child_builder.rs

use crate::{Children, HierarchyEvent, Parent};
use bevy_ecs::{
    bundle::Bundle,
    entity::Entity,
    event::Events,
    system::{Command, Commands, EntityCommands},
    world::{EntityMut, World},
};
use smallvec::SmallVec;

fn push_events(world: &mut World, events: SmallVec<[HierarchyEvent; 8]>) {
    if let Some(mut moved) = world.get_resource_mut::<Events<HierarchyEvent>>() {
        for evt in events {
            moved.send(evt);
        }
    }
}

fn push_child_unchecked(world: &mut World, parent: Entity, child: Entity) {
    let mut parent = world.entity_mut(parent);
    if let Some(mut children) = parent.get_mut::<Children>() {
        children.0.push(child);
    } else {
        parent.insert(Children(smallvec::smallvec![child]));
    }
}

fn update_parent(world: &mut World, child: Entity, new_parent: Entity) -> Option<Entity> {
    let mut child = world.entity_mut(child);
    if let Some(mut parent) = child.get_mut::<Parent>() {
        let previous = parent.0;
        *parent = Parent(new_parent);
        Some(previous)
    } else {
        child.insert(Parent(new_parent));
        None
    }
}

fn remove_from_children(world: &mut World, parent: Entity, child: Entity) {
    let mut parent = world.entity_mut(parent);
    if let Some(mut children) = parent.get_mut::<Children>() {
        children.0.retain(|x| *x != child);
        if children.is_empty() {
            parent.remove::<Children>();
        }
    }
}

fn update_old_parents(world: &mut World, parent: Entity, children: &[Entity]) {
    let mut moved: SmallVec<[HierarchyEvent; 8]> = SmallVec::with_capacity(children.len());
    for child in children {
        if let Some(previous) = update_parent(world, *child, parent) {
            debug_assert!(parent != previous);
            remove_from_children(world, previous, *child);
            moved.push(HierarchyEvent::ChildMoved {
                child: *child,
                previous_parent: previous,
                new_parent: parent,
            });
        }
    }
    push_events(world, moved);
}

fn remove_children(parent: Entity, children: &[Entity], world: &mut World) {
    let mut events: SmallVec<[HierarchyEvent; 8]> = SmallVec::new();
    if let Some(parent_children) = world.get::<Children>(parent) {
        for &child in children {
            if parent_children.contains(&child) {
                events.push(HierarchyEvent::ChildRemoved { child, parent });
            }
        }
    } else {
        return;
    }
    for event in &events {
        if let &HierarchyEvent::ChildRemoved { child, .. } = event {
            world.entity_mut(child).remove::<Parent>();
        }
    }
    push_events(world, events);

    let mut parent = world.entity_mut(parent);
    if let Some(mut parent_children) = parent.get_mut::<Children>() {
        parent_children
            .0
            .retain(|parent_child| !children.contains(parent_child));

        if parent_children.is_empty() {
            parent.remove::<Children>();
        }
    }
}

/// Command that adds a child to an entity
#[derive(Debug)]
pub struct AddChild {
    /// Parent entity to add the child to
    pub parent: Entity,
    /// Child entity to add
    pub child: Entity,
}

impl Command for AddChild {
    fn write(self, world: &mut World) {
        let previous = update_parent(world, self.child, self.parent);
        if let Some(previous) = previous {
            if previous == self.parent {
                return;
            }
            remove_from_children(world, previous, self.child);
            if let Some(mut events) = world.get_resource_mut::<Events<HierarchyEvent>>() {
                events.send(HierarchyEvent::ChildMoved {
                    child: self.child,
                    previous_parent: previous,
                    new_parent: self.parent,
                });
            }
        } else if let Some(mut events) = world.get_resource_mut::<Events<HierarchyEvent>>() {
            events.send(HierarchyEvent::ChildAdded {
                child: self.child,
                parent: self.parent,
            });
        }
        let mut parent = world.entity_mut(self.parent);
        if let Some(mut children) = parent.get_mut::<Children>() {
            if !children.contains(&self.child) {
                children.0.push(self.child);
            }
        } else {
            parent.insert(Children(smallvec::smallvec![self.child]));
        }
    }
}

