Factorial States Example

The statechart for calculating the factorial looks as follows:

In other words, the state machine calculates the factorial of 6 and prints the result.

 class Factorial : public QObject
 {
     Q_OBJECT
     Q_PROPERTY(int x READ x WRITE setX)
     Q_PROPERTY(int fac READ fac WRITE setFac)
 public:
     using QObject::QObject;

     int x() const
     {
         return m_x;
     }

     void setX(int x)
     {
         if (x == m_x)
             return;
         m_x = x;
         emit xChanged(x);
     }

     int fac() const
     {
         return m_fac;
     }

     void setFac(int fac)
     {
         m_fac = fac;
     }

 Q_SIGNALS:
     void xChanged(int value);

 private:
     int m_x = -1;
     int m_fac = 1;
 };

The Factorial class is used to hold the data of the computation, x and fac. It also provides a signal that's emitted whenever the value of x changes.

 class FactorialLoopTransition : public QSignalTransition
 {
 public:
     FactorialLoopTransition(Factorial *fact)
         : QSignalTransition(fact, &Factorial::xChanged), m_fact(fact)
     {}

     bool eventTest(QEvent *e) override
     {
         if (!QSignalTransition::eventTest(e))
             return false;
         QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
         return se->arguments().at(0).toInt() > 1;
     }

     void onTransition(QEvent *e) override
     {
         QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
         int x = se->arguments().at(0).toInt();
         int fac = m_fact->property("fac").toInt();
         m_fact->setProperty("fac",  x * fac);
         m_fact->setProperty("x",  x - 1);
     }

 private:
     Factorial *m_fact;
 };

The FactorialLoopTransition class implements the guard (x > 1) and calculations (fac = x * fac; x = x - 1) of the factorial loop.

 class FactorialDoneTransition : public QSignalTransition
 {
 public:
     FactorialDoneTransition(Factorial *fact)
         : QSignalTransition(fact, &Factorial::xChanged), m_fact(fact)
     {}

     bool eventTest(QEvent *e) override
     {
         if (!QSignalTransition::eventTest(e))
             return false;
         QStateMachine::SignalEvent *se = static_cast<QStateMachine::SignalEvent*>(e);
         return se->arguments().at(0).toInt() <= 1;
     }

     void onTransition(QEvent *) override
     {
         qInfo() << m_fact->property("fac").toInt();
     }

 private:
     Factorial *m_fact;
 };

The FactorialDoneTransition class implements the guard (x <= 1) that terminates the factorial computation. It also prints the final result to standard output.

 int main(int argc, char **argv)
 {
     QCoreApplication app(argc, argv);
     Factorial factorial;
     QStateMachine machine;

The application's main() function first creates the application object, a Factorial object and a state machine.

     QState *compute = new QState(&machine);
     compute->assignProperty(&factorial, "fac", 1);
     compute->assignProperty(&factorial, "x", 6);
     compute->addTransition(new FactorialLoopTransition(&factorial));

The compute state is created, and the initial values of x and fac are defined. A FactorialLoopTransition object is created and added to the state.

     QFinalState *done = new QFinalState(&machine);
     FactorialDoneTransition *doneTransition = new FactorialDoneTransition(&factorial);
     doneTransition->setTargetState(done);
     compute->addTransition(doneTransition);

A final state, done, is created, and a FactorialDoneTransition object is created with done as its target state. The transition is then added to the compute state.

     machine.setInitialState(compute);
     QObject::connect(&machine, &QStateMachine::finished, &app, QCoreApplication::quit);
     machine.start();

     return app.exec();
 }

The machine's initial state is set to be the compute state. We connect the QStateMachine::finished() signal to the QCoreApplication::quit() slot, so the application will quit when the state machine's work is done. Finally, the state machine is started, and the application's event loop is entered.

Example project @ code.qt.io