Statecharts · Problem 3

P3 · The Elevator Controller

Two genuine regions, a queue that must not be states, and the run-to-completion ordering that will catch you exactly once. Eleven tests.

The system

An elevator. The doors go closed → opening → open → closing → closed, with an obstruction sensor that sends a closing door back to opening. The car goes idle → up or down → idle. People press buttons for floors, and those requests queue up.

Three rules do the teaching. The car never moves unless the doors are closed. A request made while the doors are open is remembered, not dropped, and served the moment they shut. And nobody sends a “go” event — the car departs because there is somewhere to be and it is safe to go.

Step 1 — draw it

An eventless transition out of idle, guarded on “doors shut” and “there is a target above/below”. It fires the instant both become true, which may be when a request arrives or when the doors finish closing. You do not have to enumerate those two occasions.

Step 2 — build it

exercises/p3_elevator/machine.py — the skeleton. Its docstring is the contract: exact state ids, event names, and context attributes.

exercises/p3_elevator/test_machine.py — 11 acceptance tests. Do not edit them.

exercises/p3_elevator/_solution.py — a reference solution with commentary. Open it after yours is green, not before.

cd statecharts/exercises/p3_elevator
pytest -q

The trap

You will arrive at a floor, raise an internal “open the doors” event, and watch the car set off for the next floor anyway. Work out why before opening this.

The macrostep loop checks eventless transitions before it drains the internal queue (SCXML, and lesson 05). So at the moment the dispatcher's guard is evaluated, the “open the doors” event is still sitting in the queue and the doors are still closed. The guard is reading a true fact about a configuration that is about to be wrong.

Make the pending door-opening a value rather than a message: a flag in context, set by the arrival action. The dispatcher's guard reads it; the door region has its own eventless transition out of closed guarded on it; entering opening clears it. Both regions now see the same fact at the same instant, and neither names the other's states in a transition.

When it is green

Count the guards that mention the doors. There should be one. In the twelve-state product version that same rule has to appear on every arrow into every moving state, and again on every arrow you add next year — which is the difference between a rule and a convention.

← Lesson 05 Status map P4 →

Sources: SCXML Appendix D · processing model · RESOURCES.md