bevy-ecs-patterns

# Bevy ECS Patterns Advanced patterns and techniques for Bevy's Entity Component System, focusing on performance, maintainability, and complex game architectures. ## When to Use This Skill | Use this skill when... | Use bevy-game-engine instead when... | |------------------------|--------------------------------------| | Optimizing ECS query performance | Setting up a new Bevy project from scratch | | Implementing complex entity relationships or hierarchies | Learning basic ECS concepts (components, systems, resources) | | Designing system scheduling and ordering | Handling input, assets, or game states | | Using change detection (`Changed<T>`, `Added<T>`) | Writing basic game logic or app structure | | Working with `ParamSet`, parallel iteration, or batch operations | Configuring rendering, UI, or audio | | Debugging archetype fragmentation or storage strategies | Adding plugins or common Bevy dependencies | ## Core Expertise **Advanced Queries** - Complex query filters and combinations - Query state and caching - Entity relationships and hierarchies - Parallel iteration strategies **System Scheduling** - System ordering and dependencies - Run conditions and gating - System sets and scheduling - Fixed timestep systems **Change Detection** - `Changed<T>` and `Added<T>` filters - `Ref<T>` for change tracking - Efficient reactive systems ## Advanced Query Patterns **Query Filters** ```rust use bevy::prelude::*; // Multiple filters fn targeting_system( query: Query< (&Transform, &mut Target), (With<Enemy>, Without<Dead>, Changed<Health>) >, ) { for (transform, mut target) in &query { // Only enemies that are alive and recently damaged } } // Optional components fn flexible_system( query: Query<(&Transform, Option<&Velocity>, Option<&Acceleration>)>, ) { for (transform, velocity, acceleration) in &query { if let Some(vel) = velocity { // Has velocity } } } // Or filters fn pickup_system( query: Query<Entity, Or<(With<HealthPickup>, With<AmmoPickup>)>>, ) { for entity in &query { // Process any pickup type } } ``` **Multiple Queries with Conflicts** ```rust // Use ParamSet when queries have conflicting access fn combat_system( mut param_set: ParamSet<( Query<&mut Health, With<Player>>, Query<&mut Health, With<Enemy>>, )>, ) { // Access one query at a time for mut health in param_set.p0().iter_mut() { health.0 += 1.0; // Heal player } for mut health in param_set.p1().iter_mut() { health.0 -= 10.0; // Damage enemies } } // Query transmutation for flexible access fn dynamic_query( all_entities: Query<(Entity, &Transform)>, players: Query<&Player>, ) { for (entity, transform) in &all_entities { if players.get(entity).is_ok() { // This is a player } } } ``` **Entity Relationships** ```rust // Parent-child hierarchies #[derive(Component)] struct Inventory; #[derive(Component)] struct InventorySlot(usize); fn spawn_inventory(mut commands: Commands) { commands.spawn((Inventory, SpatialBundle::default())) .with_children(|parent| { for i in 0..10 { parent.spawn((InventorySlot(i), SpriteBundle::default())); } }); } // Query parent from child fn child_system( children: Query<(&InventorySlot, &Parent)>, parents: Query<&Inventory>, ) { for (slot, parent) in &children { if let Ok(inventory) = parents.get(parent.get()) { // Access parent inventory } } } // Query children from parent fn parent_system( parents: Query<&Children, With<Inventory>>, slots: Query<&InventorySlot>, ) { for children in &parents { for &child in children.iter() { if let Ok(slot) = slots.get(child) { // Access child slot } } } } ``` ## Agentic Optimizations | Context | Command | |---------|---------| | Quick compile check | `cargo check 2>&1 \| head -30` | | Fast test run | `cargo test --lib -- --test-threads=1 -q` | | Check ECS usage errors | `cargo clippy -- -W clippy::all 2>&1 \| head -50` | | List components in project | `grep -rn "derive(Component)" src/ --include="*.rs"` | | List systems in project | `grep -rn "fn.*Query<" src/ --include="*.rs"` | | Find system ordering | `grep -rn "\.add_systems\|\.chain()\|\.after(\|\.before(" src/ --include="*.rs"` | | Check for ParamSet usage | `grep -rn "ParamSet" src/ --include="*.rs"` | For system scheduling, change detection, performance patterns, custom entity commands, and design best practices, see [REFERENCE.md](REFERENCE.md).