'Start/stop the task depending on how many subscribers we have
using it.
Args:
start_delay (int): Time to way before starting.'

def validate(self, start_delay=None):

    subs = self.subscriptions
    if self.task.running:
        if (not subs):
            self.task.stop()
    elif subs:
        self.task.start(self.interval, now=False, start_delay=start_delay)
