gecs

name

gecs

description

gecs ECS framework API reference — the Entity-Component-System addon for Godot 4.x used by this project. Covers Entity, Component, System, World, QueryBuilder, Relationship, Observer, CommandBuffer, SystemTimer. Use when the task involves ECS architecture: creating entities with components, defining component data classes (C_ prefix), writing game logic systems, querying entities by component composition, entity relationships or links, reactive observers for component changes, safe structural changes during iteration, system tick rates, ECS world setup, debugging ECS queries or cache, or deferred entity destruction. Also trigger on gecs API calls: q.with_all(), ECS.world, ECS.process(), define_components(), cmd.add_component(). gecs has zero LLM training data — without this skill all ECS API calls will be fabricated. NOT relevant for standard Godot subsystems (physics, animation, UI, tilemap, shader, particles, navigation, signals, audio) unless explicitly connected to ECS.

gecs — ECS Framework for Godot 4.x

$ARGUMENTS

gecs is the ECS backend for GodotMaker. It has zero LLM training data coverage — all API knowledge must come from this skill.

Core Concept Mapping

gecs Class	Godot Base	Key Insight
Entity	extends Node	Entity IS a Node — lives in scene tree, can have child nodes
Component	extends Resource	Pure data, @export properties with defaults, no logic
System	extends Node	Contains game logic, queries entities, placed in scene tree
World	extends Node	Manages all entities/systems, archetype storage, query engine
QueryBuilder	extends RefCounted	Chain API: with_all/with_any/with_none, auto-cached
Relationship	extends Resource	Pair (relation_component, target), archetype-level indexing
Observer	extends Node	Reactive: fires on component add/remove/change events
CommandBuffer	extends RefCounted	Safe structural changes during iteration via cmd
ECS	Autoload singleton	Global access: ECS.world, ECS.process(delta, group)

Quick Start

# --- Component (pure data, extends Resource) ---
class_name C_Health extends Component
@export var current: float = 100.0
@export var maximum: float = 100.0

class_name C_Velocity extends Component
@export var direction: Vector3 = Vector3.ZERO
@export var speed: float = 100.0

# --- Entity (extends Node, define default components) ---
class_name Player extends Entity
func define_components() -> Array:
    return [C_Health.new(), C_Velocity.new()]

func on_ready():
    add_to_group("player")

# --- System (game logic, extends Node) ---
class_name MovementSystem extends System
func query() -> QueryBuilder:
    return q.with_all([C_Velocity])

func process(entities: Array[Entity], components: Array, delta: float) -> void:
    for entity in entities:
        var vel = entity.get_component(C_Velocity)
        var pos = entity.get_component(C_Position)
        pos.value += vel.direction * vel.speed * delta  # Entity is Node, not Node2D!

# --- Main scene processing ---
# main.gd
func _process(delta):
    ECS.process(delta, "input")
    ECS.process(delta, "gameplay")

func _physics_process(delta):
    ECS.process(delta, "physics")
    ECS.process(delta, "run-last")

Naming Conventions

Type	Class Name	File Name	Example
Component	C_Name	c_name.gd	C_Health / c_health.gd
System	NameSystem	s_name.gd	MovementSystem / s_movement.gd
Entity	Name	e_name.gd	Player / e_player.gd
Observer	NameObserver	o_name.gd	HealthUIObserver / o_health_ui.gd
Relationship component	R_Action	r_action.gd	R_ChildOf / r_child_of.gd

Common Operations

Entity

# Create programmatically
var entity = Player.new()
ECS.world.add_entity(entity)

# Instantiate from scene prefab (.tscn with Entity root)
var entity = preload("res://entities/e_player.tscn").instantiate()
get_tree().current_scene.add_child(entity)
ECS.world.add_entity(entity)

# Component operations (pass CLASS to get/has, INSTANCE to add/remove)
entity.add_component(C_Health.new(100))
var health = entity.get_component(C_Health)      # returns instance or null
var has = entity.has_component(C_Health)           # bool check
entity.remove_component(health)                    # pass the instance

# Enable/disable
entity.enabled = false     # excluded from queries
ECS.world.disable_entity(entity)
ECS.world.enable_entity(entity)

# Destroy (calls on_destroy, queue_free, cleans up relationships)
ECS.world.remove_entity(entity)

Query

# In a System — use q shorthand
func query() -> QueryBuilder:
    return q.with_all([C_Health, C_Velocity])   # must have ALL
             .with_any([C_Player, C_Enemy])     # must have at least ONE
             .with_none([C_Dead])                # must NOT have
             .enabled()                           # only enabled entities

# Batch component access (faster — avoids per-entity get_component):
func query() -> QueryBuilder:
    return q.with_all([C_Velocity]).iterate([C_Velocity])

func process(entities: Array[Entity], components: Array, delta: float):
    var velocities = components[0]   # Array of C_Velocity, same order as entities
    for i in entities.size():
        var pos = entities[i].get_component(C_Position)
        pos.value += velocities[i].direction * delta  # Entity is Node, not Node2D!

