bevy_gearbox

Statecharts for Bevy, designed for ECS-first workflows and data-driven tools. Pairs well with the visual editor bevy_gearbox_editor.

Overview

bevy_gearbox provides a state machine runtime modeled after XState’s core ideas (LCA-based transitions, entry/exit/transition phases, eventless transitions), adapted to Bevy’s ECS. It’s not a perfect logical match to XState; it prioritizes Bevy ergonomics, serialization, and editor integration.

Why use bevy_gearbox

• ECS-first design: states, transitions, and hierarchy are plain components and entities.
• Deterministic state changes: exit → transition actions → entry; LCA-based pathing.
• Data-driven: author machines in scenes; integrate with tools out of the box.
• Editor ecosystem: pairs with bevy_gearbox_editor for rapid iteration and live statechart inspection.

Core concepts

• Logical hierarchy (independent of scene graph): StateChildOf / StateChildren.
• Activation semantics:
  • Root remains active for the machine lifetime.
  • Entering a state enters all ancestors down to the leaf.
  • Self-transitions are external by default; use EdgeKind::Internal to suppress re-entry.
• Transition entities:
  • Source(Entity), Target(Entity), EdgeKind.
  • Event-driven transitions via EventEdge<E> components on edges.
  • Eventless transitions: AlwaysEdge.
  • Delayed transitions: After { duration } with cancellation on exit.
• Guards:
  • ECS-managed guard failure sets with per-edge checks.
  • Composition support planned via a small Logic tree.
• Actions envelope:
  • TransitionActions emitted between exits and entries.
  • Author “assign/raise/send/cancel” patterns as normal systems.

Quick start

1. Add the plugin and your event observers

app.add_plugins(GearboxPlugin)
   .add_observer(edge_event_listener::<YourEvent>);

2. Send events to the machine root; the listener routes to active leaves internally

commands.trigger_targets(YourEvent, machine_root);

3. Author via code or scenes

• Spawn states and edges with Source, Target, EdgeKind, EventEdge<E>, AlwaysEdge, After.
• Or load a scene authored with bevy_gearbox_editor.

Compatibility

License

Dual-licensed under MIT or Apache-2.0, at your option.