Struct bevy_ecs::query::Changed

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

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>.

Examples

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

Trait Implementations

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

type Fetch<'w> = ChangedFetch<'w>

Per archetype/table state used by this WorldQuery to fetch Self::Item

type Item<'w> = bool

The item returned by this WorldQuery

type ReadOnly = Changed<T>

The read-only variant of this WorldQuery, which satisfies the ReadOnlyWorldQuery trait.

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.

fn shrink<'wlong: 'wshort, 'wshort>( item: Self::Item<'wlong> ) -> Self::Item<'wshort>

This function manually implements subtyping for the query items.

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.

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.

const IS_ARCHETYPAL: bool = false

Returns true if (and only if) this Fetch relies strictly on archetypes to limit which components are accessed by the Query.

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.

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.

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.

unsafe fn filter_fetch( fetch: &mut Self::Fetch<'_>, entity: Entity, table_row: TableRow ) -> bool

Safety

fn update_component_access( id: &ComponentId, access: &mut FilteredAccess<ComponentId> )

Adds any component accesses used by this WorldQuery to access.

fn update_archetype_component_access( id: &ComponentId, archetype: &Archetype, access: &mut Access<ArchetypeComponentId> )

For the given archetype, adds any component accessed used by this WorldQuery to access.

fn init_state(world: &mut World) -> ComponentId

Creates and initializes a State for this WorldQuery type.

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.

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

SAFETY: read-only access