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§
source§impl<T: Component> QueryData for Has<T>
impl<T: Component> QueryData for Has<T>
SAFETY: Self
is the same as Self::ReadOnly
§type ReadOnly = Has<T>
type ReadOnly = Has<T>
QueryData
, which satisfies the ReadOnlyQueryData
trait.source§impl<T: Component> WorldQuery for Has<T>
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.
§type Item<'w> = bool
type Item<'w> = bool
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.§type Fetch<'w> = bool
type Fetch<'w> = bool
WorldQuery
to fetch Self::Item
§type State = ComponentId
type State = ComponentId
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
.source§fn shrink<'wlong: 'wshort, 'wshort>(
item: Self::Item<'wlong>
) -> Self::Item<'wshort>
fn shrink<'wlong: 'wshort, 'wshort>( item: Self::Item<'wlong> ) -> Self::Item<'wshort>
source§unsafe fn init_fetch<'w>(
_world: UnsafeWorldCell<'w>,
_state: &Self::State,
_last_run: Tick,
_this_run: Tick
) -> Self::Fetch<'w>
unsafe fn init_fetch<'w>( _world: UnsafeWorldCell<'w>, _state: &Self::State, _last_run: Tick, _this_run: Tick ) -> Self::Fetch<'w>
source§const IS_DENSE: bool = _
const IS_DENSE: bool = _
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.source§unsafe fn set_archetype<'w>(
fetch: &mut Self::Fetch<'w>,
state: &Self::State,
archetype: &'w Archetype,
_table: &Table
)
unsafe fn set_archetype<'w>( fetch: &mut Self::Fetch<'w>, state: &Self::State, archetype: &'w Archetype, _table: &Table )
Archetype
. This will always be called on
archetypes that match this WorldQuery
. Read moresource§unsafe fn set_table<'w>(
fetch: &mut Self::Fetch<'w>,
state: &Self::State,
table: &'w Table
)
unsafe fn set_table<'w>( fetch: &mut Self::Fetch<'w>, state: &Self::State, table: &'w Table )
Table
. This will always be called on tables
that match this WorldQuery
. Read moresource§unsafe fn fetch<'w>(
fetch: &mut Self::Fetch<'w>,
_entity: Entity,
_table_row: TableRow
) -> Self::Item<'w>
unsafe fn fetch<'w>( fetch: &mut Self::Fetch<'w>, _entity: Entity, _table_row: TableRow ) -> Self::Item<'w>
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 moresource§fn update_component_access(
component_id: &Self::State,
access: &mut FilteredAccess<ComponentId>
)
fn update_component_access( component_id: &Self::State, access: &mut FilteredAccess<ComponentId> )
source§fn init_state(world: &mut World) -> ComponentId
fn init_state(world: &mut World) -> ComponentId
State
for this WorldQuery
type.source§fn matches_component_set(
_state: &Self::State,
_set_contains_id: &impl Fn(ComponentId) -> bool
) -> bool
fn matches_component_set( _state: &Self::State, _set_contains_id: &impl Fn(ComponentId) -> bool ) -> bool
source§fn set_access(_state: &mut Self::State, _access: &FilteredAccess<ComponentId>)
fn set_access(_state: &mut Self::State, _access: &FilteredAccess<ComponentId>)
FilteredEntityRef
or FilteredEntityMut
. Read moreimpl<T: Component> ReadOnlyQueryData for Has<T>
SAFETY: Has
is read only
Auto Trait Implementations§
impl<T> Freeze for Has<T>
impl<T> RefUnwindSafe for Has<T>where
T: RefUnwindSafe,
impl<T> Send for Has<T>where
T: Send,
impl<T> Sync for Has<T>where
T: Sync,
impl<T> Unpin for Has<T>where
T: Unpin,
impl<T> UnwindSafe for Has<T>where
T: UnwindSafe,
Blanket Implementations§
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> Downcast for Twhere
T: Any,
impl<T> Downcast for Twhere
T: Any,
source§fn into_any(self: Box<T>) -> Box<dyn Any>
fn into_any(self: Box<T>) -> Box<dyn Any>
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
.source§fn into_any_rc(self: Rc<T>) -> Rc<dyn Any>
fn into_any_rc(self: Rc<T>) -> Rc<dyn Any>
Rc<Trait>
(where Trait: Downcast
) to Rc<Any>
. Rc<Any>
can then be
further downcast
into Rc<ConcreteType>
where ConcreteType
implements Trait
.source§fn as_any(&self) -> &(dyn Any + 'static)
fn as_any(&self) -> &(dyn Any + 'static)
&Trait
(where Trait: Downcast
) to &Any
. This is needed since Rust cannot
generate &Any
’s vtable from &Trait
’s.source§fn as_any_mut(&mut self) -> &mut (dyn Any + 'static)
fn as_any_mut(&mut self) -> &mut (dyn Any + 'static)
&mut Trait
(where Trait: Downcast
) to &Any
. This is needed since Rust cannot
generate &mut Any
’s vtable from &mut Trait
’s.