Struct bevy::ecs::system::Query

pub struct Query<'world, 'state, D, F = ()>
where
    D: QueryData,
    F: QueryFilter,
{ /* private fields */ }

System parameter that provides selective access to the Component data stored in a World.

Enables access to entity identifiers and components from a system, without the need to directly access the world. Its iterators and getter methods return query items. Each query item is a type containing data relative to an entity.

Query is a generic data structure that accepts two type parameters:

• D (query data). The type of data contained in the query item. Only entities that match the requested data will generate an item. Must implement the QueryData trait.
• F (query filter). A set of conditions that determines whether query items should be kept or discarded. Must implement the QueryFilter trait. This type parameter is optional.

System parameter declaration

A query should always be declared as a system parameter. This section shows the most common idioms involving the declaration of Query.

Component access

A query defined with a reference to a component as the query fetch type parameter can be used to generate items that refer to the data of said component.

// A component can be accessed by shared reference...
query: Query<&ComponentA>

// ... or by mutable reference.
query: Query<&mut ComponentA>

Query filtering

Setting the query filter type parameter will ensure that each query item satisfies the given condition.

// Just `ComponentA` data will be accessed, but only for entities that also contain
// `ComponentB`.
query: Query<&ComponentA, With<ComponentB>>

QueryData or QueryFilter tuples

Using tuples, each Query type parameter can contain multiple elements.

In the following example, two components are accessed simultaneously, and the query items are filtered on two conditions.

query: Query<(&ComponentA, &ComponentB), (With<ComponentC>, Without<ComponentD>)>

Entity identifier access

The identifier of an entity can be made available inside the query item by including Entity in the query fetch type parameter.

query: Query<(Entity, &ComponentA)>

Optional component access

A component can be made optional in a query by wrapping it into an Option. In this way, a query item can still be generated even if the queried entity does not contain the wrapped component. In this case, its corresponding value will be None.

// Generates items for entities that contain `ComponentA`, and optionally `ComponentB`.
query: Query<(&ComponentA, Option<&ComponentB>)>

See the documentation for AnyOf to idiomatically declare many optional components.

See the performance section to learn more about the impact of optional components.

Disjoint queries

A system cannot contain two queries that break Rust’s mutability rules. In this case, the Without filter can be used to disjoint them.

In the following example, two queries mutably access the same component. Executing this system will panic, since an entity could potentially match the two queries at the same time by having both Player and Enemy components. This would violate mutability rules.

fn randomize_health(
    player_query: Query<&mut Health, With<Player>>,
    enemy_query: Query<&mut Health, With<Enemy>>,
)

Adding a Without filter will disjoint the queries. In this way, any entity that has both Player and Enemy components is excluded from both queries.

fn randomize_health(
    player_query: Query<&mut Health, (With<Player>, Without<Enemy>)>,
    enemy_query: Query<&mut Health, (With<Enemy>, Without<Player>)>,
)

An alternative to this idiom is to wrap the conflicting queries into a ParamSet.

Whole Entity Access

EntityRefs can be fetched from a query. This will give read-only access to any component on the entity, and can be use to dynamically fetch any component without baking it into the query type. Due to this global access to the entity, this will block any other system from parallelizing with it. As such these queries should be sparingly used.

query: Query<(EntityRef, &ComponentA)>

As EntityRef can read any component on an entity, a query using it will conflict with any mutable access. It is strongly advised to couple EntityRef queries with the use of either With/Without filters or ParamSets. This also limits the scope of the query, which will improve iteration performance and also allows it to parallelize with other non-conflicting systems.

// This will panic!
query: Query<(EntityRef, &mut ComponentA)>

// This will not panic.
query_a: Query<EntityRef, With<ComponentA>>,
query_b: Query<&mut ComponentB, Without<ComponentA>>,

Accessing query items

The following table summarizes the behavior of the safe methods that can be used to get query items.

Query methods	Effect
iter[_mut]	Returns an iterator over all query items.
for_each[_mut], par_iter[_mut]	Runs a specified function for each query item.
iter_many[_mut]	Iterates or runs a specified function over query items generated by a list of entities.
iter_combinations[_mut]	Returns an iterator over all combinations of a specified number of query items.
get[_mut]	Returns the query item for the specified entity.
many[_mut], get_many[_mut]	Returns the query items for the specified entities.
single[_mut], get_single[_mut]	Returns the query item while verifying that there aren’t others.

