Tracking Mouse Movement in Qt Widgets: Alternatives to oldPosF()

Purpose

• This information is useful in various scenarios where you want to track the mouse movement within your widget and react accordingly.
• QHoverEvent::oldPosF() is a member function that retrieves the previous position of the mouse cursor relative to the widget that received the hover event.
• In Qt event handling, QHoverEvent is a class that encapsulates information about mouse hover events within a widget.

Data Type

• oldPosF() returns a const QPointF& (reference to a constant floating-point 2D point). This means you can access the X and Y coordinates of the previous cursor position using the dot operator (.) on the returned reference.

Usage

1. Include the Header
   Make sure your code includes the <QHoverEvent> header:

   #include <QHoverEvent>
   

2. Handle Hover Events
   

   • Override the hoverEvent() method in your widget class to receive hover events:

   class MyWidget : public QWidget {
       Q_OBJECT
   
   public:
       void hoverEvent(QHoverEvent* event) override {
           QPointF oldPos = event->oldPosF();
           // Use oldPos.x() and oldPos.y() to access previous coordinates
           // ...
       }
   
       // ... other widget methods
   };
   

Example

#include <QApplication>
#include <QWidget>
#include <QDebug>

class MyWidget : public QWidget {
    Q_OBJECT

public:
    MyWidget(QWidget* parent = nullptr) : QWidget(parent) {}

protected:
    void hoverEvent(QHoverEvent* event) override {
        QPointF oldPos = event->oldPosF();
        qDebug() << "Mouse moved from (" << oldPos.x() << ", " << oldPos.y() << ")";
    }
};

int main(int argc, char* argv[]) {
    QApplication app(argc, argv);
    MyWidget widget;
    widget.show();
    return app.exec();
}

Important Note

• As of Qt version 6.0, QHoverEvent::oldPosF() is marked as deprecated. It's recommended to use the integer-based oldPos() function (QPoint QHoverEvent::oldPos() const) instead for better performance and consistency.

Key Points

• By tracking mouse movement within your widget using oldPosF() or oldPos(), you can create interactive and dynamic user experiences in your Qt GUI applications.
• Choose the appropriate function based on your application's requirements and Qt version.
• oldPos() offers integer-based coordinates, which might be sufficient for most use cases.
• oldPosF() provides floating-point precision for the previous cursor position (deprecated in Qt 6).

---

#include <QApplication>
#include <QWidget>
#include <QDebug>

class MyWidget : public QWidget {
    Q_OBJECT

public:
    MyWidget(QWidget* parent = nullptr) : QWidget(parent) {}

protected:
    void hoverEvent(QHoverEvent* event) override {
        // Using oldPosF() (deprecated in Qt 6)
        QPointF oldPosF = event->oldPosF();
        qDebug() << "Previous position (floating-point): (" << oldPosF.x() << ", " << oldPosF.y() << ")";

        // Using oldPos()
        QPoint oldPos = event->oldPos();
        qDebug() << "Previous position (integer): (" << oldPos.x() << ", " << oldPos.y() << ")";
    }
};

int main(int argc, char* argv[]) {
    QApplication app(argc, argv);
    MyWidget widget;
    widget.show();
    return app.exec();
}

In this code:

1. We override the hoverEvent() method to receive hover events.
2. Inside hoverEvent(), we call both oldPosF() and oldPos() to retrieve the previous cursor position.
3. oldPosF() provides the coordinates as floating-point values, offering more precise calculations if needed.
4. oldPos() returns integer coordinates, which might be sufficient for most scenarios.
5. The debug output (qDebug()) displays both sets of coordinates for comparison.

---

1. QHoverEvent::oldPos()
   This function is the preferred replacement for oldPosF() in Qt 6 and later. It returns a QPoint object containing the previous cursor position as integer coordinates (X and Y). This is generally sufficient for most use cases as it provides accurate positioning for common GUI interactions.

This approach allows you to keep track of the cursor's movement with floating-point precision if needed, but it requires more manual handling compared to using oldPos().

Choosing the Right Alternative

• If you specifically require floating-point precision for calculations, consider manual tracking using your own state variables.
• For most scenarios, using QHoverEvent::oldPos() (Qt 6 and later) is the recommended choice due to its simplicity and performance benefits.