/// Command that inserts a child at a given index of a parent's children, shifting following children back
#[derive(Debug)]
pub struct InsertChildren {
    parent: Entity,
    children: SmallVec<[Entity; 8]>,
    index: usize,
}

impl Command for InsertChildren {
    fn write(self, world: &mut World) {
        update_old_parents(world, self.parent, &self.children);
        let mut parent = world.entity_mut(self.parent);
        if let Some(mut children) = parent.get_mut::<Children>() {
            children.0.retain(|value| !self.children.contains(value));
            children.0.insert_from_slice(self.index, &self.children);
        } else {
            parent.insert(Children(self.children));
        }
    }
}

/// Command that pushes children to the end of the entity's [`Children`].
#[derive(Debug)]
pub struct PushChildren {
    parent: Entity,
    children: SmallVec<[Entity; 8]>,
}

impl Command for PushChildren {
    fn write(mut self, world: &mut World) {
        update_old_parents(world, self.parent, &self.children);
        let mut parent = world.entity_mut(self.parent);
        if let Some(mut children) = parent.get_mut::<Children>() {
            children.0.retain(|child| !self.children.contains(child));
            children.0.append(&mut self.children);
        } else {
            parent.insert(Children(self.children));
        }
    }
}

/// Command that removes children from an entity, and removes that child's parent.
pub struct RemoveChildren {
    parent: Entity,
    children: SmallVec<[Entity; 8]>,
}

impl Command for RemoveChildren {
    fn write(self, world: &mut World) {
        remove_children(self.parent, &self.children, world);
    }
}

/// Command that removes the parent of an entity, and removes that entity from the parent's [`Children`].
pub struct RemoveParent {
    child: Entity,
}

impl Command for RemoveParent {
    fn write(self, world: &mut World) {
        if let Some(parent) = world.get::<Parent>(self.child) {
            let parent_entity = parent.get();
            remove_from_children(world, parent_entity, self.child);
            world.entity_mut(self.child).remove::<Parent>();
            if let Some(mut events) = world.get_resource_mut::<Events<_>>() {
                events.send(HierarchyEvent::ChildRemoved {
                    child: self.child,
                    parent: parent_entity,
                });
            }
        }
    }
}

/// Struct for building children onto an entity
pub struct ChildBuilder<'w, 's, 'a> {
    commands: &'a mut Commands<'w, 's>,
    push_children: PushChildren,
}

impl<'w, 's, 'a> ChildBuilder<'w, 's, 'a> {
    /// Spawns an entity with the given bundle and inserts it into the children defined by the [`ChildBuilder`]
    #[deprecated(
        since = "0.9.0",
        note = "Use `spawn` instead, which now accepts bundles, components, and tuples of bundles and components."
    )]
    pub fn spawn_bundle(&mut self, bundle: impl Bundle) -> EntityCommands<'w, 's, '_> {
        self.spawn(bundle)
    }

    /// Spawns an entity with the given bundle and inserts it into the children defined by the [`ChildBuilder`]
    pub fn spawn(&mut self, bundle: impl Bundle) -> EntityCommands<'w, 's, '_> {
        let e = self.commands.spawn(bundle);
        self.push_children.children.push(e.id());
        e
    }

    /// Spawns an [`Entity`] with no components and inserts it into the children defined by the [`ChildBuilder`] which adds the [`Parent`] component to it.
    pub fn spawn_empty(&mut self) -> EntityCommands<'w, 's, '_> {
        let e = self.commands.spawn_empty();
        self.push_children.children.push(e.id());
        e
    }

    /// Returns the parent entity of this [`ChildBuilder`]
    pub fn parent_entity(&self) -> Entity {
        self.push_children.parent
    }

    /// Adds a command to this [`ChildBuilder`]
    pub fn add_command<C: Command + 'static>(&mut self, command: C) -> &mut Self {
        self.commands.add(command);
        self
    }
}

