Event Transitions Example

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

The Event Transitions Example illustrates how states change when a user enters or leaves the area of a button. The states are handled by a QStateMachine object. The screen consists of a QVBoxLayout with a central button.

When the mouse is outside the button, the text in the button displays “Outside”. When the mouse enters the button, it displays “Inside”.

class Window : public QWidget
{
public:
    Window(QWidget *parent = nullptr)
        : 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. This button is added to layout , which is a QVBoxLayout object. Then two states are created: s1 is the state “Outside”, and s2 is the state “Inside”.

QStateMachine *machine = new QStateMachine(this);

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

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

State s1 is the state “Outside” and state s2 is state “Inside”.

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

When the button receives an event of type 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 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, state s3 is created. s3 will be entered when the button receives an event of type MouseButtonPress and the state machine is in state s2 . When the button receives an event of type MouseButtonRelease and the state machine is in state s3 , the machine will revert 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 that displays the QVBoxLayout object layout with its button .

Example project @ code.qt.io