Qt Reference Documentation

# Event Transitions Example

Files:

- statemachine/eventtransitions/main.cpp
- statemachine/eventtransitions/eventtransitions.pro

The Event Transitions example shows how to use event transitions, a feature of The State Machine Framework.

     class Window : public QWidget
     {
     public:
         Window(QWidget *parent = 0)
             : QWidget(parent)
         {
             QPushButton *button = new QPushButton(this);
             button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

             QVBoxLayout *layout = new QVBoxLayout;
             layout->addWidget(button);
             layout->setContentsMargins(80, 80, 80, 80);
             setLayout(layout);

The `Window` class's constructors begins by creating a button.

             QStateMachine *machine = new QStateMachine(this);

             QState *s1 = new QState();
             s1->assignProperty(button, "text", "Outside");

             QState *s2 = new QState();
             s2->assignProperty(button, "text", "Inside");

Two states, `s1` and `s2`, are created; upon entry they will assign "Outside" and "Inside" to the button's text, respectively.

             QEventTransition *enterTransition = new QEventTransition(button, QEvent::Enter);
             enterTransition->setTargetState(s2);
             s1->addTransition(enterTransition);

When the button receives an event of type QEvent::Enter and the state machine is in state `s1`, the machine will transition to state `s2`.

             QEventTransition *leaveTransition = new QEventTransition(button, QEvent::Leave);
             leaveTransition->setTargetState(s1);
             s2->addTransition(leaveTransition);

When the button receives an event of type QEvent::Leave and the state machine is in state `s2`, the machine will transition back to state `s1`.

             QState *s3 = new QState();
             s3->assignProperty(button, "text", "Pressing...");

             QEventTransition *pressTransition = new QEventTransition(button, QEvent::MouseButtonPress);
             pressTransition->setTargetState(s3);
             s2->addTransition(pressTransition);

             QEventTransition *releaseTransition = new QEventTransition(button, QEvent::MouseButtonRelease);
             releaseTransition->setTargetState(s2);
             s3->addTransition(releaseTransition);

Next, the state `s3` is created. `s3` will be entered when the button receives an event of type QEvent::MouseButtonPress and the state machine is in state `s2`. When the button receives an event of type QEvent::MouseButtonRelease and the state machine is in state `s3`, the machine will transition back to state `s2`.

             machine->addState(s1);
             machine->addState(s2);
             machine->addState(s3);

             machine->setInitialState(s1);
             machine->start();
         }
     };

Finally, the states are added to the machine as top-level states, the initial state is set to be `s1` ("Outside"), and the machine is started.

     int main(int argc, char **argv)
     {
         QApplication app(argc, argv);
         Window window;
         window.resize(300, 300);
     #if defined(Q_OS_SYMBIAN)
         window.showMaximized();
     #else
         window.show();
     #endif

         return app.exec();
     }

The main() function constructs a Window object and shows it.