>`
pub fn propagate_inherited<C: Component + Clone + PartialEq, F: QueryFilter, R: Relationship>(
    mut commands: Commands,
    changed: Query<
        (&Inherited<C>, &R::RelationshipTarget),
        (Changed<Inherited<C>>, Without<PropagateStop<C>>, F),
    >,
    recurse: Query<
        (
            Option<&R::RelationshipTarget>,
            Option<&Inherited<C>>,
            Option<&PropagateStop<C>>,
        ),
        (Without<Propagate<C>>, F),
    >,
    mut removed: RemovedComponents<Inherited<C>>,
    mut to_process: Local<Vec<(Entity, Option<Inherited<C>>)>>,
) {
    // gather changed
    for (inherited, targets) in &changed {
        to_process.extend(
            targets
                .iter()
                .map(|target| (target, Some(inherited.clone()))),
        );
    }

    // and removed
    for entity in removed.read() {
        if let Ok((Some(targets), None, _)) = recurse.get(entity) {
            to_process.extend(targets.iter().map(|target| (target, None)));
        }
    }

    // propagate
    while let Some((entity, maybe_inherited)) = (*to_process).pop() {
        let Ok((maybe_targets, maybe_current, maybe_stop)) = recurse.get(entity) else {
            continue;
        };

        if maybe_current == maybe_inherited.as_ref() {
            continue;
        }

        // update children if required
        if maybe_stop.is_none()
            && let Some(targets) = maybe_targets
        {
            to_process.extend(
                targets
                    .iter()
                    .map(|target| (target, maybe_inherited.clone())),
            );
        }

        // update this node's `Inherited<C>`
        if let Some(inherited) = maybe_inherited {
            commands.entity(entity).try_insert(inherited);
        } else {
            commands.entity(entity).try_remove::<Inherited<C>>();
        }
    }
}

/// add/remove `C` on entities with `Inherited::<C>`
pub fn propagate_output<C: Component + Clone + PartialEq, F: QueryFilter>(
    mut commands: Commands,
    changed: Query<
        (Entity, &Inherited<C>, Option<&C>),
        (Changed<Inherited<C>>, Without<PropagateOver<C>>, F),
    >,
    mut inherited_removed: RemovedComponents<Inherited<C>>,
    without_propagation_components: Query<(), (Without<PropagateOver<C>>, Without<Inherited<C>>)>,
) {
    for (entity, inherited, maybe_current) in &changed {
        if maybe_current.is_some_and(|c| &inherited.0 == c) {
            continue;
        }

        commands.entity(entity).try_insert(inherited.0.clone());
    }

    for inherited_removed in inherited_removed.read() {
        // Skip removal if propagation components were re-added this update
        if without_propagation_components.contains(inherited_removed) {
            commands.entity(inherited_removed).try_remove::<C>();
        }
    }
}

#[cfg(test)]
mod tests {
    use bevy_ecs::schedule::Schedule;

    use crate::{App, Update};

    use super::*;

    #[derive(Component, Clone, PartialEq, Debug)]
    struct TestValue(u32);

    #[test]
    fn test_simple_propagate() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let intermediate = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(intermediate))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, intermediate, propagatee]),
            Ok([&TestValue(1), &TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_remove_propagate() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(1)));

        app.world_mut()
            .commands()
            .entity(propagator)
            .remove::<Propagate<TestValue>>();
        app.update();

        assert!(query.get(app.world(), propagator).is_err());
        assert!(query.get(app.world(), propagatee).is_err());
    }

    #[test]
    fn test_remove_and_reinsert_propagate() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let parent = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let child = app.world_mut().spawn_empty().insert(ChildOf(parent)).id();

        app.update();

        app.world_mut()
            .entity_mut(parent)
            .remove::<Propagate<TestValue>>()
            .insert(Propagate(TestValue(2)));

        app.update();

        assert_eq!(
            app.world_mut()
                .query::<&TestValue>()
                .get_many(app.world(), [parent, child]),
            Ok([&TestValue(2), &TestValue(2)])
        );
    }

