The core of Home Assistant is the event bus. The event bus allows any component to fire or listen for events. It is the core of everything. For example, any state change will be announced on the event bus as a state_changed event containing the previous and the new state of an entity.

Home Assistant contains a few built-in events that are used to coordinate between various components.

Event homeassistant_start

Event homeassistant_start is fired when all components from the configuration have been intitialized. This is the event that will start the timer firing off time_changed events.

Event homeassistant_stop

Event homeassistant_stop is fired when Home Assistant is shutting down. It should be used to close any open connection or release any resources.

Event state_changed

Event state_changed is fired when a state changes. Both old_state and new_state are state objects. Documentation about state objects.

Field Description
entity_id Entity ID of the changed entity. Example:
old_state The previous state of the entity before it changed. This field is ommitted if the entity is new.
new_state The new state of the entity. This field is ommitted if the entity is removed from the state machine.

Event time_changed

Event time_changed is fired every second by the timer and contains the current time.

Field Description
now A datetime object containing the current time in UTC.

Event service_registered

Event service_registered is fired when a new service has been registered within Home Assistant.

Field Description
domain Domain of the service. Example: light.
service The service to call. Example: turn_on

Event call_service

Event call_service is fired to call a service.

Field Description
domain Domain of the service. Example: light.
service The service to call. Example: turn_on
service_data Dictionary with the service call parameters. Example: { 'brightness': 120 }.
service_call_id String with a unique call id. Example: 23123-4.

Event service_executed

Event service_executed is fired by the service handler to indicate the service is done.

Field Description
service_call_id String with the unique call id of the service call that was executed. Example: 23123-4.

Event platform_discovered

Event platform_discovered is fired when a new platform has been discovered by the discovery component.

Field Description
service The service that is discovered. Example: zwave.
discovered Information that is discovered. Can be a dict, tuple etc. Example: (, 8889).

Event component_loaded

Event component_loaded is fired when a new component has been loaded and initialized.

Field Description
component Domain of the component that has just been initialized. Example: light.