It is not unusual to have to deal with multiple distinct components in which the behavior of a component is driven by things that happen in the other components. One can model such a situation using a single statechart with parallel states, or by plugging several statecharts into one main statechart (see :pysismic.model.Statechart.copy_from_statechart). The communication and synchronization between the components can be done either by using active(state_name) in guards, or by sending internal events that address other components.
However, we believe that this approach is not very convenient:
- all the components must be defined in a single statechart;
- state name collision could occur;
- components must share a single execution context;
- component composition is not easy to achieve
- ...
Sismic allows to define multiple components in multiple statecharts, and brings a way for those statecharts to communicate and synchronize via events.
Every instance of :py~sismic.interpreter.Interpreter exposes a :py~sismic.interpreter.Interpreter.bind method which allows to bind statecharts.
sismic.interpreter.Interpreter.bind
When an interpreter interpreter_1 is bound to an interpreter interpreter_2 using interpreter_1.bind(interpreter_2), the internal events that are sent by interpreter_1 are automatically propagated as external events to interpreter_2. The binding is not restricted to only two statecharts. For example, assume we have three instances of :py~sismic.interpreter.Interpreter:
bind
from sismic.io import import_from_yaml from sismic.interpreter import Interpreter
interpreter_1 = Interpreter(import_from_yaml(filepath='examples/elevator/elevator.yaml')) interpreter_2 = Interpreter(import_from_yaml(filepath='examples/elevator/elevator_buttons.yaml')) interpreter_3 = Interpreter(import_from_yaml(filepath='examples/elevator/elevator_buttons.yaml'))
bind
assert isinstance(interpreter_1, Interpreter) assert isinstance(interpreter_2, Interpreter) assert isinstance(interpreter_3, Interpreter)
We define a bidirectional communication between the two first interpreters:
bind
interpreter_1.bind(interpreter_2) interpreter_2.bind(interpreter_1)
We also bind the third interpreters with the two first ones.
bind
interpreter_3.bind(interpreter_1) interpreter_3.bind(interpreter_2)
When an internal event is sent by an interpreter, the bound interpreters also receive this event as an external event. In the last example, when an internal event is sent by interpreter_3, then a corresponding external event is sent both to interpreter_1 and interpreter_2.
Note
Practically, unless you subclassed :py~sismic.interpreter.Interpreter, the only difference between internal and external events are the priority order in which they are processed by the interpreter.
bind
from sismic.interpreter import InternalEvent, Event
# Manually create and raise an internal event interpreter_3._raise_event(InternalEvent('test'))
print('Events for interpreter_1:', interpreter_1._external_events.pop()) print('Events for interpreter_2:', interpreter_2._external_events.pop()) print('Events for interpreter_3:', interpreter_3._internal_events.pop())
bind
Events for interpreter_1: Event('test') Events for interpreter_2: Event('test') Events for interpreter_3: InternalEvent('test')
Consider our running example, the elevator statechart. This statechart expects to receive floorSelected events (with a floor parameter representing the selected floor). The statechart operates autonomously, provided that we send such events.
Let us define a new statechart that models a panel of buttons for our elevator. For example, we consider that our panel has 4 buttons numbered 0 to 3.
/examples/elevator/elevator_buttons.yaml
As you can see in the YAML version of this statechart, the panel expects an event for each button: button_0_pushed, button_1_pushed, button_2_pushed and button_3_pushed. Each of those event causes the execution of a transition which, in turn, creates and sends a floorSelected event. The floor parameter of this event corresponds to the button number.
We bind our panel with our elevator, such that the panel can control the elevator:
buttons
from sismic.io import import_from_yaml from sismic.interpreter import Interpreter, Event, InternalEvent
elevator = Interpreter(import_from_yaml(filepath='examples/elevator/elevator.yaml')) buttons = Interpreter(import_from_yaml(filepath='examples/elevator/elevator_buttons.yaml'))
# Elevator will receive events from buttons buttons.bind(elevator)
Events that are sent to buttons are not propagated, but events that are sent by buttons are automatically propagated to elevator:
buttons
print('Awaiting events in buttons:', list(buttons._external_events)) # Empty buttons.queue(Event('button_2_pushed'))
print('Awaiting events in buttons:', list(buttons._external_events)) # External event
buttons.execute(max_steps=2) # (1) initialize buttons, and (2) consume button_2_pushed print('Awaiting events in buttons:', list(buttons._internal_events)) print('Awaiting events in elevator:', list(elevator._external_events))
buttons
Awaiting events in buttons: [] Awaiting events in buttons: [Event('button_2_pushed')] Awaiting events in buttons: [InternalEvent('floorSelected', floor=2)] Awaiting events in elevator: [Event('floorSelected', floor=2)]
The execution of bound statecharts does not differ from the execution of unbound statecharts:
buttons
elevator.execute() print('Current floor:', elevator.context.get('current'))
buttons
Current floor: 2