Struct bevy_ecs::query::Has

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

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§

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impl<T> Debug for Has<T>

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

Formats the value using the given formatter. Read more
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impl<T: Component> QueryData for Has<T>

SAFETY: Self is the same as Self::ReadOnly

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type ReadOnly = Has<T>

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

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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> = bool

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>, _state: &Self::State, _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>, 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. Read more
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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. 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( component_id: &Self::State, access: &mut FilteredAccess<ComponentId> )

Adds any component accesses used by this WorldQuery to access. Read more
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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(components: &Components) -> Option<Self::State>

Attempts to initialize a State for this WorldQuery type using read-only access to Components.
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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. Read more
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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
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impl<T: Component> ReadOnlyQueryData for Has<T>

SAFETY: Has is read only

Auto Trait Implementations§

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impl<T> Freeze for Has<T>

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

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

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

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

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

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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 + Sync + Send>

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