There are two methods for each type of query operation: immutable and mutable (ending with _mut). When using immutable methods, the query items returned are of type ROQueryItem, a read-only version of the query item. In this circumstance, every mutable reference in the query fetch type parameter is substituted by a shared reference.

Performance

Creating a Query is a low-cost constant operation. Iterating it, on the other hand, fetches data from the world and generates items, which can have a significant computational cost.

Table component storage type is much more optimized for query iteration than SparseSet.

Two systems cannot be executed in parallel if both access the same component type where at least one of the accesses is mutable. This happens unless the executor can verify that no entity could be found in both queries.

Optional components increase the number of entities a query has to match against. This can hurt iteration performance, especially if the query solely consists of only optional components, since the query would iterate over each entity in the world.

The following table compares the computational complexity of the various methods and operations, where:

• n is the number of entities that match the query,
• r is the number of elements in a combination,
• k is the number of involved entities in the operation,
• a is the number of archetypes in the world,
• C is the binomial coefficient, used to count combinations. _nC_r is read as “n choose r” and is equivalent to the number of distinct unordered subsets of r elements that can be taken from a set of n elements.

Query operation	Computational complexity
iter[_mut]	O(n)
for_each[_mut], par_iter[_mut]	O(n)
iter_many[_mut]	O(k)
iter_combinations[_mut]	O(nCr)
get[_mut]	O(1)
(get_)many	O(k)
(get_)many_mut	O(k2)
single[_mut], get_single[_mut]	O(a)
Archetype based filtering (With, Without, Or)	O(a)
Change detection filtering (Added, Changed)	O(a + n)

for_each methods are seen to be generally faster than their iter version on worlds with high archetype fragmentation. As iterators are in general more flexible and better integrated with the rest of the Rust ecosystem, it is advised to use iter methods over for_each. It is strongly advised to only use for_each if it tangibly improves performance: be sure profile or benchmark both before and after the change.

Implementations

impl<'w, 's, D, F> Query<'w, 's, D, F>

where D: QueryData, F: QueryFilter,

pub fn to_readonly(&self) -> Query<'_, 's, <D as QueryData>::ReadOnly, F>

Returns another Query from this that fetches the read-only version of the query items.

For example, Query<(&mut D1, &D2, &mut D3), With<F>> will become Query<(&D1, &D2, &D3), With<F>>. This can be useful when working around the borrow checker, or reusing functionality between systems via functions that accept query types.

pub fn iter(&self) -> QueryIter<'_, 's, <D as QueryData>::ReadOnly, F>

Returns an Iterator over the read-only query items.

Example

Here, the report_names_system iterates over the Player component of every entity that contains it:

fn report_names_system(query: Query<&Player>) {
    for player in &query {
        println!("Say hello to {}!", player.name);
    }
}

See also

• iter_mut for mutable query items.
• for_each for the closure based alternative.

pub fn iter_mut(&mut self) -> QueryIter<'_, 's, D, F>

Returns an Iterator over the query items.

Example

Here, the gravity_system updates the Velocity component of every entity that contains it:

fn gravity_system(mut query: Query<&mut Velocity>) {
    const DELTA: f32 = 1.0 / 60.0;
    for mut velocity in &mut query {
        velocity.y -= 9.8 * DELTA;
    }
}

See also

• iter for read-only query items.
• for_each_mut for the closure based alternative.

pub fn iter_combinations<const K: usize>( &self ) -> QueryCombinationIter<'_, 's, <D as QueryData>::ReadOnly, F, K>

Returns a QueryCombinationIter over all combinations of K read-only query items without repetition.

Example

fn some_system(query: Query<&ComponentA>) {
    for [a1, a2] in query.iter_combinations() {
        // ...
    }
}

See also

• iter_combinations_mut for mutable query item combinations.

pub fn iter_combinations_mut<const K: usize>( &mut self ) -> QueryCombinationIter<'_, 's, D, F, K>

Returns a QueryCombinationIter over all combinations of K query items without repetition.

