Example implementation of a simplified elevator logic, using TinyFSM.

Imagine a elevator having:

• "Call" button on each floor,
• "Floor Sensor" on each floor, triggering an event as soon as the elevator arrives there,
• "Alarm" button.

The elevator example implements two state machines interacting with each other:

1. Elevator

   • State: Idle
   • State: Moving
   • State: Panic

2. Motor

   • State: Stopped
   • State: Up
   • State: Down

A good state machine design avoids circular dependencies at all cost: While the elevator sends events to the motor, the motor NEVER sends events to the elevator (top-down only).

Did you notice the motor starting twice? This is by design, let's have a look at the call stack of fsm_list::start() in main.cpp:

FsmList<Motor, Elevator>::start()
   Motor::set_initial_state()
      Motor::current_state = Stopped
   Elevator::set_initial_state()
      Elevator::current_state = Idle
   Motor::enter()
      Motor:Stopped->entry()
         cout << "Motor: stopped"               <-- HERE
         Motor::direction = 0
   Elevator::enter()
      Elevator:Idle->entry()
         send_event(MotorStop)
            Motor::react(MotorStop)
               Motor:Stopped->transit<Stopped>
                  Motor:Stopped->exit()
                  Motor::current_state = Stopped
                  Motor:Stopped->entry()
                     cout << "Motor: stopped"   <-- HERE
                     Motor::direction = 0
            Elevator::react(MotorStop)

If we really had to work around this, we could either:

1. Change the initialization (bad design practice!) in main.cpp:

   - fsm_list::start();
   + fsm_list::set_initial_state();
   + Elevator::enter();
   

2. Modify the Motor:Stopped->entry() function in motor.cpp:

     class Stopped : public Motor {
       void entry() override {
   +     if(direction == 0)
   +       return;
         cout << "Motor: stopped" << endl;
         direction = 0;
       };