Struct bevy::ecs::system::Query

pub struct Query<'world, 'state, Q, F = ()>where
    Q: WorldQuery,
    F: ReadOnlyWorldQuery,{ /* private fields */ }
Expand description

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, both of which must implement the WorldQuery trait:

  • Q (query fetch). The type of data contained in the query item. Only entities that match the requested data will generate an item.
  • F (query filter). A set of conditions that determines whether query items should be kept or discarded. 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, emerging by combining WorldQuery implementors.

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>>

WorldQuery 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.

Accessing query items

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

Query methodsEffect
iter(_mut)Returns an iterator over all query items.
for_each(_mut),
par_for_each(_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. nCr 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 operationComputational complexity
iter(_mut)O(n)
for_each(_mut),
par_for_each(_mut)		O(n)
iter_many(_mut)O(k)
iter_combinations(_mut)O(nCr)
get(_mut)O(1)
(get_)manyO(k)
(get_)many_mutO(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§

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

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

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

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.

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

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

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

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;
        }
    }
}

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

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

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

Runs f on each read-only query item.

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.

Runs f on each query item.

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.

Runs f on each read-only query item in parallel.

Parallelization is achieved by using the World’s ComputeTaskPool.

Tasks and batch size

The items in the query get sorted into batches. Internally, this function spawns a group of futures that each take on a batch_size sized section of the items (or less if the division is not perfect). Then, the tasks in the ComputeTaskPool work through these futures.

You can use this value to tune between maximum multithreading ability (many small batches) and minimum parallelization overhead (few big batches). Rule of thumb: If the function body is (mostly) computationally expensive but there are not many items, a small batch size (=more batches) may help to even out the load. If the body is computationally cheap and you have many items, a large batch size (=fewer batches) avoids spawning additional futures that don’t help to even out the load.

Panics

This method panics if the ComputeTaskPool resource is added to the World before using this method. If using this from a query that is being initialized and run from the Schedule, this never panics.

See also

Runs f on each read-only query item in parallel.

Parallelization is achieved by using the World’s ComputeTaskPool.

Panics

This method panics if the ComputeTaskPool resource is added to the World before using this method. If using this from a query that is being initialized and run from the Schedule, this never panics.

See also

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.

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

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

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.

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.

Returns the query items for the given array of Entity.

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

See also

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.

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

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

Returns a mutable 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_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

Returns a mutable reference to the component T of 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

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

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

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

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

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;
    }
}

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!")
    }
}

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);
    }
}

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§

Formats the value using the given formatter. Read more
Returns an Iterator of Entitys over all of entitys descendants. Read more
Returns an Iterator of Entitys over all of entitys ancestors. Read more
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Creates an iterator from a value. Read more
The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Creates an iterator from a value. Read more

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