Bevy Ecs Queries

Bevy 0.18 — ECS Queries

When to use this skill

• Reading or writing components from a system.
• Filtering by presence (With/Without), alternation (Or), or change detection (Changed/Added).
• Parallelising over a large entity set with par_iter_mut.
• Borrowing a subset of a query via a lens (transmute_lens).
• Compiler error mentioning ArchetypeQueryData (new bound in 0.18).

Canonical pattern

use bevy::prelude::*;

#[derive(Component, Default)]
struct Health(f32);

#[derive(Component, Default)]
struct Velocity(Vec3);

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Enemy;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, (
            move_things,
            on_health_changed,
            damage_visible_enemies,
            integrate_in_parallel,
        ))
        .run();
}

// Sequential iteration. `&` for read, `&mut` for write.
fn move_things(time: Res, mut q: Query) {
    let dt = time.delta_secs();
    for (vel, mut tf) in &mut q {
        tf.translation += vel.0 * dt;
    }
}

// Change detection. `Changed` triggers on insert OR mutation.
// `Added` triggers only on insert.
fn on_health_changed(q: Query>) {
    for (entity, hp) in &q {
        info!("entity {:?} now has {} hp", entity, hp.0);
    }
}

// Combined filters. `With`/`Without` constrain entities;
// `Or` alternates over filters (not components).
fn damage_visible_enemies(
    mut q: Query, Without, Or, Changed)>)>,
) {
    for mut hp in &mut q {
        hp.0 -= 1.0;
    }
}

// Parallel iteration. Use when N is large (>10k) and per-entity work is non-trivial.
// Cannot use `Commands` or external mutable state — task pool runs items in parallel.
fn integrate_in_parallel(mut q: Query) {
    q.par_iter_mut().for_each(|(vel, mut tf)| {
        tf.translation += vel.0 * 0.016;
    });
}

// Query lens: temporarily view a query as a narrower one. Useful for
// passing a stricter query into a helper without re-binding the system's
// SystemParam list.
#[allow(dead_code)]
fn use_lens(mut q: Query) {
    // Read-only narrowed view of just the Transform column.
    let mut lens = q.transmute_lens::();
    let _read_only: Query = lens.query();
}

Gotchas (0.18)

• ArchetypeQueryData is a new trait that bounds query data types where the exact item count must be known at compile time (e.g. for_each-style ergonomics). If you get a "trait ArchetypeQueryData not implemented" error, you're using a dynamic query (FilteredEntityRef/FilteredEntityMut) where a static one is required. Re-shape the query.
• EntityMut::get_components_mut::() is the safe way to grab two &muts out of one entity in 0.18 — returns Result. Don't reach for unsafe World::get_mut aliasing tricks.
• Query::get/get_mut returns Result, not Option. The error type carries the entity, so don't swallow it with .ok() if you actually need to know why a lookup missed.
• Or, With)> — Or alternates over filters, not raw component types. Or does not compile.
• Changed/Added are tick-based. A system that runs every other frame can miss changes only seen in the skipped frame. If you must not miss a change, use observers or a buffered queue.
• Don't pair par_iter_mut with Commands. Spawn/despawn from a sequential system that consumes a Resource queue written by the parallel one.

See also

• bevy-ecs-components — declaring the components queried here.
• bevy-ecs-systems — using queries inside SystemParam and run conditions.
• bevy-migration-0-17-to-0-18 — EntityMut::get_components_mut and tick-type move.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: chrisgliddon
• Source: chrisgliddon/bevy-skills
• License: MIT

Install and usage instructions live in the source repository linked above.