Example

fn some_system(mut query: Query<&mut ComponentA>) {
    let mut combinations = query.iter_combinations_mut();
    while let Some([mut a1, mut a2]) = combinations.fetch_next() {
        // mutably access components data
    }
}

See also

• iter_combinations for read-only query item combinations.

pub fn iter_many<EntityList>( &self, entities: EntityList ) -> QueryManyIter<'_, 's, <D as QueryData>::ReadOnly, F, <EntityList as IntoIterator>::IntoIter>

where EntityList: IntoIterator, <EntityList as IntoIterator>::Item: Borrow<Entity>,

Returns an Iterator over the read-only query items generated from an Entity list.

Items are returned in the order of the list of entities. Entities that don’t match the query are skipped.

Example

// A component containing an entity list.
#[derive(Component)]
struct Friends {
    list: Vec<Entity>,
}

fn system(
    friends_query: Query<&Friends>,
    counter_query: Query<&Counter>,
) {
    for friends in &friends_query {
        for counter in counter_query.iter_many(&friends.list) {
            println!("Friend's counter: {:?}", counter.value);
        }
    }
}

See also

• iter_many_mut to get mutable query items.

pub fn iter_many_mut<EntityList>( &mut self, entities: EntityList ) -> QueryManyIter<'_, 's, D, F, <EntityList as IntoIterator>::IntoIter>

where EntityList: IntoIterator, <EntityList as IntoIterator>::Item: Borrow<Entity>,

Returns an iterator over the query items generated from an Entity list.

Items are returned in the order of the list of entities. Entities that don’t match the query are skipped.

Examples

#[derive(Component)]
struct Counter {
    value: i32
}

#[derive(Component)]
struct Friends {
    list: Vec<Entity>,
}

fn system(
    friends_query: Query<&Friends>,
    mut counter_query: Query<&mut Counter>,
) {
    for friends in &friends_query {
        let mut iter = counter_query.iter_many_mut(&friends.list);
        while let Some(mut counter) = iter.fetch_next() {
            println!("Friend's counter: {:?}", counter.value);
            counter.value += 1;
        }
    }
}

pub unsafe fn iter_unsafe(&self) -> QueryIter<'_, 's, D, F>

Returns an Iterator over the query items.

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.

See also

• iter and iter_mut for the safe versions.

pub unsafe fn iter_combinations_unsafe<const K: usize>( &self ) -> QueryCombinationIter<'_, 's, D, F, K>

Iterates over all possible combinations of K query items without repetition.

Safety

This allows aliased mutability. You must make sure this call does not result in multiple mutable references to the same component.

See also

• iter_combinations and iter_combinations_mut for the safe versions.

pub unsafe fn iter_many_unsafe<EntityList>( &self, entities: EntityList ) -> QueryManyIter<'_, 's, D, F, <EntityList as IntoIterator>::IntoIter>

where EntityList: IntoIterator, <EntityList as IntoIterator>::Item: Borrow<Entity>,

Returns an Iterator over the query items generated from an Entity list.

Safety

This allows aliased mutability and does not check for entity uniqueness. You must make sure this call does not result in multiple mutable references to the same component. Particular care must be taken when collecting the data (rather than iterating over it one item at a time) such as via Iterator::collect.

See also

• iter_many_mut to safely access the query items.

pub fn for_each<'this>( &'this self, f: impl FnMut(<<D as QueryData>::ReadOnly as WorldQuery>::Item<'this>) )

👎Deprecated since 0.13.0: Query::for_each was not idiomatic Rust and has been moved to query.iter().for_each()

Runs f on each read-only query item.

Shorthand for query.iter().for_each(..).

Example

Here, the report_names_system iterates over the Player component of every entity that contains it:

fn report_names_system(query: Query<&Player>) {
    query.for_each(|player| {
        println!("Say hello to {}!", player.name);
    });
}

See also

• for_each_mut to operate on mutable query items.
• iter for the iterator based alternative.

pub fn for_each_mut<'a>( &'a mut self, f: impl FnMut(<D as WorldQuery>::Item<'a>) )

👎Deprecated since 0.13.0: Query::for_each_mut was not idiomatic Rust and has been moved to query.iter_mut().for_each()

Runs f on each query item.

