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Bevy Events Messaging
Docs-first stub for Bevy messages and observers. Godot counterpart: godot-signal-architecture. Not a full port. Keywords: Bevy, Message, Event, Observer.
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 events and messaging (docs-first stub) Not a full port. Two opened mechanisms: buffered messages, and events that run observers immediately. Neither is a signal bus. ## When to use Load this when one system should notice that something happened without storing that fact on every entity. Pick the mechanism from the pages below. Do not invent a third channel. ## ECS shape - **Messages (pull).** `bevy::ecs::message` 0.20.0 defines the `Message` trait as a buffered message for pull-based handling. `MessageWriter` writes. `MessageReader` reads and tracks what it already saw. `Messages` holds what occurred within the last two `Messages::update` calls. `message_update_system` calls `Messages::update` on registered messages. `signal_message_update_system` signals that update after `FixedUpdate` systems. Registration helpers on `App` were not opened. Do not invent an `add_message` name. - **Events (push).** The `Event` trait page: derive `Event`, add an observer, trigger. The sample uses `World::trigger` and `world.add_observer` with `On<Speak>`. `Commands::trigger` is also named. Observers run immediately inside the trigger call. `Event::Trigger` defaults to `GlobalTrigger` for derived events. - **Observers.** `bevy::ecs::observer`: an `Observer` is a component that holds a system. `On` is the system parameter that processes the event. `EntityEvent` is the page's pointer for events that target one entity. The entity-event trait page was not opened. - **State transitions.** `bevy::state` documents `StateTransitionEvent<S>` when state `S` changes. That type lives with states. See bevy-state-machines. ## Godot idea to Bevy idiom | Idea | Bevy idiom (idea only) | | --- | --- | | Something happened; readers poll later | A `Message` plus `MessageWriter` / `MessageReader` | | Something happened; listeners run now | An `Event` plus an `Observer` (`On<...>`) and `trigger` | | Notice on one entity | `EntityEvent` (trait page not opened; do not invent methods) | | Global signal singleton | Not this. Use a message type or a global event | ## Gaps - `App` methods that register a message type were not opened. - `EntityEvent` methods, entity-targeted trigger helpers, and observer run-condition helpers were not opened beyond the module index names. - Ordering between `message_update_system` and gameplay systems was not opened beyond the `FixedUpdate` signal function name. ## 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 signal types or a signal bus autoload. WHY: a Godot port, an unopened API, or a public lie fails the docs-first bar. - No invented message-registration or trigger 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/) - Unversioned API index. - [bevy::ecs::message](https://docs.rs/bevy/latest/bevy/ecs/message/index.html) - `Message`, `MessageWriter`, `MessageReader`, `Messages`, `message_update_system`. - [Event](https://docs.rs/bevy/latest/bevy/ecs/event/trait.Event.html) - `Event`, `On`, `World::trigger`, `Commands::trigger`, `add_observer`. - [bevy::ecs::observer](https://docs.rs/bevy/latest/bevy/ecs/observer/index.html) - `Observer` and `On`. - [Examples](https://github.com/bevyengine/bevy/tree/latest/examples#examples) - Example tree on the `latest` branch. ### Related Skills #### Prerequisites - [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md) - ECS ownership before adding a notice channel. - [bevy-ecs-composition](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-ecs-composition/SKILL.md) - Systems that write or read the notice. #### Complements - [bevy-state-machines](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-state-machines/SKILL.md) - `StateTransitionEvent` for app-wide state changes. - [godot-signal-architecture](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/godot-signal-architecture/SKILL.md) - Godot counterpart. Source idea only. Do not import Godot types. #### Downstream / consumers - None yet. Gameplay skills are not ported. #### Master - [bevy-master](https://github.com/thedivergentai/gd-agentic-skills/blob/main/skills/bevy-master/SKILL.md) - Bevy router.