update component cloning documentation to mention moving components

crates/bevy_ecs/src/component.rs

@@ -1035,16 +1035,16 @@ impl ComponentDescriptor {
     }
 }
 
-/// Function type that can be used to clone an entity.
+/// Function type that can be used to clone a component of an entity.
 pub type ComponentCloneFn = fn(&SourceComponent, &mut ComponentCloneCtx);
 
-/// The clone behavior to use when cloning a [`Component`].
+/// The clone behavior to use when cloning or moving a [`Component`].
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub enum ComponentCloneBehavior {
     /// Uses the default behavior (which is passed to [`ComponentCloneBehavior::resolve`])
     #[default]
     Default,
-    /// Do not clone this component.
+    /// Do not clone/move this component.
     Ignore,
     /// Uses a custom [`ComponentCloneFn`].
     Custom(ComponentCloneFn),

crates/bevy_ecs/src/entity/clone_entities.rs

@@ -349,6 +349,19 @@ impl<'a, 'b> ComponentCloneCtx<'a, 'b> {
 /// 2. component-defined handler using [`Component::clone_behavior`]
 /// 3. default handler override using [`EntityClonerBuilder::with_default_clone_fn`].
 /// 4. reflect-based or noop default clone handler depending on if `bevy_reflect` feature is enabled or not.
+///
+/// # Moving components
+/// [`EntityCloner`] can be configured to move components instead of cloning them by using [`EntityClonerBuilder::move_components`].
+/// In this mode components will be moved - removed from source entity and added to the target entity.
+///
+/// Components with [`ComponentCloneBehavior::Ignore`] clone behavior will not be moved, while components that
+/// have a [`ComponentCloneBehavior::Custom`] clone behavior will be cloned using it and then removed from the source entity.
+/// All other components will be bitwise copied from the source entity onto the target entity and then removed without dropping.
+///
+/// Choosing to move components instead of cloning makes [`EntityClonerBuilder::with_default_clone_fn`] ineffective since it's replaced by
+/// move handler for components that have [`ComponentCloneBehavior::Default`] clone behavior.
+///
+/// Note that moving components still triggers `on_remove` hooks/observers on source entity and `on_insert`/`on_add` hooks/observers on the target entity.
 #[derive(Default)]
 pub struct EntityCloner {
     filter: EntityClonerFilter,
@@ -400,6 +413,7 @@ impl<'a> BundleScratch<'a> {
     ///
     /// # Safety
     /// All [`ComponentId`] values in this instance must come from `world`.
+    #[track_caller]
     pub(crate) unsafe fn write(
         self,
         world: &mut World,
@@ -709,7 +723,7 @@ impl EntityCloner {
                         }
                     }
 
-                    (/* don't drop */ false, ())
+                    (/* should drop? */ false, ())
                 },
             );
         }
@@ -776,6 +790,8 @@ impl<'w, Filter: CloneByFilter> EntityClonerBuilder<'w, Filter> {
     }
 
     /// Sets the default clone function to use.
+    ///
+    /// Will be overridden if [`EntityClonerBuilder::move_components`] is enabled.
     pub fn with_default_clone_fn(&mut self, clone_fn: ComponentCloneFn) -> &mut Self {
         self.state.default_clone_fn = clone_fn;
         self
@@ -788,6 +804,8 @@ impl<'w, Filter: CloneByFilter> EntityClonerBuilder<'w, Filter> {
     ///
     /// The setting only applies to components that are allowed through the filter
     /// at the time [`EntityClonerBuilder::clone_entity`] is called.
+    ///
+    /// Enabling this overrides any custom function set with [`EntityClonerBuilder::with_default_clone_fn`].
     pub fn move_components(&mut self, enable: bool) -> &mut Self {
         self.state.move_components = enable;
         self

crates/bevy_ecs/src/system/commands/entity_command.rs

@@ -310,8 +310,20 @@ pub fn clone_components<B: Bundle>(target: Entity) -> impl EntityCommand {
     }
 }
 
-/// An [`EntityCommand`] that clones the specified components of an entity
-/// and inserts them into another entity, then removes them from the original entity.
+/// An [`EntityCommand`] moves the specified components of this entity into another entity.
+///
+/// Components with [`Ignore`] clone behavior will not be moved, while components that
+/// have a [`Custom`] clone behavior will be cloned using it and then removed from the source entity.
+/// All other components will be moved without any other special handling.
+///
+/// Note that this will trigger `on_remove` hooks/observers on this entity and `on_insert`/`on_add` hooks/observers on the target entity.
+///
+/// # Panics
+///
+/// The command will panic when applied if the target entity does not exist.
+///
+/// [`Ignore`]: crate::component::ComponentCloneBehavior::Ignore
+/// [`Custom`]: crate::component::ComponentCloneBehavior::Custom
 pub fn move_components<B: Bundle>(target: Entity) -> impl EntityCommand {
     move |mut entity: EntityWorldMut| {
         entity.move_components::<B>(target);

crates/bevy_ecs/src/system/commands/mod.rs

@@ -2234,15 +2234,20 @@ impl<'a> EntityCommands<'a> {
         self.queue(entity_command::clone_components::<B>(target))
     }
 
-    /// Clones the specified components of this entity and inserts them into another entity,
-    /// then removes the components from this entity.
+    /// Moves the specified components of this entity into another entity.
     ///
-    /// Components can only be cloned if they implement
-    /// [`Clone`] or [`Reflect`](bevy_reflect::Reflect).
+    /// Components with [`Ignore`] clone behavior will not be moved, while components that
+    /// have a [`Custom`] clone behavior will be cloned using it and then removed from the source entity.
+    /// All other components will be moved without any other special handling.
+    ///
+    /// Note that this will trigger `on_remove` hooks/observers on this entity and `on_insert`/`on_add` hooks/observers on the target entity.
     ///
     /// # Panics
     ///
     /// The command will panic when applied if the target entity does not exist.
+    ///
+    /// [`Ignore`]: crate::component::ComponentCloneBehavior::Ignore
+    /// [`Custom`]: crate::component::ComponentCloneBehavior::Custom
     pub fn move_components<B: Bundle>(&mut self, target: Entity) -> &mut Self {
         self.queue(entity_command::move_components::<B>(target))
     }