Struct Single 

pub struct Single<'world, 'state, D: QueryData, F: QueryFilter = (), Desc: SingleDescriptor<'world, 'state, D, F> = ()>(pub <Desc::D as WorldQuery>::Item<'world>);

Get single entity from query

Example

Before using Single:

pub fn snap_camera(
    players: Query<&Transform, With<Player>>,
    mut cameras: Query<&mut Transform, (With<Camera>, Without<Player>)>,
) {
    let player = players.single();
    let mut camera = cameras.single_mut();
 
    camera.translation = player.translation.xy().extend(100.0);
}

After using Single:

pub fn snap_camera(
    player: Single<&Transform, With<Player>>,
    mut camera: Single<&mut Transform, (With<Camera>, Without<Player>)>,
) {
    camera.translation = player.translation.xy().extend(100.0);
}

Or like this:

pub fn snap_camera(
    Single(player): Single<&Transform, With<Player>>,
    Single(mut camera): Single<&mut Transform, (With<Camera>, Without<Player>)>,
) {
    camera.translation = player.translation.xy().extend(100.0);
}

Example with multiple components

Before using Single:

pub fn move_with_wasd(
    mut players: Query<(&mut KinematicCharacterController, &Player)>,
    input: Res<ButtonInput<KeyCode>>,
    time: Res<Time>,
) {
    let (mut controller, player) = players.single_mut();
 
    const WASD_DISPATCH: [(KeyCode, Vec2); 4] = [
        (KeyCode::KeyW, Vec2::Y),
        (KeyCode::KeyA, Vec2::NEG_X),
        (KeyCode::KeyS, Vec2::NEG_Y),
        (KeyCode::KeyD, Vec2::X),
    ];
 
    let mut translation = None;
 
    for (key, vector) in WASD_DISPATCH {
        if input.pressed(key) {
            translation = Some(translation.unwrap_or(Vec2::ZERO) + vector);
        }
    }
 
    controller.translation =
        translation.map(|t| t.normalize() * player.speed * time.delta_seconds());
}

After using Single:

pub fn move_with_wasd(
    Single((
        mut controller, 
        player
    )): Single<(&mut KinematicCharacterController, &Player)>,
    input: Res<ButtonInput<KeyCode>>,
    time: Res<Time>,
) {
    const WASD_DISPATCH: [(KeyCode, Vec2); 4] = [
        (KeyCode::KeyW, Vec2::Y),
        (KeyCode::KeyA, Vec2::NEG_X),
        (KeyCode::KeyS, Vec2::NEG_Y),
        (KeyCode::KeyD, Vec2::X),
    ];
 
    let mut translation = None;
 
    for (key, vector) in WASD_DISPATCH {
        if input.pressed(key) {
            translation = Some(translation.unwrap_or(Vec2::ZERO) + vector);
        }
    }
 
    controller.translation =
        translation.map(|t| t.normalize() * player.speed * time.delta_seconds());
}

Example with SystemParam

Before using Single:

pub fn example(
    mut param_set: ParamSet<(
        Query<Entity>,
        Query<&mut SpritesheetAnimation>
    )>,
) {
    let entities = param_set.p0();
    let mut animation = param_set.p1().single_mut();
 
    // ...
}

After using Single:

pub fn example(
    mut param_set: ParamSet<(
        Query<Entity>,
        Single<&mut SpritesheetAnimation>
    )>,
) {
    let entities = param_set.p0();
    let Single(mut animation) = param_set.p1();
 
    // ...
}

Tuple Fields

0: <Desc::D as WorldQuery>::Item<'world>

Trait Implementations

impl<'world, 'state, D: QueryData, F: QueryFilter, Desc: SingleDescriptor<'world, 'state, D, F>> Deref for Single<'world, 'state, D, F, Desc>

type Target = <<Desc as SingleDescriptor<'world, 'state, D, F>>::D as WorldQuery>::Item<'world>

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<'world, 'state, D: QueryData, F: QueryFilter, Desc: SingleDescriptor<'world, 'state, D, F>> DerefMut for Single<'world, 'state, D, F, Desc>

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl<'ww, 'ss, D: QueryData + 'static, F: QueryFilter + 'static> SystemParam for Single<'ww, 'ss, D, F>

type State = QueryState<D, F>

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

type Item<'w, 's> = Single<'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) -> Self::State

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

unsafe fn new_archetype( state: &mut Self::State, archetype: &Archetype, system_meta: &mut SystemMeta, )

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

unsafe fn get_param<'w, 's>( state: &'s mut Self::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'w>, change_tick: Tick, ) -> Self::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.

fn queue( state: &mut Self::State, system_meta: &SystemMeta, world: DeferredWorld<'_>, )

Queues any deferred mutations to be applied at the next apply_deferred.

impl<'w, 's, D: ReadOnlyQueryData + 'static, F: QueryFilter + 'static> ReadOnlySystemParam for Single<'w, 's, D, F>