Bevy Game Loop Collection Agent Skill

Bevy collection loop

When to use

Load this when the run is about picking up objects and reaching a count.

ECS shape

• Components: a collected flag and a quota resource you define.
• Resources: run-wide tables live in a resource (bevy-data-resources), not an autoload.
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Engine counterparts

| Godot idea | Bevy idiom |

| --- | --- |

| Pickup | A component marker. Detection waits on spatial queries. |

| Quota | A resource count. |

| Win | A state change via bevy-state-machines. |

Scope

A collectible is a marker component. Overlap is a distance check you own until a physics skill supplies contacts. Mark each entity once per pass: Commands::despawn applies later, so a second collector would otherwise score the same pickup. The quota is a resource. Reaching it writes NextState, which the state schedule applies after this system.

[dependencies]
bevy = "0.20"

#[derive(Component)]
struct Collectible {
    points: u32,
}

#[derive(Component)]
struct Collector;

#[derive(Resource)]
struct CollectionScore {
    points: u32,
    quota: u32,
}

#[derive(States, Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
enum RunState {
    #[default]
    Playing,
    Won,
}

fn collect_by_distance(
    mut commands: Commands,
    mut score: ResMut<CollectionScore>,
    collectors: Query<&Transform, With<Collector>>,
    pickups: Query<(Entity, &Transform, &Collectible)>,
) {
    let mut taken = std::collections::HashSet::new();
    for collector in &collectors {
        for (entity, transform, item) in &pickups {
            if taken.contains(&entity) {
                continue;
            }
            let distance = collector.translation.distance(transform.translation);
            if distance <= 1.0 {
                taken.insert(entity);
                score.points += item.points;
                commands.entity(entity).despawn();
            }
        }
    }
}

fn reach_quota(score: Res<CollectionScore>, mut next: ResMut<NextState<RunState>>) {
    if score.points >= score.quota {
        next.set(RunState::Won);
    }
}

fn spawn_pickups(mut commands: Commands) {
    commands.insert_resource(CollectionScore {
        points: 0,
        quota: 3,
    });
    commands.spawn((Collector, Transform::default()));
    for index in 0..3 {
        commands.spawn((
            Collectible { points: 1 },
            Transform::from_xyz(index as f32, 0.0, 0.0),
        ));
    }
}

pub struct CollectionPlugin;

impl Plugin for CollectionPlugin {
    fn build(&self, app: &mut App) {
        app.init_state::<RunState>()
            .add_systems(Startup, spawn_pickups)
            .add_systems(Update, (collect_by_distance, reach_quota).chain());
    }
}

#[derive(Component)]
struct Remaining(u32);

#[derive(Resource, Default)]
struct Quota {
    collected: u32,
    need: u32,
}

fn tally(mut quota: ResMut<Quota>, mut counters: Query<&mut Remaining>) {
    for mut remaining in &mut counters {
        if remaining.0 > 0 {
            remaining.0 -= 1;
            quota.collected += 1;
        }
    }
}

fn quota_met(quota: Res<Quota>) -> bool {
    quota.collected >= quota.need && quota.need > 0
}

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | A Godot node script |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside this skill | Load the skill that owns it | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not treat NextState::set as already applied in the same system. WHY: the state schedule performs the transition after this system returns.
• Do not score a pickup from two collectors in one pass. WHY: despawn is deferred, so the entity is still in the query.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot node, resource, or signal types for this mechanic. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No Godot types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned site.

Official Documentation

• ECS - Component, systems, Query, Startup, Update, and .chain().
• States - States, NextState, and init_state on the same ECS schedule page.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-state-machines - Win and lose states.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-game-loop-collection - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.