GECS Observers in Godot 4.3+

Implement reactive, event-driven game logic using GECS v9 Observers. Unlike systems which tick every frame, an Observer activates only when specific lifecycle transitions occur (components added/removed, properties mutated, query match sets changed, or custom events dispatched).

Addon: GECS · version v9 · Godot 4.3+ · MIT
Related skills: gecs-core for primitives, gecs-systems for frame-based logic, gecs-queries for query construction.

1. Anatomy of an Observer

An observer extends Observer (Node) and implements query() with event modifiers and the each() callback:

class_name HealthObserver
extends Observer