Struct bevy_ecs::query::Changed

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pub struct Changed<T>(/* private fields */);
Expand description

A filter on a component that only retains results added or mutably dereferenced after the system last ran.

A common use for this filter is avoiding redundant work when values have not changed.

Note that simply mutably dereferencing a component is considered a change (DerefMut). Bevy does not compare components to their previous values.

To retain all results without filtering but still check whether they were changed after the system last ran, use Ref<T>.

Deferred

Note, that entity modifications issued with Commands (like entity creation or entity component addition or removal) are visible only after deferred operations are applied, typically at the end of the schedule iteration.

Time complexity

Changed is not ArchetypeFilter, which practically means that if query (with T component filter) matches million entities, Changed<T> filter will iterate over all of them even if none of them were changed.

For example, these two systems are roughly equivalent in terms of performance:


fn system1(q: Query<&MyComponent, Changed<Transform>>) {
    for item in &q { /* component changed */ }
}

fn system2(q: Query<(&MyComponent, Ref<Transform>)>) {
    for item in &q {
        if item.1.is_changed() { /* component changed */ }
    }
}

Examples


fn print_moving_objects_system(query: Query<&Name, Changed<Transform>>) {
    for name in &query {
        println!("Entity Moved: {:?}", name);
    }
}

Trait Implementations§

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impl<T: Component> QueryFilter for Changed<T>

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const IS_ARCHETYPAL: bool = false

Returns true if (and only if) this Filter relies strictly on archetypes to limit which components are accessed by the Query. Read more
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unsafe fn filter_fetch( fetch: &mut Self::Fetch<'_>, entity: Entity, table_row: TableRow ) -> bool

Safety Read more
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impl<T: Component> WorldQuery for Changed<T>

SAFETY: fetch accesses a single component in a readonly way. This is sound because update_component_access and update_archetype_component_access add read access for that component and panic when appropriate. update_component_access adds a With filter for a component. This is sound because matches_component_set returns whether the set contains that component.

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type Item<'w> = bool

The item returned by this WorldQuery For QueryData this will be the item returned by the query. For QueryFilter this will be either (), or a bool indicating whether the entity should be included or a tuple of such things.
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type Fetch<'w> = ChangedFetch<'w>

Per archetype/table state used by this WorldQuery to fetch Self::Item
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type State = ComponentId

State used to construct a Self::Fetch. This will be cached inside QueryState, so it is best to move as much data / computation here as possible to reduce the cost of constructing Self::Fetch.
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fn shrink<'wlong: 'wshort, 'wshort>( item: Self::Item<'wlong> ) -> Self::Item<'wshort>

This function manually implements subtyping for the query items.
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unsafe fn init_fetch<'w>( world: UnsafeWorldCell<'w>, id: &ComponentId, last_run: Tick, this_run: Tick ) -> Self::Fetch<'w>

Creates a new instance of this fetch. Read more
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const IS_DENSE: bool = _

Returns true if (and only if) every table of every archetype matched by this fetch contains all of the matched components. This is used to select a more efficient “table iterator” for “dense” queries. If this returns true, WorldQuery::set_table must be used before WorldQuery::fetch can be called for iterators. If this returns false, WorldQuery::set_archetype must be used before WorldQuery::fetch can be called for iterators.
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unsafe fn set_archetype<'w>( fetch: &mut Self::Fetch<'w>, component_id: &ComponentId, _archetype: &'w Archetype, table: &'w Table )

Adjusts internal state to account for the next Archetype. This will always be called on archetypes that match this WorldQuery. Read more
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unsafe fn set_table<'w>( fetch: &mut Self::Fetch<'w>, component_id: &ComponentId, table: &'w Table )

Adjusts internal state to account for the next Table. This will always be called on tables that match this WorldQuery. Read more
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unsafe fn fetch<'w>( fetch: &mut Self::Fetch<'w>, entity: Entity, table_row: TableRow ) -> Self::Item<'w>

Fetch Self::Item for either the given entity in the current Table, or for the given entity in the current Archetype. This must always be called after WorldQuery::set_table with a table_row in the range of the current Table or after WorldQuery::set_archetype with a entity in the current archetype. Read more
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fn update_component_access( id: &ComponentId, access: &mut FilteredAccess<ComponentId> )

Adds any component accesses used by this WorldQuery to access.
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fn init_state(world: &mut World) -> ComponentId

Creates and initializes a State for this WorldQuery type.
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fn get_state(world: &World) -> Option<ComponentId>

Attempts to initializes a State for this WorldQuery type.
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fn matches_component_set( id: &ComponentId, set_contains_id: &impl Fn(ComponentId) -> bool ) -> bool

Returns true if this query matches a set of components. Otherwise, returns false.
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fn set_access(_state: &mut Self::State, _access: &FilteredAccess<ComponentId>)

Sets available accesses for implementors with dynamic access such as FilteredEntityRef or FilteredEntityMut. Read more

Auto Trait Implementations§

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impl<T> RefUnwindSafe for Changed<T>
where T: RefUnwindSafe,

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impl<T> Send for Changed<T>
where T: Send,

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impl<T> Sync for Changed<T>
where T: Sync,

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impl<T> Unpin for Changed<T>
where T: Unpin,

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impl<T> UnwindSafe for Changed<T>
where T: UnwindSafe,

Blanket Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> Downcast for T
where T: Any,

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fn into_any(self: Box<T>) -> Box<dyn Any>

Convert Box<dyn Trait> (where Trait: Downcast) to Box<dyn Any>. Box<dyn Any> can then be further downcast into Box<ConcreteType> where ConcreteType implements Trait.
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fn into_any_rc(self: Rc<T>) -> Rc<dyn Any>

Convert Rc<Trait> (where Trait: Downcast) to Rc<Any>. Rc<Any> can then be further downcast into Rc<ConcreteType> where ConcreteType implements Trait.
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fn as_any(&self) -> &(dyn Any + 'static)

Convert &Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &Any’s vtable from &Trait’s.
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fn as_any_mut(&mut self) -> &mut (dyn Any + 'static)

Convert &mut Trait (where Trait: Downcast) to &Any. This is needed since Rust cannot generate &mut Any’s vtable from &mut Trait’s.
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impl<T> DowncastSync for T
where T: Any + Send + Sync,

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fn into_any_arc(self: Arc<T>) -> Arc<dyn Any + Send + Sync>

Convert Arc<Trait> (where Trait: Downcast) to Arc<Any>. Arc<Any> can then be further downcast into Arc<ConcreteType> where ConcreteType implements Trait.
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T> Instrument for T

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fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
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fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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impl<T> WithSubscriber for T

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fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more
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fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more