Communication between statecharts¶
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
The communication and synchronization between the components can be done either by using
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.
Bind an interpreter or a callable to the current interpreter. Each time an internal event is sent by this interpreter, any bound object will be called with the same event. If interpreter_or_callable is an Interpreter instance, its queue method is called. This is, if i1 and i2 are interpreters, i1.bind(i2) is equivalent to i1.bind(i2.queue).
Parameters: interpreter_or_callable (
Any]]) – interpreter or callable to bind
Returns: self so it can be chained
When an interpreter
interpreter_1 is bound to an interpreter
interpreter_1.bind(interpreter_2), the internal events that are sent by
interpreter_1 are automatically
propagated as external events to
The binding is not restricted to only two statecharts.
For example, assume we have three instances of
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:
We also bind the third interpreters with the two first ones.
bind() returns the current interpreter, so multiple calls
can be chained:
When an internal event is sent by an interpreter, the bound interpreters also receive this event as an external
In the last example, when an internal event is sent by
interpreter_3, then a corresponding external event
is sent both to
Practically, unless you subclassed
Interpreter, the only difference between
internal and external events are the priority order in which they are processed by the interpreter.
from sismic.model 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())
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.
statechart: name: Elevator buttons description: | Buttons that remotely control the elevator. root state: name: active parallel states: - name: button_0 transitions: - event: button_0_pushed action: send('floorSelected', floor= 0) - name: button_1 transitions: - event: button_1_pushed action: send('floorSelected', floor= 1) - name: button_2 transitions: - event: button_2_pushed action: send('floorSelected', floor= 2) - name: button_3 transitions: - event: button_3_pushed action: send('floorSelected', floor= 3)
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:
from sismic.io import import_from_yaml from sismic.interpreter import Interpreter from sismic.model import Event, InternalEvent elevator = Interpreter(import_from_yaml(open('examples/elevator/elevator.yaml'))) buttons = Interpreter(import_from_yaml(open('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
are automatically propagated to
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))
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:
elevator.execute() print('Current floor:', elevator.context.get('current'))
Current floor: 2