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use crate::{
component::Component,
entity::Entity,
query::{
Fetch, FilterFetch, QueryCombinationIter, QueryEntityError, QueryIter, QueryState,
WorldQuery,
},
world::{Mut, World},
};
use bevy_tasks::TaskPool;
use std::{any::TypeId, fmt::Debug};
use thiserror::Error;
/// Provides scoped access to components in a [`World`].
///
/// Queries enable iteration over entities and their components as well as filtering them
/// on certain conditions. A query matches its parameters against the world to produce a series
/// of results. Each *query result* is a tuple of components (the same components defined
/// in the query) that belong to the same entity.
///
/// Computational cost of queries is reduced by the fact that they have an internal archetype
/// cache to avoid re-computing archetype matches on each query access.
///
/// Query functionality is based on the [`WorldQuery`] trait. Both tuples of components
/// (up to 16 elements) and query filters implement this trait.
///
/// `Query` accepts two type parameters:
///
/// 1. **Component access:** the components that an entity must have at the same time to yield
/// a query result.
/// 2. **Query filters (optional):** a predicate that ignores query results that don't match
/// its conditions.
///
/// # Usage as system parameter
///
/// A query is defined by declaring it as a system parameter. This section shows the various
/// use cases of `Query` as a system parameter.
///
/// ## Immutable component access
///
/// The following example defines a query that gives an iterator over `(&ComponentA, &ComponentB)`
/// tuples, where `ComponentA` and `ComponentB` belong to the same entity. Accessing components
/// immutably helps system parallelization.
///
/// ```
/// # use bevy_ecs::component::Component;
/// # use bevy_ecs::system::IntoSystem;
/// # use bevy_ecs::system::Query;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # fn system(
/// query: Query<(&ComponentA, &ComponentB)>
/// # ) {}
/// # system.system();
/// ```
///
/// ## Mutable component access
///
/// The following example is similar to the previous one, with the exception of `ComponentA`
/// being accessed mutably here. Note that both mutable and immutable accesses are allowed
/// in the same query.
///
/// ```
/// # use bevy_ecs::component::Component;
/// # use bevy_ecs::system::IntoSystem;
/// # use bevy_ecs::system::Query;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # fn system(
/// // `ComponentA` is accessed mutably, while `ComponentB` is accessed immutably.
/// mut query: Query<(&mut ComponentA, &ComponentB)>
/// # ) {}
/// # system.system();
/// ```
///
/// Two systems cannot be executed in parallel if both access a certain component and
/// at least one of the accesses is mutable, unless the schedule can verify that no entity
/// could be found in both queries, as otherwise Rusts mutability Rules would be broken.
///
/// Similarly, a system cannot contain two queries that would break Rust's mutability Rules.
/// If you need such Queries, you can use Filters to make the Queries disjoint or use a
/// [`QuerySet`](super::QuerySet).
///
/// ## Entity ID access
///
/// Inserting [`Entity`](crate::entity::Entity) at any position in the type parameter tuple
/// will give access to the entity ID.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # fn system(
/// query: Query<(Entity, &ComponentA, &ComponentB)>
/// # ) {}
/// # system.system();
/// ```
///
/// ## Query filtering
///
/// The second, optional type parameter of query, is used for filters can be added to filter
/// out the query results that don't satisfy the given condition.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # #[derive(Component)]
/// # struct ComponentC;
/// # fn system(
/// // `ComponentC` data won't be accessed, but only entities that contain it will be queried.
/// query: Query<(&ComponentA, &ComponentB), With<ComponentC>>
/// # ) {}
/// # system.system();
/// ```
///
/// If you need to apply more filters in a single query, group them into a tuple:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # #[derive(Component)]
/// # struct ComponentC;
/// # fn system(
/// // Similar to the previous query, but with the addition of a `Changed` filter.