Shorthand for query.iter_mut().for_each(..).

Example

Here, the gravity_system updates the Velocity component of every entity that contains it:

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;
    });
}

See also

• for_each to operate on read-only query items.
• iter_mut for the iterator based alternative.

pub fn par_iter(&self) -> QueryParIter<'_, '_, <D as QueryData>::ReadOnly, F>

Returns a parallel iterator over the query results for the given World.

This can only be called for read-only queries, see par_iter_mut for write-queries.

Note that you must use the for_each method to iterate over the results, see par_iter_mut for an example.

pub fn par_iter_mut(&mut self) -> QueryParIter<'_, '_, D, F>

Returns a parallel iterator over the query results for the given World.

This can only be called for mutable queries, see par_iter for read-only-queries.

Example

Here, the gravity_system updates the Velocity component of every entity that contains it:

fn gravity_system(mut query: Query<&mut Velocity>) {
    const DELTA: f32 = 1.0 / 60.0;
    query.par_iter_mut().for_each(|mut velocity| {
        velocity.y -= 9.8 * DELTA;
    });
}

pub fn get( &self, entity: Entity ) -> Result<<<D as QueryData>::ReadOnly as WorldQuery>::Item<'_>, QueryEntityError>

Returns the read-only query item for the given Entity.

In case of a nonexisting entity or mismatched component, a QueryEntityError is returned instead.

Example

Here, get is used to retrieve the exact query item of the entity specified by the SelectedCharacter resource.

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);
    }
}

See also

• get_mut to get a mutable query item.

pub fn get_many<const N: usize>( &self, entities: [Entity; N] ) -> Result<[<<D as QueryData>::ReadOnly as WorldQuery>::Item<'_>; N], QueryEntityError>

Returns the read-only query items for the given array of Entity.

The returned query items are in the same order as the input. In case of a nonexisting entity or mismatched component, a QueryEntityError is returned instead. The elements of the array do not need to be unique, unlike get_many_mut.

See also

• get_many_mut to get mutable query items.
• many for the panicking version.

pub fn many<const N: usize>( &self, entities: [Entity; N] ) -> [<<D as QueryData>::ReadOnly as WorldQuery>::Item<'_>; N]

Returns the read-only query items for the given array of Entity.

Panics

This method panics if there is a query mismatch or a non-existing entity.

Examples

use bevy_ecs::prelude::*;

#[derive(Component)]
struct Targets([Entity; 3]);

#[derive(Component)]
struct Position{
    x: i8,
    y: i8
};

impl Position {
    fn distance(&self, other: &Position) -> i8 {
        // Manhattan distance is way easier to compute!
        (self.x - other.x).abs() + (self.y - other.y).abs()
    }
}

fn check_all_targets_in_range(targeting_query: Query<(Entity, &Targets, &Position)>, targets_query: Query<&Position>){
    for (targeting_entity, targets, origin) in &targeting_query {
        // We can use "destructuring" to unpack the results nicely
        let [target_1, target_2, target_3] = targets_query.many(targets.0);

        assert!(target_1.distance(origin) <= 5);
        assert!(target_2.distance(origin) <= 5);
        assert!(target_3.distance(origin) <= 5);
    }
}

See also

• get_many for the non-panicking version.

pub fn get_mut( &mut self, entity: Entity ) -> Result<<D as WorldQuery>::Item<'_>, QueryEntityError>

Returns the query item 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 item of the entity specified by the PoisonedCharacter resource.

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;
    }
}

See also

• get to get a read-only query item.

pub fn get_many_mut<const N: usize>( &mut self, entities: [Entity; N] ) -> Result<[<D as WorldQuery>::Item<'_>; N], QueryEntityError>

Returns the query items for the given array of Entity.

The returned query items are in the same order as the input. In case of a nonexisting entity, duplicate entities or mismatched component, a QueryEntityError is returned instead.

See also

• get_many to get read-only query items.
• many_mut for the panicking version.

pub fn many_mut<const N: usize>( &mut self, entities: [Entity; N] ) -> [<D as WorldQuery>::Item<'_>; N]

Returns the query items for the given array of Entity.

