Event Transitions Example

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 = 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. 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 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, 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 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.

Files:

© 2018 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.