/// query: Query<(&ComponentA, &ComponentB), (With<ComponentC>, Changed<ComponentA>)>
/// # ) {}
/// # system.system();
/// ```
///
/// The following list contains all the available query filters:
///
/// - [`Added`](crate::query::Added)
/// - [`Changed`](crate::query::Changed)
/// - [`With`](crate::query::With)
/// - [`Without`](crate::query::Without)
/// - [`Or`](crate::query::Or)
///
/// ## Optional component access
///
/// A component can be made optional in a query by wrapping it into an [`Option`]. In the
/// following example, the query will iterate over components of both entities that contain
/// `ComponentA` and `ComponentB`, and entities that contain `ComponentA` but not `ComponentB`.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// # fn system(
/// query: Query<(&ComponentA, Option<&ComponentB>)>
/// # ) {}
/// # system.system();
/// ```
///
/// If an entity does not contain a component, its corresponding query result value will be
/// `None`. Optional components increase the number of entities a query has to match against,
/// therefore they can hurt iteration performance, especially in the worst case scenario where
/// the query solely consists of only optional components, since all entities will be iterated
/// over.
///
/// ## Single component access
///
/// If just a single component needs to be accessed, using a tuple as the first type parameter
/// of `Query` can be omitted.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct MyComponent;
/// # fn tuple_system(
/// // This is correct, but can be avoided.
/// query: Query<(&MyComponent,)>
/// # ) {}
/// # tuple_system.system();
///
/// # fn non_tuple_system(
/// // This is the preferred method.
/// query: Query<&MyComponent>
/// # ) {}
/// # non_tuple_system.system();
/// ```
///
/// # Usage of query results
///
/// Inside the body of the system function, the `Query` is available as a function parameter.
/// This section shows various methods to access query results.
///
/// ## Iteration over every query result
///
/// The [`iter`](Self::iter) and [`iter_mut`](Self::iter_mut) methods are used to iterate
/// over every query result. Refer to the
/// [`Iterator` API docs](https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html)
/// for advanced iterator usage.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct ComponentA;
/// # #[derive(Component)]
/// # struct ComponentB;
/// fn immutable_query_system(query: Query<(&ComponentA, &ComponentB)>) {
/// for (a, b) in query.iter() {
/// // Here, `a` and `b` are normal references to components, relatively of
/// // `&ComponentA` and `&ComponentB` types.
/// }
/// }
/// # immutable_query_system.system();
///
/// fn mutable_query_system(mut query: Query<(&mut ComponentA, &ComponentB)>) {
/// for (mut a, b) in query.iter_mut() {
/// // Similar to the above system, but this time `ComponentA` can be accessed mutably.
/// // Note the usage of `mut` in the tuple and the call to `iter_mut` instead of `iter`.
/// }
/// }
/// # mutable_query_system.system();
/// ```
///
/// ## Getting the query result for a particular entity
///
/// If you have an [`Entity`] ID, you can use the [`get`](Self::get) or
/// [`get_mut`](Self::get_mut) methods to access the query result for that particular entity.
///
/// ## Getting a single query result
///
/// While it's possible to get a single result from a query by using `iter.next()`, a more
/// idiomatic approach would use the [`single`](Self::single) or [`single_mut`](Self::single_mut)
/// methods instead. Keep in mind though that they will return a [`QuerySingleError`] if the
/// number of query results differ from being exactly one. If that's the case, use `iter.next()`
/// (or `iter_mut.next()`) to only get the first query result.
pub struct Query<'world, 'state, Q: WorldQuery, F: WorldQuery = ()>
where
F::Fetch: FilterFetch,
{
pub(crate) world: &'world World,
pub(crate) state: &'state QueryState<Q, F>,
pub(crate) last_change_tick: u32,
pub(crate) change_tick: u32,
}
impl<'w, 's, Q: WorldQuery, F: WorldQuery> Query<'w, 's, Q, F>
where
F::Fetch: FilterFetch,
{
/// Creates a new query.
///
/// # Safety
///
/// This will create a query that could violate memory safety rules. Make sure that this is only
/// called in ways that ensure the queries have unique mutable access.
