如果互联网/设备/服务不可用，仅在不可用时记录一次，重新连接时记录一次#

class MyCoordinator(DataUpdateCoordinator[MyData]):
    """Class to manage fetching data."""

    def __init__(self, hass: HomeAssistant, client: MyClient) -> None:
        """Initialize coordinator."""
        super().__init__(
            hass,
            logger=LOGGER,
            name=DOMAIN,
            update_interval=timedelta(minutes=1),
        )
        self.client = client
    
    async def _async_update_data(self) -> MyData:
        try:
            return await self.client.get_data()
        except MyException as ex:
            raise UpdateFailed(f"The device is unavailable: {ex}")

class MySensor(SensorEntity):

    _unavailable_logged: bool = False

    async def async_update(self) -> None:
        try:
            data = await self.client.get_data()
        except MyException as ex:
            self._attr_available = False
            if not self._unavailable_logged:
                _LOGGER.info("The sensor is unavailable: %s", ex)
                self._unavailable_logged = True
        else:
            self._attr_available = True
            self._attr_native_value = data.value
            if self._unavailable_logged:
                _LOGGER.info("The sensor is back online")
                self._unavailable_logged = False