Entities event setup

Reasoning

Entities may need to subscribe to events, eg. from the integration library, and update state when a new event comes in. In order to do this correctly, the entities should subscribe and register the update callback in the entity method async_added_to_hass. This entity method is called after the entity has been registered by the entity platform helper and the entity will now have all its interfaces available to call, such as self.hass and self.async_write_ha_state. Registering an update callback before this stage will cause errors if the callback eg. tries to access self.hass or write a state update. To avoid memory leaks, the entities should unsubscribe from the events, ie. unregister the update callback, in the entity method async_will_remove_from_hass.

Example implementation

In the example below, the self.client.events.subscribe returns a function that when called, unsubscribes the entity from the event. So we subscribe to the event in async_added_to_hass and unsubscribe in async_will_remove_from_hass.

sensor.py

class MySensor(SensorEntity):
    """Representation of a sensor."""
    
    unsubscribe: Callable[[], None] = None

    def __init__(self, client: MyClient) -> None:
        """Initialize the sensor."""
        self.client = client
    
    async def async_added_to_hass(self) -> None:
        """Subscribe to the events."""
        await super().async_added_to_hass()
        self.unsubscribe = self.client.events.subscribe("my_event", self._handle_event)
    
    async def async_will_remove_from_hass(self) -> None:
        """Unsubscribe from the events."""
        if self.unsubscribe:
            self.unsubscribe()
        await super().async_will_remove_from_hass()
    
    async def _handle_event(self, event: Event) -> None:
        """Handle the event."""
        ...
        self.async_write_ha_state()

info

The above example can be simplified using lifecycle functions. This saves the need to store the callback function in the entity.

    async def async_added_to_hass(self) -> None:
        """Subscribe to the events."""
        await super().async_added_to_hass()
        self.async_on_remove(
            self.client.events.subscribe("my_event", self._handle_event)
        )

Exceptions

There are no exceptions to this rule.