#[inline]
pub(crate) unsafe fn new(
world: &'w World,
state: &'s QueryState<Q, F>,
last_change_tick: u32,
change_tick: u32,
) -> Self {
Self {
world,
state,
last_change_tick,
change_tick,
}
}
/// Returns an [`Iterator`] over the query results.
///
/// This can only return immutable data (mutable data will be cast to an immutable form).
/// See [`Self::iter_mut`] for queries that contain at least one mutable component.
///
/// # Example
///
/// Here, the `report_names_system` iterates over the `Player` component of every entity
/// that contains it:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Player { name: String }
/// #
/// fn report_names_system(query: Query<&Player>) {
/// for player in query.iter() {
/// println!("Say hello to {}!", player.name);
/// }
/// }
/// # report_names_system.system();
/// ```
#[inline]
pub fn iter(&'s self) -> QueryIter<'w, 's, Q, Q::ReadOnlyFetch, F> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state
.iter_unchecked_manual(self.world, self.last_change_tick, self.change_tick)
}
}
/// Returns an [`Iterator`] over the query results.
///
/// # Example
///
/// Here, the `gravity_system` iterates over the `Velocity` component of every entity in
/// the world that contains it in order to update it:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Velocity { x: f32, y: f32, z: f32 }
/// fn gravity_system(mut query: Query<&mut Velocity>) {
/// const DELTA: f32 = 1.0 / 60.0;
/// for mut velocity in query.iter_mut() {
/// velocity.y -= 9.8 * DELTA;
/// }
/// }
/// # gravity_system.system();
/// ```
#[inline]
pub fn iter_mut(&mut self) -> QueryIter<'_, '_, Q, Q::Fetch, F> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state
.iter_unchecked_manual(self.world, self.last_change_tick, self.change_tick)
}
}
/// Returns an [`Iterator`] over all possible combinations of `K` query results without repetition.
/// This can only return immutable data
///
/// For permutations of size K of query returning N results, you will get:
/// - if K == N: one permutation of all query results
/// - if K < N: all possible K-sized combinations of query results, without repetition
/// - if K > N: empty set (no K-sized combinations exist)
#[inline]
pub fn iter_combinations<const K: usize>(
&self,
) -> QueryCombinationIter<'_, '_, Q, Q::ReadOnlyFetch, F, K> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state.iter_combinations_unchecked_manual(
self.world,
self.last_change_tick,
self.change_tick,
)
}
}
/// Iterates over all possible combinations of `K` query results without repetition.
///
/// The returned value is not an `Iterator`, because that would lead to aliasing of mutable references.
/// In order to iterate it, use `fetch_next` method with `while let Some(..)` loop pattern.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// # struct A;
/// # fn some_system(mut query: Query<&mut A>) {
/// // iterate using `fetch_next` in while loop
/// let mut combinations = query.iter_combinations_mut();
/// while let Some([mut a, mut b]) = combinations.fetch_next() {
/// // mutably access components data
/// }
/// # }
/// ```
///
/// There is no `for_each` method, because it cannot be safely implemented
/// due to a [compiler bug](https://github.com/rust-lang/rust/issues/62529).
///
/// For immutable access see [`Query::iter_combinations`].
#[inline]
pub fn iter_combinations_mut<const K: usize>(
&mut self,
) -> QueryCombinationIter<'_, '_, Q, Q::Fetch, F, K> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state.iter_combinations_unchecked_manual(
self.world,
self.last_change_tick,
self.change_tick,
)
}
}
/// Returns an [`Iterator`] over the query results.
///
/// # Safety
///
/// This function makes it possible to violate Rust's aliasing guarantees. You must make sure
/// this call does not result in multiple mutable references to the same component
#[inline]
pub unsafe fn iter_unsafe(&'s self) -> QueryIter<'w, 's, Q, Q::Fetch, F> {
// SEMI-SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
self.state
.iter_unchecked_manual(self.world, self.last_change_tick, self.change_tick)
}
/// Iterates over all possible combinations of `K` query results without repetition.
