- Qt 5.5
- Qt Widgets
- Event Transitions Example
- Qt 5.5.1 Reference Documentation

# Event Transitions Example

    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);
        window.show();

        return app.exec();
    }

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

Files:

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