ObserversΒΆ
Observers are a way to generically add behavior to a StateMachine without changing its internal implementation.
One possible use case is to add an observer that prints a log message when the SM runs a transition or enters a new state.
Giving the Reusing transitions as example:
>>> from tests.examples.traffic_light_machine import TrafficLightMachine
>>> class LogObserver(object):
... def __init__(self, name):
... self.name = name
...
... def after_transition(self, event, source, target):
... print(f"{self.name} after: {source.id}--({event})-->{target.id}")
...
... def on_enter_state(self, target, event):
... print(f"{self.name} enter: {target.id} from {event}")
>>> sm = TrafficLightMachine()
>>> sm.add_observer(LogObserver("Paulista Avenue"))
TrafficLightMachine...
>>> sm.cycle()
Paulista Avenue enter: yellow from cycle
Paulista Avenue after: green--(cycle)-->yellow
'Running cycle from green to yellow'
Hint
The StateMachine
itself is registered as an observer, so by using .add_observer()
an
external object can have the same level of functionalities provided to the built-in class.
Tip
Domain models are also registered as an observer.
See also
See Actions, Validators and guards for a list of possible callbacks.
And also Dynamic dispatch to know more about how the lib calls methods to match their signature.