Panics

This method panics if there is a query mismatch, a non-existing entity, or the same Entity is included more than once in the array.

Examples

use bevy_ecs::prelude::*;

#[derive(Component)]
struct Spring{
    connected_entities: [Entity; 2],
    strength: f32,
}

#[derive(Component)]
struct Position {
    x: f32,
    y: f32,
}

#[derive(Component)]
struct Force {
    x: f32,
    y: f32,
}

fn spring_forces(spring_query: Query<&Spring>, mut mass_query: Query<(&Position, &mut Force)>){
    for spring in &spring_query {
         // We can use "destructuring" to unpack our query items nicely
         let [(position_1, mut force_1), (position_2, mut force_2)] = mass_query.many_mut(spring.connected_entities);

         force_1.x += spring.strength * (position_1.x - position_2.x);
         force_1.y += spring.strength * (position_1.y - position_2.y);

         // Silence borrow-checker: I have split your mutable borrow!
         force_2.x += spring.strength * (position_2.x - position_1.x);
         force_2.y += spring.strength * (position_2.y - position_1.y);
    }
}

See also

• get_many_mut for the non panicking version.
• many to get read-only query items.

pub unsafe fn get_unchecked( &self, entity: Entity ) -> Result<<D as WorldQuery>::Item<'_>, QueryEntityError>

Returns the query item 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.

See also

• get_mut for the safe version.

pub fn get_component<T>( &self, entity: Entity ) -> Result<&T, QueryComponentError>

where T: Component,

👎Deprecated since 0.13.0: Please use get and select for the exact component based on the structure of the exact query as required.

Returns a shared reference to the component T of the given Entity.

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.

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);
    }
}

See also

• component a panicking version of this function.
• get_component_mut to get a mutable reference of a component.

pub fn get_component_mut<T>( &mut self, entity: Entity ) -> Result<Mut<'_, T>, QueryComponentError>

where T: Component,

👎Deprecated since 0.13.0: Please use get_mut and select for the exact component based on the structure of the exact query as required.

Returns a mutable reference to the component T of the given entity.

In case of a nonexisting entity, mismatched component or missing write access, a QueryComponentError is returned instead.

Example

Here, get_component_mut is used to retrieve the Health component of the entity specified by the PoisonedCharacter resource.

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;
    }
}

See also

• component_mut a panicking version of this function.
• get_component to get a shared reference of a component.

pub fn component<T>(&self, entity: Entity) -> &T

where T: Component,

👎Deprecated since 0.13.0: Please use get and select for the exact component based on the structure of the exact query as required.

Returns a shared reference to the component T of the given Entity.

Panics

Panics in case of a nonexisting entity or mismatched component.

See also

• get_component a non-panicking version of this function.
• component_mut to get a mutable reference of a component.

pub fn component_mut<T>(&mut self, entity: Entity) -> Mut<'_, T>

where T: Component,

👎Deprecated since 0.13.0: Please use get_mut and select for the exact component based on the structure of the exact query as required.

Returns a mutable reference to the component T of the given entity.

Panics

Panics in case of a nonexisting entity, mismatched component or missing write access.

See also

• get_component_mut a non-panicking version of this function.
• component to get a shared reference of a component.

pub unsafe fn get_component_unchecked_mut<T>( &self, entity: Entity ) -> Result<Mut<'_, T>, QueryComponentError>

where T: Component,

👎Deprecated since 0.13.0: Please use get_unchecked and select for the exact component based on the structure of the exact query as required.

Returns a mutable reference to the component T of the given entity.

In case of a nonexisting entity or mismatched component, a QueryComponentError 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.

See also

• get_component_mut for the safe version.

pub fn single(&self) -> <<D as QueryData>::ReadOnly as WorldQuery>::Item<'_>

Returns a single read-only query item when there is exactly one entity matching the query.

Panics

This method panics if the number of query items is not exactly one.

