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Bevy State Machines
Docs-first stub for Bevy app states and per-entity state data. Godot counterpart: godot-state-machine-advanced. Not a full port. Keywords: Bevy, States, NextState, OnEnter.
Archived 2026-10-23 · Source updated 2026-10-11 · License: LGPL-3.0. Check the original repository for the latest version.
SKILL.md instructions
# Bevy state machines (docs-first stub) Not a full port. The opened API is app-wide states in `bevy::state` 0.20.0. A per-entity graph is data plus systems, not a second engine type. ## When to use Load this when the app needs a large-scale mode (paused, in a menu, loading) or when one entity needs a small mode flag. Use `bevy::state` for the first. Use a component for the second. Do not merge them into one Godot-style state graph. ## ECS shape - **App states.** `bevy::state`: states are app-wide finite state machines. Three types implement `States`. - Standard states change only by setting the `NextState<S>` resource. - `SubStates` also use `NextState<S>`, and are removed from the `World` when source states are not in the required state. - `ComputedStates` derive from other states through a `compute` method. - **Transition schedules.** `OnEnter<S>`, `OnExit<S>`, and `OnTransition<S>` run systems on matching transitions. `StateTransitionEvent<S>` fires when state `S` changes. Run conditions: `in_state<S>` and `state_changed<S>`. - **Entity lifetime.** `DespawnOnEnter<S>` and `DespawnOnExit<S>` despawn entities on those transitions (`state_scoped`). - **Feature.** The `bevy` 0.20.0 crate page includes `bevy_state` in the `default_app` collection. The state module was opened; `App` installation method names were not. Do not invent `init_state` or similar. - **Per-entity mode.** Store an enum in a `Component` and branch in a system (`Query` from the ECS page). No per-entity state-machine type was opened. ## Godot idea to Bevy idiom | Idea | Bevy idiom (idea only) | | --- | --- | | App mode (menu, pause, play) | A `States` type and `NextState<S>` | | Run logic only in one mode | `in_state` or an `OnEnter` / `OnExit` / `OnTransition` schedule | | Child mode that exists only inside a parent mode | `SubStates` | | Mode derived from other modes | `ComputedStates` | | One actor's animation or combat mode | An enum `Component` plus a system. Not `States` unless it is truly app-wide | ## Gaps - State installation methods on `App` were not opened (`bevy::state::app` is listed, not read). - The states, sub-states, and computed-state examples named by the module were not opened. - Hierarchical per-entity graphs, transition tables, and animation state machines were not opened. ## Expert Knowledge Delta Library target in prose: **Bevy 0.20.0**. Lattice URLs stay unversioned. Opened canon only: `Component`, `Resource`, `Query`, `Commands`, `Startup`, `Update`, `Fixed` / `FixedMain` (`bevy::time`), `.chain()`, `Plugin`, `DefaultPlugins`, `MinimalPlugins`. Messages and observers are an ownership slot. This file does not invent their struct APIs. Systems run in parallel unless `.chain()` orders the ones that conflict. ### Who owns state | Piece | Owner | Who may mutate | Schedule | | --- | --- | --- | --- | | Data on one entity | `Component` | One system via `Query` | `Update` or `FixedMain` | | One value for the whole app | `Resource` | One system | `Startup` to insert, then the schedule that reads it | | Spawn | `Commands` | The spawning system | `Startup` for the initial world; later schedules for runtime spawn | | Order that must not race | `.chain()` | The chained systems only | Same schedule | | Plugin boundary | `Plugin` | `build` registers systems and resources | Before the app runs | Borrow rule: two systems that both mutably query the same component must be ordered (`.chain()`) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API. ### Decision | If you need | Do this | Do not | | --- | --- | --- | | Per-entity data | `Component` + `Query` | A Godot node script | | App-wide config or clock | `Resource` | Copy the value onto every entity | | Stable simulation step | `FixedMain` and the `Fixed` clock | Tie simulation to raw frame `Update` | | Startup spawn | `Commands` in `Startup` | Spawn from a render system | | Cross-system notice that is not shared state | Message or observer slot in `bevy-events-messaging` | A new event type invented in this file | | An API this skill did not open | A Gaps row | A fabricated crate (solver, navmesh module, net, XR) | Load `references/a-gate-notes.md` for the same table plus the gap rule. ## NEVER - Do not put `/0.20.0/` in lattice URLs. WHY: permalinks stay on `latest`. The 0.20.0 pin belongs in prose once. - Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless `.chain()` orders them. - Do not invent a physics, navigation, networking, or XR crate here. WHY: those rows are gaps, blocked, or revisit. - No Godot animation-tree or node state-machine types. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - No invented `App` state-setup methods. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - No third-party crates. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - No Creation Station content. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - No umbrella skill over Godot and Bevy. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - Do not mark this row `ported`. It is `docs_mapped`. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. ## Reference docs.rs links use the unversioned `latest` permalink. The library pin on bevy-master stays **0.20.0**. ### Official Documentation - [Migration guides](https://bevy.org/learn/migration-guides/introduction/) - Unversioned hop index. - [bevy crate](https://docs.rs/bevy/latest/bevy/) - `bevy_state` ships inside the `default_app` collection. - [bevy::state](https://docs.rs/bevy/latest/bevy/state/index.html) - `States`, `NextState`, `SubStates`, `ComputedStates`, transition schedules, `StateTransitionEvent`, `in_state`, `state_changed`, `DespawnOnEnter`, `DespawnOnExit`. - [Quick Start: ECS](https://bevy.org/learn/quick-start/getting-started/ecs/) - `Component` and `Query` for a per-entity mode flag. - [Examples](https://github.com/bevyengine/bevy/tree/latest/examples#examples) - Example tree on the `latest` branch. State examples were not opened. ### Related Skills #### Prerequisites - [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md) - Schedules versus a scene tree. - [bevy-data-resources](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-data-resources/SKILL.md) - `NextState<S>` is a resource. Per-entity modes are components. #### Complements - [bevy-events-messaging](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-events-messaging/SKILL.md) - Messages and observers that are not state transitions. - [godot-state-machine-advanced](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/godot-state-machine-advanced/SKILL.md) - Godot counterpart. Source idea only. Do not import Godot types. #### Downstream / consumers - [bevy-character-movement](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-character-movement/SKILL.md) - Actor motion. A jump or ground mode stays a component unless you opened a reason to use `States`. #### Master - [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md) - Bevy router.