Struct bevy_ecs::query::Has

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

Returns a bool that describes if an entity has the component T.

This can be used in a Query if you want to know whether or not entities have the component T but don’t actually care about the component’s value.

Examples

fn food_entity_system(query: Query<(&Name, Has<IsHungry>) >) {
    for (name, is_hungry) in &query {
        if is_hungry{
            println!("{} would like some food.", name.name);
        } else {
            println!("{} has had sufficient.", name.name);
        }
    }
}

// Unlike `Option<&T>`, `Has<T>` is compatible with `&mut T`
// as it does not actually access any data.
fn alphabet_entity_system(mut alphas: Query<(&mut Alpha, Has<Beta>)>, mut betas: Query<(&mut Beta, Has<Alpha>)>) {
    for (mut alpha, has_beta) in alphas.iter_mut() {
        alpha.has_beta = has_beta;
    }
    for (mut beta, has_alpha) in betas.iter_mut() {
        beta.has_alpha = has_alpha;
    }
}

Trait Implementations

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

type Fetch<'w> = bool

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

type Item<'w> = bool

The item returned by this WorldQuery

type ReadOnly = Has<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.

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 = true

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

unsafe fn init_fetch<'w>( _world: UnsafeWorldCell<'w>, _state: &Self::State, _last_run: Tick, _this_run: Tick ) -> Self::Fetch<'w>

Creates a new instance of this fetch.

unsafe fn set_archetype<'w>( fetch: &mut Self::Fetch<'w>, state: &Self::State, archetype: &'w Archetype, _table: &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>( fetch: &mut Self::Fetch<'w>, state: &Self::State, 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 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.

fn update_component_access( _state: &Self::State, _access: &mut FilteredAccess<ComponentId> )

Adds any component accesses used by this WorldQuery to access.

fn update_archetype_component_access( _state: &Self::State, _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( _state: &Self::State, _set_contains_id: &impl Fn(ComponentId) -> bool ) -> bool

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

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

Safety

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

SAFETY: Has is read only