Event Transitions Example

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

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Publié le 26 janvier 2024 - Mis à jour le 26 janvier 2024

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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".

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

        QVBoxLayout *layout = new QVBoxLayout;
        layout-&gt;addWidget(button);
        layout-&gt;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".

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        QStateMachine *machine = new QStateMachine(this);

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

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

State s1 is the state "Outside" and state s2 is state "Inside".

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        QEventTransition *enterTransition = new QEventTransition(button, QEvent::Enter);
        enterTransition-&gt;setTargetState(s2);
        s1-&gt;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.

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        QEventTransition *leaveTransition = new QEventTransition(button, QEvent::Leave);
        leaveTransition-&gt;setTargetState(s1);
        s2-&gt;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.

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

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

        QEventTransition *releaseTransition = new QEventTransition(button, QEvent::MouseButtonRelease);
        releaseTransition-&gt;setTargetState(s2);
        s3-&gt;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.

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        machine-&gt;addState(s1);
        machine-&gt;addState(s2);
        machine-&gt;addState(s3);

        machine-&gt;setInitialState(s1);
        machine-&gt;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.

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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▲

Example project @ code.qt.io

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