'Sign up a subscriber to this ticker.
Args:
store_key (str): Unique storage hash for this ticker subscription.
args (any, optional): Arguments to call the hook method with.
Kwargs:
_start_delay (int): If set, this will be
used to delay the start of the trigger instead of
`interval`.'

def add(self, store_key, *args, **kwargs):

    if self._is_ticking:
        self._to_add.append((store_key, (args, kwargs)))
    else:
        start_delay = kwargs.pop('_start_delay', None)
        self.subscriptions[store_key] = (args, kwargs)
        self.validate(start_delay=start_delay)