/// Trait defining how to build children
pub trait BuildChildren {
    /// Creates a [`ChildBuilder`] with the given children built in the given closure
    ///
    /// Compared to [`add_children`][BuildChildren::add_children], this method returns self
    /// to allow chaining.
    fn with_children(&mut self, f: impl FnOnce(&mut ChildBuilder)) -> &mut Self;
    /// Creates a [`ChildBuilder`] with the given children built in the given closure
    ///
    /// Compared to [`with_children`][BuildChildren::with_children], this method returns the
    /// the value returned from the closure, but doesn't allow chaining.
    ///
    /// ## Example
    ///
    /// ```no_run
    /// # use bevy_ecs::prelude::*;
    /// # use bevy_hierarchy::*;
    /// #
    /// # #[derive(Component)]
    /// # struct SomethingElse;
    /// #
    /// # #[derive(Component)]
    /// # struct MoreStuff;
    /// #
    /// # fn foo(mut commands: Commands) {
    ///     let mut parent_commands = commands.spawn_empty();
    ///     let child_id = parent_commands.add_children(|parent| {
    ///         parent.spawn_empty().id()
    ///     });
    ///
    ///     parent_commands.insert(SomethingElse);
    ///     commands.entity(child_id).with_children(|parent| {
    ///         parent.spawn_bundle(MoreStuff);
    ///     });
    /// # }
    /// ```
    fn add_children<T>(&mut self, f: impl FnOnce(&mut ChildBuilder) -> T) -> T;
    /// Pushes children to the back of the builder's children
    ///
    /// If the children were previously children of another parent, that parent's [`Children`] component
    /// will have those children removed from its list. Removing all children from a parent causes its
    /// [`Children`] component to be removed from the entity.
    fn push_children(&mut self, children: &[Entity]) -> &mut Self;
    /// Inserts children at the given index
    ///
    /// If the children were previously children of another parent, that parent's [`Children`] component
    /// will have those children removed from its list. Removing all children from a parent causes its
    /// [`Children`] component to be removed from the entity.
    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self;
    /// Removes the given children
    ///
    /// Removing all children from a parent causes its [`Children`] component to be removed from the entity.
    fn remove_children(&mut self, children: &[Entity]) -> &mut Self;
    /// Adds a single child
    ///
    /// If the children were previously children of another parent, that parent's [`Children`] component
    /// will have those children removed from its list. Removing all children from a parent causes its
    /// [`Children`] component to be removed from the entity.
    fn add_child(&mut self, child: Entity) -> &mut Self;
    /// Sets the parent of this entity.
    fn set_parent(&mut self, parent: Entity) -> &mut Self;
    /// Removes the parent of this entity.
    fn remove_parent(&mut self) -> &mut Self;
}

impl<'w, 's, 'a> BuildChildren for EntityCommands<'w, 's, 'a> {
    fn with_children(&mut self, spawn_children: impl FnOnce(&mut ChildBuilder)) -> &mut Self {
        self.add_children(spawn_children);
        self
    }

    fn add_children<T>(&mut self, spawn_children: impl FnOnce(&mut ChildBuilder) -> T) -> T {
        let parent = self.id();
        let mut builder = ChildBuilder {
            commands: self.commands(),
            push_children: PushChildren {
                children: SmallVec::default(),
                parent,
            },
        };

        let result = spawn_children(&mut builder);
        let children = builder.push_children;
        self.commands().add(children);

        result
    }

    fn push_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        self.commands().add(PushChildren {
            children: SmallVec::from(children),
            parent,
        });
        self
    }

    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        self.commands().add(InsertChildren {
            children: SmallVec::from(children),
            index,
            parent,
        });
        self
    }

    fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        self.commands().add(RemoveChildren {
            children: SmallVec::from(children),
            parent,
        });
        self
    }

    fn add_child(&mut self, child: Entity) -> &mut Self {
        let parent = self.id();
        self.commands().add(AddChild { child, parent });
        self
    }

    fn set_parent(&mut self, parent: Entity) -> &mut Self {
        let child = self.id();
        self.commands().add(AddChild { child, parent });
        self
    }

    fn remove_parent(&mut self) -> &mut Self {
        let child = self.id();
        self.commands().add(RemoveParent { child });
        self
    }
}