/// See [`Query::iter_combinations`].
///
/// # Safety
/// This allows aliased mutability. You must make sure this call does not result in multiple
/// mutable references to the same component
#[inline]
pub unsafe fn iter_combinations_unsafe<const K: usize>(
&self,
) -> QueryCombinationIter<'_, '_, Q, Q::Fetch, F, K> {
// SEMI-SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
self.state.iter_combinations_unchecked_manual(
self.world,
self.last_change_tick,
self.change_tick,
)
}
/// Runs `f` on each query result. This is faster than the equivalent iter() method, but cannot
/// be chained like a normal [`Iterator`].
///
/// This can only pass in immutable data, see [`Self::for_each_mut`] for mutable access.
///
/// # Example
///
/// Here, the `report_names_system` iterates over the `Player` component of every entity
/// that contains it:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Player { name: String }
/// #
/// fn report_names_system(query: Query<&Player>) {
/// query.for_each(|player| {
/// println!("Say hello to {}!", player.name);
/// });
/// }
/// # report_names_system.system();
/// ```
#[inline]
pub fn for_each<FN: FnMut(<Q::ReadOnlyFetch as Fetch<'w, 's>>::Item)>(&'s self, f: FN) {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state
.for_each_unchecked_manual::<Q::ReadOnlyFetch, FN>(
self.world,
f,
self.last_change_tick,
self.change_tick,
)
};
}
/// Runs `f` on each query result. This is faster than the equivalent iter() method, but cannot
/// be chained like a normal [`Iterator`].
///
/// # Example
///
/// Here, the `gravity_system` iterates over the `Velocity` component of every entity in
/// the world that contains it in order to update it:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Velocity { x: f32, y: f32, z: f32 }
/// fn gravity_system(mut query: Query<&mut Velocity>) {
/// const DELTA: f32 = 1.0 / 60.0;
/// query.for_each_mut(|mut velocity| {
/// velocity.y -= 9.8 * DELTA;
/// });
/// }
/// # gravity_system.system();
/// ```
#[inline]
pub fn for_each_mut<'a, FN: FnMut(<Q::Fetch as Fetch<'a, 'a>>::Item)>(&'a mut self, f: FN) {
// SAFE: system runs without conflicts with other systems. same-system queries have runtime
// borrow checks when they conflict
unsafe {
self.state.for_each_unchecked_manual::<Q::Fetch, FN>(
self.world,
f,
self.last_change_tick,
self.change_tick,
)
};
}
/// Runs `f` on each query result in parallel using the given task pool.
///
/// This can only be called for immutable data, see [`Self::par_for_each_mut`] for
/// mutable access.
#[inline]
pub fn par_for_each<FN: Fn(<Q::ReadOnlyFetch as Fetch<'w, 's>>::Item) + Send + Sync + Clone>(
&'s self,
task_pool: &TaskPool,
batch_size: usize,
f: FN,
) {
// SAFE: system runs without conflicts with other systems. same-system queries have runtime
// borrow checks when they conflict
unsafe {
self.state
.par_for_each_unchecked_manual::<Q::ReadOnlyFetch, FN>(
self.world,
task_pool,
batch_size,
f,
self.last_change_tick,
self.change_tick,
)
};
}
/// Runs `f` on each query result in parallel using the given task pool.
#[inline]
pub fn par_for_each_mut<'a, FN: Fn(<Q::Fetch as Fetch<'a, 'a>>::Item) + Send + Sync + Clone>(
&'a mut self,
task_pool: &TaskPool,
batch_size: usize,
f: FN,
) {
// SAFE: system runs without conflicts with other systems. same-system queries have runtime
// borrow checks when they conflict
unsafe {
self.state.par_for_each_unchecked_manual::<Q::Fetch, FN>(
self.world,
task_pool,
batch_size,
f,
self.last_change_tick,
self.change_tick,
)
};
}
/// Returns the query result for the given [`Entity`].
