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Skillsbevy-game-loop-time-trial

bevy-game-loop-time-trial

Coding & development

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Updated 10-16 06:08

About this 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.

```rust
#[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

Complements

Downstream / consumers

  • None yet.

Master