/// Struct for adding children to an entity directly through the [`World`] for use in exclusive systems
#[derive(Debug)]
pub struct WorldChildBuilder<'w> {
    world: &'w mut World,
    current_entity: Option<Entity>,
    parent_entities: Vec<Entity>,
}

impl<'w> WorldChildBuilder<'w> {
    /// Spawns an entity with the given bundle and inserts it into the children defined by the [`WorldChildBuilder`]
    pub fn spawn(&mut self, bundle: impl Bundle + Send + Sync + 'static) -> EntityMut<'_> {
        let parent_entity = self.parent_entity();
        let entity = self.world.spawn((bundle, Parent(parent_entity))).id();
        push_child_unchecked(self.world, parent_entity, entity);
        self.current_entity = Some(entity);
        if let Some(mut added) = self.world.get_resource_mut::<Events<HierarchyEvent>>() {
            added.send(HierarchyEvent::ChildAdded {
                child: entity,
                parent: parent_entity,
            });
        }
        self.world.entity_mut(entity)
    }

    #[deprecated(
        since = "0.9.0",
        note = "Use `spawn` instead, which now accepts bundles, components, and tuples of bundles and components."
    )]
    /// Spawns an entity with the given bundle and inserts it into the children defined by the [`WorldChildBuilder`]
    pub fn spawn_bundle(&mut self, bundle: impl Bundle + Send + Sync + 'static) -> EntityMut<'_> {
        self.spawn(bundle)
    }

    /// Spawns an [`Entity`] with no components and inserts it into the children defined by the [`WorldChildBuilder`] which adds the [`Parent`] component to it.
    pub fn spawn_empty(&mut self) -> EntityMut<'_> {
        let parent_entity = self.parent_entity();
        let entity = self.world.spawn(Parent(parent_entity)).id();
        push_child_unchecked(self.world, parent_entity, entity);
        self.current_entity = Some(entity);
        if let Some(mut added) = self.world.get_resource_mut::<Events<HierarchyEvent>>() {
            added.send(HierarchyEvent::ChildAdded {
                child: entity,
                parent: parent_entity,
            });
        }
        self.world.entity_mut(entity)
    }

    /// Returns the parent entity of this [`WorldChildBuilder`]
    pub fn parent_entity(&self) -> Entity {
        self.parent_entities
            .last()
            .cloned()
            .expect("There should always be a parent at this point.")
    }
}

/// Trait that defines adding children to an entity directly through the [`World`]
pub trait BuildWorldChildren {
    /// Creates a [`WorldChildBuilder`] with the given children built in the given closure
    fn with_children(&mut self, spawn_children: impl FnOnce(&mut WorldChildBuilder)) -> &mut Self;
    /// Pushes children to the back of the builder's children
    fn push_children(&mut self, children: &[Entity]) -> &mut Self;
    /// Inserts children at the given index
    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self;
    /// Removes the given children
    fn remove_children(&mut self, children: &[Entity]) -> &mut Self;
}

impl<'w> BuildWorldChildren for EntityMut<'w> {
    fn with_children(&mut self, spawn_children: impl FnOnce(&mut WorldChildBuilder)) -> &mut Self {
        {
            let entity = self.id();
            let mut builder = WorldChildBuilder {
                current_entity: None,
                parent_entities: vec![entity],
                // SAFETY: self.update_location() is called below. It is impossible to make EntityMut
                // function calls on `self` within the scope defined here
                world: unsafe { self.world_mut() },
            };

            spawn_children(&mut builder);
        }
        self.update_location();
        self
    }

    fn push_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        {
            // SAFETY: parent entity is not modified and its location is updated manually
            let world = unsafe { self.world_mut() };
            update_old_parents(world, parent, children);
            // Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
            self.update_location();
        }
        if let Some(mut children_component) = self.get_mut::<Children>() {
            children_component
                .0
                .retain(|value| !children.contains(value));
            children_component.0.extend(children.iter().cloned());
        } else {
            self.insert(Children::from_entities(children));
        }
        self
    }