    #[test]
    fn test_remove_orphan() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(1)));

        app.world_mut()
            .commands()
            .entity(propagatee)
            .remove::<ChildOf>();
        app.update();

        assert!(query.get(app.world(), propagatee).is_err());
    }

    #[test]
    fn test_reparented() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator_one = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let other_parent = app.world_mut().spawn_empty().id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator_one))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(1)));

        app.world_mut()
            .commands()
            .entity(propagatee)
            .insert(ChildOf(other_parent));

        app.update();

        assert!(query.get(app.world(), propagatee).is_err());
    }

    #[test]
    fn test_reparented_with_prior() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator_a = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagator_b = app.world_mut().spawn(Propagate(TestValue(2))).id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator_a))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(1)));

        app.world_mut()
            .commands()
            .entity(propagatee)
            .insert(ChildOf(propagator_b));
        app.update();

        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(2)));
    }

    #[test]
    fn test_detach_and_reattach_propagates_to_descendants() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let intermediate = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(intermediate))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [intermediate, propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );

        app.world_mut()
            .entity_mut(intermediate)
            .remove::<ChildOf>()
            .insert(ChildOf(propagator));
        app.update();

        assert_eq!(
            query.get_many(app.world(), [intermediate, propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_propagate_over() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_over = app
            .world_mut()
            .spawn(TestValue(2))
            .insert((PropagateOver::<TestValue>::default(), ChildOf(propagator)))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagate_over))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagate_over), Ok(&TestValue(2)));
        assert_eq!(query.get(app.world(), propagatee), Ok(&TestValue(1)));
    }

    #[test]
    fn test_remove_propagate_over() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_over = app
            .world_mut()
            .spawn(TestValue(2))
            .insert((PropagateOver::<TestValue>::default(), ChildOf(propagator)))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagate_over))
            .id();

        app.update();
        assert_eq!(
            app.world_mut()
                .query::<&Inherited<TestValue>>()
                .get(app.world(), propagate_over),
            Ok(&Inherited(TestValue(1)))
        );
        assert_eq!(
            app.world_mut()
                .query::<&Inherited<TestValue>>()
                .get(app.world(), propagatee),
            Ok(&Inherited(TestValue(1)))
        );
        assert_eq!(
            query.get_many(app.world(), [propagate_over, propagatee]),
            Ok([&TestValue(2), &TestValue(1)])
        );

        app.world_mut()
            .commands()
            .entity(propagate_over)
            .remove::<PropagateOver<TestValue>>();
        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagate_over, propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_propagate_over_parent_removed() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_over = app
            .world_mut()
            .spawn(TestValue(2))
            .insert((PropagateOver::<TestValue>::default(), ChildOf(propagator)))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, propagate_over]),
            Ok([&TestValue(1), &TestValue(2)])
        );

        app.world_mut()
            .commands()
            .entity(propagator)
            .remove::<Propagate<TestValue>>();
        app.update();

        assert!(query.get(app.world(), propagator).is_err(),);
        assert_eq!(query.get(app.world(), propagate_over), Ok(&TestValue(2)));
    }

    #[test]
    fn test_orphaned_propagate_over() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_over = app
            .world_mut()
            .spawn(TestValue(2))
            .insert((PropagateOver::<TestValue>::default(), ChildOf(propagator)))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagate_over))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagate_over, propagatee]),
            Ok([&TestValue(2), &TestValue(1)])
        );

        app.world_mut()
            .commands()
            .entity(propagate_over)
            .remove::<ChildOf>();
        app.update();

        assert_eq!(query.get(app.world(), propagate_over), Ok(&TestValue(2)));
        assert!(query.get(app.world(), propagatee).is_err());
    }

    #[test]
    fn test_propagate_stop() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_stop = app
            .world_mut()
            .spawn(PropagateStop::<TestValue>::default())
            .insert(ChildOf(propagator))
            .id();
        let no_propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagate_stop))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagate_stop), Ok(&TestValue(1)));
        assert!(query.get(app.world(), no_propagatee).is_err());
    }