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// This can only return immutable data (mutable data will be cast to an immutable form).
/// See [`get_mut`](Self::get_mut) for queries that contain at least one mutable component.
///
/// # Example
///
/// Here, `get` is used to retrieve the exact query result of the entity specified by the
/// `SelectedCharacter` resource.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # struct SelectedCharacter { entity: Entity }
/// # #[derive(Component)]
/// # struct Character { name: String }
/// #
/// fn print_selected_character_name_system(
/// query: Query<&Character>,
/// selection: Res<SelectedCharacter>
/// )
/// {
/// if let Ok(selected_character) = query.get(selection.entity) {
/// println!("{}", selected_character.name);
/// }
/// }
/// # print_selected_character_name_system.system();
/// ```
#[inline]
pub fn get(
&'s self,
entity: Entity,
) -> Result<<Q::ReadOnlyFetch as Fetch<'w, 's>>::Item, QueryEntityError> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state.get_unchecked_manual::<Q::ReadOnlyFetch>(
self.world,
entity,
self.last_change_tick,
self.change_tick,
)
}
}
/// Returns the query result for the given [`Entity`].
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// # Example
///
/// Here, `get_mut` is used to retrieve the exact query result of the entity specified by the
/// `PoisonedCharacter` resource.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # struct PoisonedCharacter { character_id: Entity }
/// # #[derive(Component)]
/// # struct Health(u32);
/// #
/// fn poison_system(mut query: Query<&mut Health>, poisoned: Res<PoisonedCharacter>) {
/// if let Ok(mut health) = query.get_mut(poisoned.character_id) {
/// health.0 -= 1;
/// }
/// }
/// # poison_system.system();
/// ```
#[inline]
pub fn get_mut(
&mut self,
entity: Entity,
) -> Result<<Q::Fetch as Fetch>::Item, QueryEntityError> {
// SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
unsafe {
self.state.get_unchecked_manual::<Q::Fetch>(
self.world,
entity,
self.last_change_tick,
self.change_tick,
)
}
}
/// Returns the query result for the given [`Entity`].
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// # Safety
///
/// This function makes it possible to violate Rust's aliasing guarantees. You must make sure
/// this call does not result in multiple mutable references to the same component
#[inline]
pub unsafe fn get_unchecked(
&'s self,
entity: Entity,
) -> Result<<Q::Fetch as Fetch<'w, 's>>::Item, QueryEntityError> {
// SEMI-SAFE: system runs without conflicts with other systems.
// same-system queries have runtime borrow checks when they conflict
self.state.get_unchecked_manual::<Q::Fetch>(
self.world,
entity,
self.last_change_tick,
self.change_tick,
)
}
/// Returns a reference to the [`Entity`]'s [`Component`] of the given type.
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// # Example
///
/// Here, `get_component` is used to retrieve the `Character` component of the entity
/// specified by the `SelectedCharacter` resource.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # struct SelectedCharacter { entity: Entity }
/// # #[derive(Component)]
/// # struct Character { name: String }
/// #
/// fn print_selected_character_name_system(
/// query: Query<&Character>,
/// selection: Res<SelectedCharacter>
/// )
/// {
/// if let Ok(selected_character) = query.get_component::<Character>(selection.entity) {
/// println!("{}", selected_character.name);
/// }
/// }
/// # print_selected_character_name_system.system();
/// ```
#[inline]
pub fn get_component<T: Component>(&self, entity: Entity) -> Result<&T, QueryComponentError> {
let world = self.world;
let entity_ref = world
.get_entity(entity)
.ok_or(QueryComponentError::NoSuchEntity)?;
let component_id = world
.components()
.get_id(TypeId::of::<T>())
.ok_or(QueryComponentError::MissingComponent)?;
let archetype_component = entity_ref
.archetype()
.get_archetype_component_id(component_id)
.ok_or(QueryComponentError::MissingComponent)?;
if self
.state
.archetype_component_access
.has_read(archetype_component)
{
entity_ref
.get::<T>()
.ok_or(QueryComponentError::MissingComponent)
} else {
Err(QueryComponentError::MissingReadAccess)
}
}
/// Returns a mutable reference to the [`Entity`]'s [`Component`] of the given type.
