Bevy Game Loop Time Trial Agent Skill

Bevy time-trial loop

When to use

Load this when a run records elapsed time, splits, or a finish line.

ECS shape

• Components: a racer marker and a finish volume.
• Resources: the race clock, split list, and best time live on one resource.
• Systems: tick the clock, record a split, then test the finish line, in that order with .chain().
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

[package]
name = "time-trial"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

The race clock is one resource. Three systems write it, so they share one chain.

#[derive(Resource)]
struct RaceClock {
    elapsed: f32,
    splits: Vec<f32>,
    finished: bool,
    best: f32,
}

#[derive(Component)]
struct Racer;

#[derive(Component)]
struct FinishLine;

fn spawn_course(mut commands: Commands) {
    commands.insert_resource(RaceClock {
        elapsed: 0.0,
        splits: Vec::new(),
        finished: false,
        best: f32::MAX,
    });
    commands.spawn((Racer, Transform::from_xyz(0.0, 0.0, 0.0)));
    commands.spawn((FinishLine, Transform::from_xyz(10.0, 0.0, 0.0)));
    commands.spawn(SplitGate {
        at_x: 5.0,
        taken: false,
    });
}

fn tick_race(time: Res<Time>, mut clock: ResMut<RaceClock>) {
    if !clock.finished {
        clock.elapsed += time.delta_secs();
    }
}

#[derive(Component)]
struct SplitGate {
    at_x: f32,
    taken: bool,
}

fn record_split(
    mut clock: ResMut<RaceClock>,
    racers: Query<&Transform, With<Racer>>,
    mut gates: Query<&mut SplitGate>,
) {
    if clock.finished {
        return;
    }
    let Some(racer) = racers.iter().next() else {
        return;
    };
    for mut gate in &mut gates {
        if !gate.taken && racer.translation.x >= gate.at_x {
            gate.taken = true;
            let elapsed = clock.elapsed;
            clock.splits.push(elapsed);
        }
    }
}

fn cross_finish(
    mut clock: ResMut<RaceClock>,
    racers: Query<&Transform, With<Racer>>,
    lines: Query<&Transform, With<FinishLine>>,
) {
    if clock.finished {
        return;
    }
    let Some(racer) = racers.iter().next() else {
        return;
    };
    let Some(line) = lines.iter().next() else {
        return;
    };
    if racer.translation.x >= line.translation.x {
        clock.finished = true;
        if clock.elapsed < clock.best {
            clock.best = clock.elapsed;
        }
    }
}

pub struct TimeTrialPlugin;

impl Plugin for TimeTrialPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, spawn_course).add_systems(
            Update,
            (tick_race, record_split, cross_finish).chain(),
        );
    }
}

A fixed step moves the racer. It does not own the race clock.

#[derive(Component)]
struct Racer;

fn drive(time: Res<Time<Fixed>>, mut racers: Query<&mut Transform, With<Racer>>) {
    let step = time.delta_secs() * 8.0;
    for mut transform in &mut racers {
        transform.translation.x += step;
    }
}

pub struct DrivePlugin;

impl Plugin for DrivePlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(FixedUpdate, drive);
    }
}

Race data

| Need | Bevy idiom |

| --- | --- |

| Race clock | Elapsed on a resource. The fixed clock is a stepper, not a race format. |

| Split | Your own list on that resource, written when the racer passes a SplitGate. |

| Finish | A finished flag on that resource. |

Scope

• Race timer, split, and ghost data are yours to build as components.
• Ghost replay belongs on bevy-save-load.

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 | One struct that stores every entity's fields |

| 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 the documented set | Read the linked Bevy pages | 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.

• No signal bus for splits or the finish. WHY: the clock resource is shared state; one-shot notices belong in bevy-events-messaging.
• No node 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 links stay on the unversioned learn pages.

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 - Countdown and finish.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.