Struct 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.

Footguns

Note that a Query<Has<T>> will match all existing entities. Beware! Even if it matches all entities, it doesn’t mean that query.get(entity) will always return Ok(bool).

In the case of a non-existent entity, such as a despawned one, it will return Err. A workaround is to replace query.get(entity).unwrap() by query.get(entity).unwrap_or_default().

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> Debug for Has<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

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

SAFETY: Self is the same as Self::ReadOnly

const IS_READ_ONLY: bool = true

True if this query is read-only and may not perform mutable access.

type ReadOnly = Has<T>

The read-only variant of this QueryData, which satisfies the ReadOnlyQueryData trait.

type Item<'w> = bool

The item returned by this WorldQuery This will be the data retrieved by the query, and is visible to the end user when calling e.g. Query<Self>::get.

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

This function manually implements subtyping for the query items.

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 an entity in the current archetype. Accesses components registered in WorldQuery::update_component_access.

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

SAFETY: update_component_access and update_archetype_component_access do nothing. This is sound because fetch does not access components.

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> = bool

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, 'wshort>( fetch: Self::Fetch<'wlong>, ) -> Self::Fetch<'wshort>

This function manually implements subtyping for the query fetches.

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 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>( 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.

fn update_component_access( component_id: &Self::State, access: &mut FilteredAccess<ComponentId>, )

Adds any component accesses 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<Self::State>

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

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.

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

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

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

SAFETY: Has is read only