Example

fn player_system(query: Query<&Position, With<Player>>) {
    let player_position = query.single();
    // do something with player_position
}

See also

• get_single for the non-panicking version.
• single_mut to get the mutable query item.

pub fn get_single( &self ) -> Result<<<D as QueryData>::ReadOnly as WorldQuery>::Item<'_>, QuerySingleError>

Returns a single read-only query item when there is exactly one entity matching the query.

If the number of query items is not exactly one, a QuerySingleError is returned instead.

Example

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!");
        }
    }
}

See also

• get_single_mut to get the mutable query item.
• single for the panicking version.

pub fn single_mut(&mut self) -> <D as WorldQuery>::Item<'_>

Returns a single query item when there is exactly one entity matching the query.

Panics

This method panics if the number of query item is not exactly one.

Example

fn regenerate_player_health_system(mut query: Query<&mut Health, With<Player>>) {
    let mut health = query.single_mut();
    health.0 += 1;
}

See also

• get_single_mut for the non-panicking version.
• single to get the read-only query item.

pub fn get_single_mut( &mut self ) -> Result<<D as WorldQuery>::Item<'_>, QuerySingleError>

Returns a single query item when there is exactly one entity matching the query.

If the number of query items is not exactly one, a QuerySingleError is returned instead.

Example

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;
}

See also

• get_single to get the read-only query item.
• single_mut for the panicking version.

pub fn is_empty(&self) -> bool

Returns true if there are no query items.

Example

Here, the score is increased only if an entity with a Player component is present in the world:

fn update_score_system(query: Query<(), With<Player>>, mut score: ResMut<Score>) {
    if !query.is_empty() {
        score.0 += 1;
    }
}

pub fn contains(&self, entity: Entity) -> bool

Returns true if the given Entity matches the query.

Example

fn targeting_system(in_range_query: Query<&InRange>, target: Res<Target>) {
    if in_range_query.contains(target.entity) {
        println!("Bam!")
    }
}

pub fn transmute_lens<NewD>(&mut self) -> QueryLens<'_, NewD>

where NewD: QueryData,

Returns a QueryLens that can be used to get a query with a more general fetch.

For example, this can transform a Query<(&A, &mut B)> to a Query<&B>. This can be useful for passing the query to another function. Note that since filter terms are dropped, non-archetypal filters like Added and Changed will not be respected. To maintain or change filter terms see Self::transmute_lens_filtered

Panics

This will panic if NewD is not a subset of the original fetch Q

Example

fn reusable_function(lens: &mut QueryLens<&A>) {
    assert_eq!(lens.query().single().0, 10);
}

// We can use the function in a system that takes the exact query.
fn system_1(mut query: Query<&A>) {
    reusable_function(&mut query.as_query_lens());
}

// We can also use it with a query that does not match exactly
// by transmuting it.
fn system_2(mut query: Query<(&mut A, &B)>) {
    let mut lens = query.transmute_lens::<&A>();
    reusable_function(&mut lens);
}

Allowed Transmutes

Besides removing parameters from the query, you can also make limited changes to the types of paramters.

• Can always add/remove Entity
• Ref<T> <-> &T
• &mut T -> &T
• &mut T -> Ref<T>
• EntityMut -> EntityRef

pub fn transmute_lens_filtered<NewD, NewF>( &mut self ) -> QueryLens<'_, NewD, NewF>

where NewD: QueryData, NewF: QueryFilter,

Equivalent to Self::transmute_lens but also includes a QueryFilter type.

Note that the lens will iterate the same tables and archetypes as the original query. This means that additional archetypal query terms like With and Without will not necessarily be respected and non-archetypal terms like Added and Changed will only be respected if they are in the type signature.

pub fn as_query_lens(&mut self) -> QueryLens<'_, D>

Gets a QueryLens with the same accesses as the existing query

impl<'w, 's, D, F> Query<'w, 's, D, F>

where D: ReadOnlyQueryData, F: QueryFilter,

pub fn get_inner( &self, entity: Entity ) -> Result<<<D as QueryData>::ReadOnly as WorldQuery>::Item<'w>, QueryEntityError>

Returns the query item for the given Entity, with the actual “inner” world lifetime.

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 for queries that contain at least one mutable component.

Example

Here, get is used to retrieve the exact query item of the entity specified by the SelectedCharacter resource.

