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