Bevy events and messaging

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. Check the App docs for message registration helpers before writing a method 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. Check that trait page for method names before writing them.
• State transitions. bevy::state documents StateTransitionEvent<S> when state S changes. That type lives with states. See bevy-state-machines.

Coming from Godot

Godot idea	Bevy idiom
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. Check the trait page for methods before writing them.
Global signal singleton	Not this. Use a message type or a global event

Scope

• Check the App docs for methods that register a message type before writing them.
• Check the observer module for EntityEvent methods, entity-targeted trigger helpers, and observer run-condition helpers beyond the module index names.
• Check the message docs for ordering between message_update_system and gameplay systems beyond the FixedUpdate signal function name.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their 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 outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• 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: this skill only names documented Bevy crates and modules.

• No Godot signal types or a signal bus autoload. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented message-registration or trigger methods. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No third-party crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

docs.rs links use the unversioned latest permalink. The library pin on bevy-master stays 0.20.0.

Official Documentation

• Migration guides - Unversioned hop index.
• bevy crate - Unversioned API index.
• bevy::ecs::message - Message, MessageWriter, MessageReader, Messages, message_update_system.
• Event - Event, On, World::trigger, Commands::trigger, add_observer.
• bevy::ecs::observer - Observer and On.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - ECS ownership before adding a notice channel.
• bevy-ecs-composition - Systems that write or read the notice.

Complements

• bevy-state-machines - StateTransitionEvent for app-wide state changes.
• godot-signal-architecture - Equivalent Godot skill. Do not import Godot types.

Downstream / consumers

• Gameplay skills cover combat, abilities, and similar loops.

Master

• bevy-master - Bevy router.