Struct Changed 

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

A filter on a component that only retains results the first time after they have been added or mutably dereferenced.

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>.

Note that this includes changes that happened before the first time this Query was run.

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 after the system that queued them.

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

impl<T> QueryFilter for Changed<T>

where T: Component,

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.

unsafe fn filter_fetch( _state: &<Changed<T> as WorldQuery>::State, fetch: &mut <Changed<T> as WorldQuery>::Fetch<'_>, entity: Entity, table_row: TableRow, ) -> bool

Returns true if the provided Entity and [TableRow] should be included in the query results. If false, the entity will be skipped.

impl<T> WorldQuery for Changed<T>

where T: Component,

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.

type Fetch<'w> = ChangedFetch<'w, T>

Per archetype/table state retrieved by this [WorldQuery] to compute Self::Item for each entity.

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_fetch<'wlong, 'wshort>( fetch: <Changed<T> as WorldQuery>::Fetch<'wlong>, ) -> <Changed<T> as WorldQuery>::Fetch<'wshort>

where 'wlong: 'wshort,

This function manually implements subtyping for the query fetches.

unsafe fn init_fetch<'w, 's>( world: UnsafeWorldCell<'w>, _: &'s ComponentId, last_run: Tick, this_run: Tick, ) -> <Changed<T> as WorldQuery>::Fetch<'w>

Creates a new instance of Self::Fetch, by combining data from the World with the cached Self::State. Readonly accesses resources registered in [WorldQuery::update_component_access].

unsafe fn set_archetype<'w, 's>( fetch: &mut <Changed<T> as WorldQuery>::Fetch<'w>, component_id: &'s 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 set_table<'w, 's>( fetch: &mut <Changed<T> as WorldQuery>::Fetch<'w>, _: &'s ComponentId, table: &'w Table, )

Adjusts internal state to account for the next [Table]. This will always be called on tables that match this [WorldQuery].

fn update_component_access(_: &ComponentId, access: &mut FilteredAccess)

Adds any component accesses to the current entity used by this [WorldQuery] to access.

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

Creates and initializes a State for this [WorldQuery] type.

fn get_state(components: &Components) -> Option<ComponentId>

Attempts to initialize a State for this [WorldQuery] type using read-only access to [Components].

fn matches_component_set( _: &ComponentId, set_contains_id: &impl Fn(ComponentId) -> bool, ) -> bool

Returns true if this query matches a set of components. Otherwise, returns false.

fn init_nested_access( _state: &Self::State, _system_name: Option<&str>, _component_access_set: &mut FilteredAccessSet, _world: UnsafeWorldCell<'_>, )

Adds any component accesses to other entities used by this [WorldQuery].

fn update_archetypes(_state: &mut Self::State, _world: UnsafeWorldCell<'_>)

Called when the query state is updating its archetype cache. This can be used by nested queries to update their internal archetype caches.