Bevy Turn System Agent Skill

Bevy turn system

When to use

Load this when play advances in turns rather than continuous time. Time still exists; you choose not to step the simulation every frame.

ECS shape

• Components: initiative and the active entity id you define.
• Resources: the active entity and the round live in one Resource (bevy-data-resources).
• 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.

Turn ownership

| Fact | Owner |

| --- | --- |

| Initiative | A component number, sorted when the round opens. |

| Active actor | TurnClock.order[index]. |

| Round | A counter on that same resource. Advance it in the chained end system. |

Scope

• Initiative is a component. The active entity and the round counter are one resource so a single system advances them.
• Rebuild the queue when a round starts or when a speed stat changes. Do not sort again on every action.
• bevy::state is for app modes, not an initiative queue. See bevy-state-machines.

Examples

[dependencies]
bevy = "0.20"

Each fighter stores speed and action points. The clock names who is active.

#[derive(Component)]
struct Fighter {
    speed: i32,
    tie_break: u32,
    action_points: u32,
}

#[derive(Resource)]
struct TurnClock {
    round: u32,
    order: Vec<Entity>,
    index: usize,
}

fn spawn_match(mut commands: Commands) {
    let a = commands
        .spawn(Fighter {
            speed: 12,
            tie_break: 0,
            action_points: 2,
        })
        .id();
    let b = commands
        .spawn(Fighter {
            speed: 12,
            tie_break: 1,
            action_points: 2,
        })
        .id();
    commands.insert_resource(TurnClock {
        round: 1,
        order: vec![a, b],
        index: 0,
    });
}

Sort once. Equal speed falls through to a stable id so replays stay deterministic.

#[derive(Component, Clone, Copy)]
struct Fighter {
    speed: i32,
    tie_break: u32,
}

#[derive(Resource)]
struct TurnClock {
    order: Vec<Entity>,
}

fn build_order(mut clock: ResMut<TurnClock>, fighters: Query<(Entity, &Fighter)>) {
    let mut rows: Vec<_> = fighters.iter().collect();
    rows.sort_by(|left, right| {
        right
            .1
            .speed
            .cmp(&left.1.speed)
            .then(left.1.tie_break.cmp(&right.1.tie_break))
    });
    clock.order = rows.into_iter().map(|(entity, _)| entity).collect();
}

Spend points only after the cost check. take_action sets end_requested. end_turn refills points and advances the index only when that flag is set. Both systems mutably use TurnClock and Fighter, so .chain() is required or the flag and the points race.

#[derive(Component)]
struct Fighter {
    action_points: u32,
}

#[derive(Resource)]
struct TurnClock {
    order: Vec<Entity>,
    index: usize,
    round: u32,
    end_requested: bool,
}

fn spend(cost: u32, fighter: &mut Fighter) -> bool {
    if fighter.action_points < cost {
        return false;
    }
    fighter.action_points -= cost;
    true
}

fn take_action(mut clock: ResMut<TurnClock>, mut fighters: Query<&mut Fighter>) {
    if clock.end_requested {
        return;
    }
    let Some(&current) = clock.order.get(clock.index) else {
        return;
    };
    let Ok(mut fighter) = fighters.get_mut(current) else {
        return;
    };
    if spend(1, &mut fighter) {
        clock.end_requested = true;
    }
}

fn end_turn(mut clock: ResMut<TurnClock>, mut fighters: Query<&mut Fighter>) {
    if !clock.end_requested || clock.order.is_empty() {
        return;
    }
    let Some(&current) = clock.order.get(clock.index) else {
        return;
    };
    if let Ok(mut fighter) = fighters.get_mut(current) {
        fighter.action_points = 2;
    }
    clock.end_requested = false;
    clock.index += 1;
    if clock.index >= clock.order.len() {
        clock.index = 0;
        clock.round += 1;
    }
}

pub struct TurnPlugin;

impl Plugin for TurnPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Update, (take_action, end_turn).chain());
    }
}

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 field stored outside the world |

| 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 |

| Initiative queue | TurnClock order built when the round opens | States or a sort on every action |

| Spend then end | take_action then end_turn with .chain() | Two unordered systems writing Fighter and TurnClock |

| 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 the documented set | Check the official documentation | 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 invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• Do not sort initiative on every action. WHY: the queue is rebuilt when a round opens or a speed stat changes, so replays stay stable.
• Do not use bevy::state as the initiative queue. WHY: states are app modes; the active entity is TurnClock.
• No invented APIs or unnamed third-party crates as the chosen solver. WHY: this skill only names documented Bevy crates and modules.

Reference

Official links stay on unversioned latest pages. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-state-machines - App mode is not turn order.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-turn-system - Godot package for turn order.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.