# Standalone query (outside a System):
var enemies = ECS.world.query.with_all([C_Health, C_Enemy]).execute()
var player = ECS.world.query.with_all([C_Player]).execute_one()

CommandBuffer (safe structural changes during iteration)

class_name LifetimeSystem extends System

func query():
    return q.with_all([C_Lifetime])

func process(entities: Array[Entity], components: Array, delta: float):
    for entity in entities:          # safe forward iteration
        var lt = entity.get_component(C_Lifetime)
        lt.time -= delta
        if lt.time <= 0:
            cmd.remove_entity(entity)                     # queued
        if should_upgrade(entity):
            cmd.remove_component(entity, C_OldState)      # queued
            cmd.add_component(entity, C_NewState.new())   # queued
    # auto-executes after system completes (FlushMode.PER_SYSTEM default)

Relationships

# Add a relationship
entity.add_relationship(Relationship.new(R_ChildOf.new(), parent_entity))

# Query entities with a relationship
var children = ECS.world.query.with_relationship([
    Relationship.new(R_ChildOf.new(), parent_entity)
]).execute()

# Wildcard query (any target)
var has_allies = entity.has_relationship(Relationship.new(R_AllyTo.new(), null))

# Remove with limit
entity.remove_relationship(Relationship.new(R_Buff.new(), null), 1)   # remove 1
entity.remove_relationship(Relationship.new(R_Effect.new(), null))    # remove all

System Groups & Scene Architecture

Main.tscn
+-- World (World node)
+-- Systems (Node)
|   +-- input (SystemGroup)
|   |   +-- PlayerControlsSystem
|   +-- gameplay (SystemGroup)
|   |   +-- HealthSystem
|   |   +-- DeathSystem
|   +-- physics (SystemGroup)
|   |   +-- MovementSystem
|   |   +-- CollisionSystem
|   +-- run-last (SystemGroup)
|       +-- PendingDeleteSystem
+-- Entities (Node — spawned entities go here)
+-- Level (Node3D — level geometry)

SystemGroup nodes auto-assign their name as the group property of child Systems.

Reference Files

Read the relevant file when you need detailed API beyond this quick reference:

Need	File	When to read
Entity lifecycle, prefabs, spawning	references/entity.md	Creating entities, scene prefab setup, on_ready/on_destroy
Component design, @export patterns	references/component.md	Defining new components, constructor patterns
System impl, CommandBuffer, timers	references/system.md	Writing systems, sub_systems, tick rates, deps, parallel
World setup, entity management	references/world.md	World init, add/remove entities/systems, process groups
Queries, Relationships, Observers	references/query.md	Complex queries, entity linking, reactive systems
Debug tools, profiling	references/debug.md	Runtime inspection, editor debugger, performance
Naming, file org, scene architecture	references/patterns.md	Project structure, cross-cutting patterns, ECS_DESIGN adaptation

MANDATORY: Read gotchas.md Before Writing ECS Code

Before writing ANY gecs code, read gotchas.md. It contains 19 hard-won pitfalls with wrong→correct code examples.

If you hit a compile or runtime error, check gotchas.md first — most ECS errors are covered there.

Critical Gotchas (summary)

1. All @export properties MUST have default values — Godot errors on Resource export without defaults
2. get_component() takes the CLASS, not an instance — entity.get_component(C_Health) not entity.get_component(health_instance)
3. Never put logic in Components — all behavior belongs in Systems
4. Use cmd for structural changes during iteration — direct add/remove during process() causes entity skipping
5. Avoid with_group() in queries — ~50x slower than with_all([C_Tag]) due to SceneTree traversal. Use tag components instead
6. define_components() must return fresh .new() instances — returning cached/shared instances causes state leakage between entities
7. Entity extends Node, NOT Node2D — entity.position does NOT work. Store position in a C_Position component. See gotchas.md G1
8. Component _init() must have default params — func _init(v: float = 0.0) so Component.new() works for duplication
9. Components are inaccessible before add_entity() — get_component() returns null until entity enters scene tree. See gotchas.md G2
10. Never call system.process() directly in tests — causes Array[Entity] type error + CommandBuffer not flushed. Never write test_system_has_query — q is null outside World. See G10, G14
11. add_system() second param is bool, not group name — set system.group before add_system(). See G15
12. world.process(delta) without group skips grouped systems — must pass group name. See G16
13. NodePath @export in .tscn unreliable for World — set entity_nodes_root/system_nodes_root in _init(). See G17
14. Area2D overlap data stale in _process — overlap systems must run in physics group. See G18
15. := type inference fails with ternary + null — use explicit type annotation. See G19