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// # Example
///
/// Here, `get_component_mut` is used to retrieve the `Health` component of the entity
/// specified by the `PoisonedCharacter` resource.
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # struct PoisonedCharacter { character_id: Entity }
/// # #[derive(Component)]
/// # struct Health(u32);
/// #
/// fn poison_system(mut query: Query<&mut Health>, poisoned: Res<PoisonedCharacter>) {
/// if let Ok(mut health) = query.get_component_mut::<Health>(poisoned.character_id) {
/// health.0 -= 1;
/// }
/// }
/// # poison_system.system();
/// ```
#[inline]
pub fn get_component_mut<T: Component>(
&mut self,
entity: Entity,
) -> Result<Mut<'_, T>, QueryComponentError> {
// SAFE: unique access to query (preventing aliased access)
unsafe { self.get_component_unchecked_mut(entity) }
}
/// Returns a mutable reference to the [`Entity`]'s [`Component`] of the given type.
///
/// In case of a nonexisting entity or mismatched component, a [`QueryEntityError`] is
/// returned instead.
///
/// # Safety
///
/// This function makes it possible to violate Rust's aliasing guarantees. You must make sure
/// this call does not result in multiple mutable references to the same component
#[inline]
pub unsafe fn get_component_unchecked_mut<T: Component>(
&self,
entity: Entity,
) -> Result<Mut<'_, T>, QueryComponentError> {
let world = self.world;
let entity_ref = world
.get_entity(entity)
.ok_or(QueryComponentError::NoSuchEntity)?;
let component_id = world
.components()
.get_id(TypeId::of::<T>())
.ok_or(QueryComponentError::MissingComponent)?;
let archetype_component = entity_ref
.archetype()
.get_archetype_component_id(component_id)
.ok_or(QueryComponentError::MissingComponent)?;
if self
.state
.archetype_component_access
.has_write(archetype_component)
{
entity_ref
.get_unchecked_mut::<T>(self.last_change_tick, self.change_tick)
.ok_or(QueryComponentError::MissingComponent)
} else {
Err(QueryComponentError::MissingWriteAccess)
}
}
/// Returns a single immutable query result when there is exactly one entity matching
/// the query.
///
/// This can only return immutable data. Use [`single_mut`](Self::single_mut) for
/// queries that contain at least one mutable component.
///
/// # Example
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component)]
/// # struct Player;
/// # #[derive(Component)]
/// # struct Position(f32, f32);
/// fn player_system(query: Query<&Position, With<Player>>) {
/// let player_position = query.single();
/// // do something with player_position
/// }
/// # player_system.system();
/// ```
///
/// # Panics
///
/// Panics if the number of query results is not exactly one. Use
/// [`get_single`](Self::get_single) to return a `Result` instead of panicking.
#[track_caller]
pub fn single(&'s self) -> <Q::ReadOnlyFetch as Fetch<'w, 's>>::Item {
self.get_single().unwrap()
}
/// Returns a single immutable query result when there is exactly one entity matching
/// the query.
///
/// This can only return immutable data. Use [`get_single_mut`](Self::get_single_mut)
/// for queries that contain at least one mutable component.
///
/// If the number of query results is not exactly one, a [`QuerySingleError`] is returned
/// instead.