    #[test]
    fn test_remove_propagate_stop() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagate_stop = app
            .world_mut()
            .spawn(PropagateStop::<TestValue>::default())
            .insert(ChildOf(propagator))
            .id();
        let no_propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagate_stop))
            .id();

        app.update();

        assert_eq!(query.get(app.world(), propagate_stop), Ok(&TestValue(1)));
        assert!(query.get(app.world(), no_propagatee).is_err());

        app.world_mut()
            .commands()
            .entity(propagate_stop)
            .remove::<PropagateStop<TestValue>>();
        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagate_stop, no_propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_intermediate_override() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let intermediate = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(intermediate))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, intermediate, propagatee]),
            Ok([&TestValue(1), &TestValue(1), &TestValue(1)])
        );

        app.world_mut()
            .entity_mut(intermediate)
            .insert(Propagate(TestValue(2)));
        app.update();

        assert_eq!(
            app.world_mut()
                .query::<&TestValue>()
                .get_many(app.world(), [propagator, intermediate, propagatee]),
            Ok([&TestValue(1), &TestValue(2), &TestValue(2)])
        );
    }

    #[test]
    fn test_filter() {
        #[derive(Component)]
        struct Marker;

        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue, With<Marker>>::new(
            Update,
        ));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagatee = app
            .world_mut()
            .spawn_empty()
            .insert(ChildOf(propagator))
            .id();

        app.update();

        assert!(query.get(app.world(), propagator).is_err());
        assert!(query.get(app.world(), propagatee).is_err());

        // NOTE: changes to the filter condition are not rechecked
        app.world_mut().entity_mut(propagator).insert(Marker);
        app.update();

        assert!(query.get(app.world(), propagator).is_err());
        assert!(query.get(app.world(), propagatee).is_err());

        app.world_mut()
            .entity_mut(propagator)
            .insert(Propagate(TestValue(1)));
        app.update();

        assert_eq!(query.get(app.world(), propagator), Ok(&TestValue(1)));
        assert!(query.get(app.world(), propagatee).is_err());

        app.world_mut().entity_mut(propagatee).insert(Marker);
        app.update();

        assert_eq!(query.get(app.world(), propagator), Ok(&TestValue(1)));
        assert!(query.get(app.world(), propagatee).is_err());

        app.world_mut()
            .entity_mut(propagator)
            .insert(Propagate(TestValue(1)));
        app.update();

        assert_eq!(
            app.world_mut()
                .query::<&TestValue>()
                .get_many(app.world(), [propagator, propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_removed_propagate_still_inherits() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app.world_mut().spawn(Propagate(TestValue(1))).id();
        let propagatee = app
            .world_mut()
            .spawn(Propagate(TestValue(2)))
            .insert(ChildOf(propagator))
            .id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, propagatee]),
            Ok([&TestValue(1), &TestValue(2)])
        );

        app.world_mut()
            .commands()
            .entity(propagatee)
            .remove::<Propagate<TestValue>>();
        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, propagatee]),
            Ok([&TestValue(1), &TestValue(1)])
        );
    }

    #[test]
    fn test_reparent_respects_stop() {
        let mut app = App::new();
        app.add_schedule(Schedule::new(Update));
        app.add_plugins(HierarchyPropagatePlugin::<TestValue>::new(Update));

        let mut query = app.world_mut().query::<&TestValue>();

        let propagator = app
            .world_mut()
            .spawn((
                Propagate(TestValue(1)),
                PropagateStop::<TestValue>::default(),
            ))
            .id();
        let propagatee = app.world_mut().spawn(TestValue(2)).id();

        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, propagatee]),
            Ok([&TestValue(1), &TestValue(2)])
        );

        app.world_mut()
            .commands()
            .entity(propagatee)
            .insert(ChildOf(propagator));
        app.update();

        assert_eq!(
            query.get_many(app.world(), [propagator, propagatee]),
            Ok([&TestValue(1), &TestValue(2)])
        );
    }
}