ENH: Cleaner LEEM triggering definition

[johnsinsheimer]

This is a revisit of this pull request below, as a cleaner way of using the callback
NSLS-II-ESM-1/profile_collection#6

I ran into this problem again this past week, and Tom suggested using subscribe with run=False, to get around the problem calling the function and finishing before the acquire put is even made.

Previously, there was also some code

def __init__(self, *args, **kwargs):
        def check_if_done(value, old_value, **kwargs):
            st = self.st
            if st is not None:
                if value == 0 and old_value == 1:
                    st._finished()
                    self.st = None

        super().__init__(*args, **kwargs)
        # On a background thread, listen for the server's response.
        self.start_acq.subscribe(check_if_done)
        self.st = None

But I wasn't sure of the purpose of this. Is this to deal with the possibility of the run engine starting when the acquisition in progress? Or why would we need this code in the init?

[mrakitin]

Looks good, thanks for the update, @johnsinsheimer!

I tend to use the approach applied some time ago at QAS using SubscriptionStatus: https://github.com/NSLS-II-QAS/profile_collection/blob/325105dbe5b5429e84f0a24b5f7efc5cbfd521c6/startup/20-motors.py#L77-L119.