///
/// # Example
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # use bevy_ecs::system::QuerySingleError;
/// # #[derive(Component)]
/// # struct PlayerScore(i32);
/// fn player_scoring_system(query: Query<&PlayerScore>) {
/// match query.get_single() {
/// Ok(PlayerScore(score)) => {
/// println!("Score: {}", score);
/// }
/// Err(QuerySingleError::NoEntities(_)) => {
/// println!("Error: There is no player!");
/// }
/// Err(QuerySingleError::MultipleEntities(_)) => {
/// println!("Error: There is more than one player!");
/// }
/// }
/// }
/// # player_scoring_system.system();
/// ```
pub fn get_single(
&'s self,
) -> Result<<Q::ReadOnlyFetch as Fetch<'w, 's>>::Item, QuerySingleError> {
let mut query = self.iter();
let first = query.next();
let extra = query.next().is_some();
match (first, extra) {
(Some(r), false) => Ok(r),
(None, _) => Err(QuerySingleError::NoEntities(std::any::type_name::<Self>())),
(Some(_), _) => Err(QuerySingleError::MultipleEntities(std::any::type_name::<
Self,
>())),
}
}
/// Returns a single mutable query result when there is exactly one entity matching
/// the query.
///
/// # Example
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Player;
/// # #[derive(Component)]
/// # struct Health(u32);
/// #
/// fn regenerate_player_health_system(mut query: Query<&mut Health, With<Player>>) {
/// let mut health = query.single_mut();
/// health.0 += 1;
/// }
/// # regenerate_player_health_system.system();
/// ```
///
/// # Panics
///
/// Panics if the number of query results is not exactly one. Use
/// [`get_single_mut`](Self::get_single_mut) to return a `Result` instead of panicking.
#[track_caller]
pub fn single_mut(&mut self) -> <Q::Fetch as Fetch<'_, '_>>::Item {
self.get_single_mut().unwrap()
}
/// Returns a single mutable query result when there is exactly one entity matching
/// the query.
///
/// If the number of query results is not exactly one, a [`QuerySingleError`] is returned
/// instead.
///
/// # Example
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Player;
/// # #[derive(Component)]
/// # struct Health(u32);
/// #
/// fn regenerate_player_health_system(mut query: Query<&mut Health, With<Player>>) {
/// let mut health = query.get_single_mut().expect("Error: Could not find a single player.");
/// health.0 += 1;
/// }
/// # regenerate_player_health_system.system();
/// ```
pub fn get_single_mut(
&mut self,
) -> Result<<Q::Fetch as Fetch<'_, '_>>::Item, QuerySingleError> {
let mut query = self.iter_mut();
let first = query.next();
let extra = query.next().is_some();
match (first, extra) {
(Some(r), false) => Ok(r),
(None, _) => Err(QuerySingleError::NoEntities(std::any::type_name::<Self>())),
(Some(_), _) => Err(QuerySingleError::MultipleEntities(std::any::type_name::<
Self,
>())),
}
}
/// Returns `true` if there are no query results.
///
/// # Example
///
/// Here, the score is increased only if an entity with a `Player` component is present
/// in the world:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #
/// # #[derive(Component)]
/// # struct Player;
/// # #[derive(Component)]
/// # struct Score(u32);
/// fn update_score_system(query: Query<(), With<Player>>, mut score: ResMut<Score>) {
/// if !query.is_empty() {
/// score.0 += 1;
/// }
/// }
/// # update_score_system.system();
/// ```
#[inline]
pub fn is_empty(&self) -> bool {
self.state
.is_empty(self.world, self.last_change_tick, self.change_tick)
}
}
/// An error that occurs when retrieving a specific [`Entity`]'s component from a [`Query`]
#[derive(Error, Debug)]
pub enum QueryComponentError {
#[error("This query does not have read access to the requested component.")]
MissingReadAccess,
#[error("This query does not have write access to the requested component.")]
MissingWriteAccess,
#[error("The given entity does not have the requested component.")]
MissingComponent,
#[error("The requested entity does not exist.")]
NoSuchEntity,
}
/// An error that occurs when evaluating a [`Query`] as a single expected resulted via
/// [`Query::single`] or [`Query::single_mut`].
#[derive(Debug, Error)]
pub enum QuerySingleError {
#[error("No entities fit the query {0}")]
NoEntities(&'static str),
#[error("Multiple entities fit the query {0}!")]
MultipleEntities(&'static str),
}