bevy-ecs-expert

name	bevy-ecs-expert
description	ALWAYS use this when the request matches Bevy ECS Expert: Master Bevy's Entity Component System (ECS) in Rust, covering Systems, Queries, Resources, and parallel scheduling.

Bevy ECS Expert

Selective Reading Rule

Start with:

• references/senior-master-standard.md
• references/usage-routing.md
• references/quality-checklist.md

Then load only the inherited docs, scripts, assets, or examples that match the user's actual task.

Overview

A guide to building high-performance game logic using Bevy's data-oriented ECS architecture. Learn how to structure systems, optimize queries, manage resources, and leverage parallel execution.

When to Use This Skill

• Use when developing games with the Bevy engine in Rust.
• Use when designing game systems that need to run in parallel.
• Use when optimizing game performance by minimizing cache misses.
• Use when refactoring object-oriented logic into data-oriented ECS patterns.

Step-by-Step Guide

1. Defining Components

Use simple structs for data. Derive Component and Reflect.

#[derive(Component, Reflect, Default)]
#[reflect(Component)]
struct Velocity {
    x: f32,
    y: f32,
}

#[derive(Component)]
struct Player;

2. Writing Systems

Systems are regular Rust functions that query components.

fn movement_system(
    time: Res<Time>,
    mut query: Query<(&mut Transform, &Velocity), With<Player>>,
) {
    for (mut transform, velocity) in &mut query {
        transform.translation.x += velocity.x * time.delta_seconds();
        transform.translation.y += velocity.y * time.delta_seconds();
    }
}

3. Managing Resources

Use Resource for global data (score, game state).

#[derive(Resource)]
struct GameState {
    score: u32,
}

fn score_system(mut game_state: ResMut<GameState>) {
    game_state.score += 10;
}

4. Scheduling Systems

Add systems to the App builder, defining execution order if needed.

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<GameState>()
        .add_systems(Update, (movement_system, score_system).chain())
        .run();
}

Examples

Example 1: Spawning Entities with Require Component

use bevy::prelude::*;

#[derive(Component, Reflect, Default)]
#[require(Velocity, Sprite)]
struct Player;

#[derive(Component, Default)]
struct Velocity {
    x: f32,
    y: f32,
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn((
        Player,
        Velocity { x: 10.0, y: 0.0 },
        Sprite::from_image(asset_server.load("player.png")), 
    ));
}

Example 2: Query Filters

Use With and Without to filter entities efficiently.

fn enemy_behavior(
    query: Query<&Transform, (With<Enemy>, Without<Dead>)>,
) {
    for transform in &query {
        // Only active enemies processed here
    }
}

Best Practices

• ✅ Do: Use Query filters (With, Without, Changed) to reduce iteration count.
• ✅ Do: Prefer Res over ResMut when read-only access is sufficient to allow parallel execution.
• ✅ Do: Use Bundle to spawn complex entities atomically.
• ❌ Don't: Store heavy logic inside Components; keep them as pure data.
• ❌ Don't: Use RefCell or interior mutability inside components; let the ECS handle borrowing.

Troubleshooting

Problem: System panic with "Conflict" error. Solution: You are likely trying to access the same component mutably in two systems running in parallel. Use .chain() to order them or split the logic.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.