    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        {
            // SAFETY: parent entity is not modified and its location is updated manually
            let world = unsafe { self.world_mut() };
            update_old_parents(world, parent, children);
            // Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
            self.update_location();
        }

        if let Some(mut children_component) = self.get_mut::<Children>() {
            children_component
                .0
                .retain(|value| !children.contains(value));
            children_component.0.insert_from_slice(index, children);
        } else {
            self.insert(Children::from_entities(children));
        }
        self
    }

    fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self.id();
        // SAFETY: This doesn't change the parent's location
        let world = unsafe { self.world_mut() };
        remove_children(parent, children, world);
        self
    }
}

impl<'w> BuildWorldChildren for WorldChildBuilder<'w> {
    fn with_children(
        &mut self,
        spawn_children: impl FnOnce(&mut WorldChildBuilder<'w>),
    ) -> &mut Self {
        let current_entity = self
            .current_entity
            .expect("Cannot add children without a parent. Try creating an entity first.");
        self.parent_entities.push(current_entity);
        self.current_entity = None;

        spawn_children(self);

        self.current_entity = self.parent_entities.pop();
        self
    }

    fn push_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self
            .current_entity
            .expect("Cannot add children without a parent. Try creating an entity first.");
        update_old_parents(self.world, parent, children);
        if let Some(mut children_component) = self.world.get_mut::<Children>(parent) {
            children_component
                .0
                .retain(|value| !children.contains(value));
            children_component.0.extend(children.iter().cloned());
        } else {
            self.world
                .entity_mut(parent)
                .insert(Children::from_entities(children));
        }
        self
    }

    fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
        let parent = self
            .current_entity
            .expect("Cannot add children without a parent. Try creating an entity first.");
        update_old_parents(self.world, parent, children);
        if let Some(mut children_component) = self.world.get_mut::<Children>(parent) {
            children_component
                .0
                .retain(|value| !children.contains(value));
            children_component.0.insert_from_slice(index, children);
        } else {
            self.world
                .entity_mut(parent)
                .insert(Children::from_entities(children));
        }
        self
    }

    fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
        let parent = self
            .current_entity
            .expect("Cannot remove children without a parent. Try creating an entity first.");

        remove_children(parent, children, self.world);
        self
    }
}

#[cfg(test)]
mod tests {
    use super::{BuildChildren, BuildWorldChildren};
    use crate::prelude::{Children, Parent};
    use smallvec::{smallvec, SmallVec};

    use bevy_ecs::{
        component::Component,
        entity::Entity,
        system::{CommandQueue, Commands},
        world::World,
    };

    #[derive(Component)]
    struct C(u32);

    #[test]
    fn build_children() {
        let mut world = World::default();
        let mut queue = CommandQueue::default();
        let mut commands = Commands::new(&mut queue, &world);

        let parent = commands.spawn(C(1)).id();
        let children = commands.entity(parent).add_children(|parent| {
            [
                parent.spawn(C(2)).id(),
                parent.spawn(C(3)).id(),
                parent.spawn(C(4)).id(),
            ]
        });

        queue.apply(&mut world);
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.as_slice(),
            children.as_slice(),
        );
        assert_eq!(*world.get::<Parent>(children[0]).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(children[1]).unwrap(), Parent(parent));

        assert_eq!(*world.get::<Parent>(children[0]).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(children[1]).unwrap(), Parent(parent));
    }

    #[test]
    fn push_and_insert_and_remove_children_commands() {
        let mut world = World::default();
        let entities = world
            .spawn_batch(vec![(C(1),), (C(2),), (C(3),), (C(4),), (C(5),)])
            .collect::<Vec<Entity>>();

        let mut queue = CommandQueue::default();
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(entities[0]).push_children(&entities[1..3]);
        }
        queue.apply(&mut world);

        let parent = entities[0];
        let child1 = entities[1];
        let child2 = entities[2];
        let child3 = entities[3];
        let child4 = entities[4];

        let expected_children: SmallVec<[Entity; 8]> = smallvec![child1, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert_eq!(*world.get::<Parent>(child1).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(child2).unwrap(), Parent(parent));

        assert_eq!(*world.get::<Parent>(child1).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(child2).unwrap(), Parent(parent));

        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent).insert_children(1, &entities[3..]);
        }
        queue.apply(&mut world);

        let expected_children: SmallVec<[Entity; 8]> = smallvec![child1, child3, child4, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert_eq!(*world.get::<Parent>(child3).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(child4).unwrap(), Parent(parent));

        let remove_children = [child1, child4];
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent).remove_children(&remove_children);
        }
        queue.apply(&mut world);

        let expected_children: SmallVec<[Entity; 8]> = smallvec![child3, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert!(world.get::<Parent>(child1).is_none());
        assert!(world.get::<Parent>(child4).is_none());
    }