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);
    }
}

pub fn iter_inner(&self) -> QueryIter<'w, 's, <D as QueryData>::ReadOnly, F>

Returns an Iterator over the query items, with the actual “inner” world lifetime.

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:

fn report_names_system(query: Query<&Player>) {
    for player in &query {
        println!("Say hello to {}!", player.name);
    }
}

Trait Implementations

impl<D, F> Debug for Query<'_, '_, D, F>

where D: QueryData, F: QueryFilter,

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

Formats the value using the given formatter.

impl<'w, 'q, Q, F> From<&'q mut Query<'w, '_, Q, F>> for QueryLens<'q, Q, F>

where Q: QueryData, F: QueryFilter,

fn from(value: &'q mut Query<'w, '_, Q, F>) -> QueryLens<'q, Q, F>

Converts to this type from the input type.

impl<'w, 's, Q, F> From<&'s mut QueryLens<'w, Q, F>> for Query<'w, 's, Q, F>

where Q: QueryData, F: QueryFilter,

fn from(value: &'s mut QueryLens<'w, Q, F>) -> Query<'w, 's, Q, F>

Converts to this type from the input type.

impl<'w, 's, D, F> HierarchyQueryExt<'w, 's, D, F> for Query<'w, 's, D, F>

where D: QueryData, F: QueryFilter,

fn iter_descendants(&'w self, entity: Entity) -> DescendantIter<'w, 's, D, F>

where <D as QueryData>::ReadOnly: WorldQuery<Item<'w> = &'w Children>,

Returns an Iterator of Entitys over all of entitys descendants.

fn iter_ancestors(&'w self, entity: Entity) -> AncestorIter<'w, 's, D, F>

where <D as QueryData>::ReadOnly: WorldQuery<Item<'w> = &'w Parent>,

Returns an Iterator of Entitys over all of entitys ancestors.

impl<'w, 's, D, F> IntoIterator for &'w Query<'_, 's, D, F>

where D: QueryData, F: QueryFilter,

type Item = <<D as QueryData>::ReadOnly as WorldQuery>::Item<'w>

The type of the elements being iterated over.

type IntoIter = QueryIter<'w, 's, <D as QueryData>::ReadOnly, F>

Which kind of iterator are we turning this into?

fn into_iter(self) -> <&'w Query<'_, 's, D, F> as IntoIterator>::IntoIter

Creates an iterator from a value.

impl<'w, 's, D, F> IntoIterator for &'w mut Query<'_, 's, D, F>

where D: QueryData, F: QueryFilter,

type Item = <D as WorldQuery>::Item<'w>

The type of the elements being iterated over.

type IntoIter = QueryIter<'w, 's, D, F>

Which kind of iterator are we turning this into?

fn into_iter(self) -> <&'w mut Query<'_, 's, D, F> as IntoIterator>::IntoIter

Creates an iterator from a value.

impl<D, F> SystemParam for Query<'_, '_, D, F>

where D: QueryData + 'static, F: QueryFilter + 'static,

type State = QueryState<D, F>

Used to store data which persists across invocations of a system.

type Item<'w, 's> = Query<'w, 's, D, F>

The item type returned when constructing this system param. The value of this associated type should be Self, instantiated with new lifetimes.

fn init_state( world: &mut World, system_meta: &mut SystemMeta ) -> <Query<'_, '_, D, F> as SystemParam>::State

Registers any World access used by this SystemParam and creates a new instance of this param’s State.

fn new_archetype( state: &mut <Query<'_, '_, D, F> as SystemParam>::State, archetype: &Archetype, system_meta: &mut SystemMeta )

For the specified Archetype, registers the components accessed by this SystemParam (if applicable).

unsafe fn get_param<'w, 's>( state: &'s mut <Query<'_, '_, D, F> as SystemParam>::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'w>, change_tick: Tick ) -> <Query<'_, '_, D, F> as SystemParam>::Item<'w, 's>

Creates a parameter to be passed into a SystemParamFunction.

fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World)

Applies any deferred mutations stored in this SystemParam’s state. This is used to apply Commands during apply_deferred.

impl<'w, 's, D, F> ReadOnlySystemParam for Query<'w, 's, D, F>

where D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static,