    #[test]
    fn push_and_insert_and_remove_children_world() {
        let mut world = World::default();
        let entities = world
            .spawn_batch(vec![(C(1),), (C(2),), (C(3),), (C(4),), (C(5),)])
            .collect::<Vec<Entity>>();

        world.entity_mut(entities[0]).push_children(&entities[1..3]);

        let parent = entities[0];
        let child1 = entities[1];
        let child2 = entities[2];
        let child3 = entities[3];
        let child4 = entities[4];

        let expected_children: SmallVec<[Entity; 8]> = smallvec![child1, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert_eq!(*world.get::<Parent>(child1).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(child2).unwrap(), Parent(parent));

        world.entity_mut(parent).insert_children(1, &entities[3..]);
        let expected_children: SmallVec<[Entity; 8]> = smallvec![child1, child3, child4, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert_eq!(*world.get::<Parent>(child3).unwrap(), Parent(parent));
        assert_eq!(*world.get::<Parent>(child4).unwrap(), Parent(parent));

        let remove_children = [child1, child4];
        world.entity_mut(parent).remove_children(&remove_children);
        let expected_children: SmallVec<[Entity; 8]> = smallvec![child3, child2];
        assert_eq!(
            world.get::<Children>(parent).unwrap().0.clone(),
            expected_children
        );
        assert!(world.get::<Parent>(child1).is_none());
        assert!(world.get::<Parent>(child4).is_none());
    }

    /// Tests what happens when all children are removed from a parent using world functions
    #[test]
    fn children_removed_when_empty_world() {
        let mut world = World::default();
        let entities = world
            .spawn_batch(vec![(C(1),), (C(2),), (C(3),)])
            .collect::<Vec<Entity>>();

        let parent1 = entities[0];
        let parent2 = entities[1];
        let child = entities[2];

        // push child into parent1
        world.entity_mut(parent1).push_children(&[child]);
        assert_eq!(
            world.get::<Children>(parent1).unwrap().0.as_slice(),
            &[child]
        );

        // move only child from parent1 with `push_children`
        world.entity_mut(parent2).push_children(&[child]);
        assert!(world.get::<Children>(parent1).is_none());

        // move only child from parent2 with `insert_children`
        world.entity_mut(parent1).insert_children(0, &[child]);
        assert!(world.get::<Children>(parent2).is_none());

        // remove only child from parent1 with `remove_children`
        world.entity_mut(parent1).remove_children(&[child]);
        assert!(world.get::<Children>(parent1).is_none());
    }

    /// Tests what happens when all children are removed form a parent using commands
    #[test]
    fn children_removed_when_empty_commands() {
        let mut world = World::default();
        let entities = world
            .spawn_batch(vec![(C(1),), (C(2),), (C(3),)])
            .collect::<Vec<Entity>>();

        let parent1 = entities[0];
        let parent2 = entities[1];
        let child = entities[2];

        let mut queue = CommandQueue::default();

        // push child into parent1
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent1).push_children(&[child]);
            queue.apply(&mut world);
        }
        assert_eq!(
            world.get::<Children>(parent1).unwrap().0.as_slice(),
            &[child]
        );

        // move only child from parent1 with `push_children`
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent2).push_children(&[child]);
            queue.apply(&mut world);
        }
        assert!(world.get::<Children>(parent1).is_none());

        // move only child from parent2 with `insert_children`
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent1).insert_children(0, &[child]);
            queue.apply(&mut world);
        }
        assert!(world.get::<Children>(parent2).is_none());

        // move only child from parent1 with `add_child`
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent2).add_child(child);
            queue.apply(&mut world);
        }
        assert!(world.get::<Children>(parent1).is_none());

        // remove only child from parent2 with `remove_children`
        {
            let mut commands = Commands::new(&mut queue, &world);
            commands.entity(parent2).remove_children(&[child]);
            queue.apply(&mut world);
        }
        assert!(world.get::<Children>(parent2).is_none());
    }

    #[test]
    fn regression_push_children_same_archetype() {
        let mut world = World::new();
        let child = world.spawn_empty().id();
        world.spawn_empty().push_children(&[child]);
    }
}