Destructor of QGraphicsSceneMoveEvent: When and Why

Understanding QGraphicsSceneMoveEvent

• Inheritance
  It inherits from QGraphicsSceneEvent, which itself inherits from the generic QEvent class.
• Purpose
  This class in Qt's graphics scene framework signals that a QGraphicsWidget (a widget embedded in a graphics scene) has been moved.

Destructor: ~QGraphicsSceneMoveEvent()

• Automatic Memory Management
  In Qt, memory management for most objects is handled automatically. This means that when a QGraphicsSceneMoveEvent object goes out of scope or is explicitly deleted, the destructor is called implicitly to free any memory it was using.
• Function
  This destructor is responsible for cleaning up any resources allocated by the QGraphicsSceneMoveEvent object when it's no longer needed.

Key Points

• You typically don't need to call this destructor manually, as Qt's memory management takes care of it.

How QGraphicsSceneMoveEvent Works

• In the context of QGraphicsSceneMoveEvent, a slot might be used to update the widget's position in your application's data model or perform other actions based on the movement.
• Slots are member functions of your classes that can be connected to signals to perform specific actions when the signal is emitted.
• This signal is then received by any objects that have connected to it using a Qt slot mechanism.
• When a QGraphicsWidget is moved within a graphics scene, it emits a QGraphicsSceneMoveEvent signal.

#include <QGraphicsScene>
#include <QGraphicsWidget>

class MyWidget : public QGraphicsWidget {
public:
    // ... other widget-related code

    void handleWidgetMove(QGraphicsSceneMoveEvent* event) {
        // Access the new and old positions using event->newPos() and event->oldPos()
        QPointF newPos = event->newPos();
        // Update your data model or perform other actions based on the new position
        // ...
    }
};

int main() {
    QGraphicsScene scene;
    MyWidget* widget = new MyWidget(&scene);
    scene.addItem(widget);

    // Connect the widget's move signal to the handleWidgetMove slot
    connect(widget, &QGraphicsWidget::itemChange, widget, &MyWidget::handleWidgetMove);

    // ... (code to move the widget)

    return 0;
}

---

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsWidget>
#include <QMouseEvent>

class MyWidget : public QGraphicsWidget {
    Q_OBJECT

public:
    MyWidget(QGraphicsScene *scene) : QGraphicsWidget(scene) {
        setFlag(QGraphicsItem::ItemIsMovable);
    }

protected:
    void mouseMoveEvent(QMouseEvent *event) override {
        QGraphicsWidget::mouseMoveEvent(event);
        update(); // Trigger repaint after movement
    }
};

class MyScene : public QGraphicsScene {
    Q_OBJECT

public:
    void mousePressEvent(QMouseEvent *event) override {
        if (event->button() == Qt::LeftButton) {
            selectedWidget = itemAt(event->pos(), QGraphicsItem::GraphicsItemSelection);
        }
        QGraphicsScene::mousePressEvent(event);
    }

    void mouseReleaseEvent(QMouseEvent *event) override {
        selectedWidget = nullptr;
        QGraphicsScene::mouseReleaseEvent(event);
    }

private:
    QGraphicsItem *selectedWidget = nullptr;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    // Create the graphics scene
    MyScene scene;

    // Add a widget to the scene
    MyWidget *widget = new MyWidget(&scene);
    scene.addItem(widget);
    widget->setPos(100, 100); // Set initial position

    // Create a graphics view to display the scene
    QGraphicsView view(&scene);
    view.setFixedSize(400, 300);
    view.show();

    return app.exec();
}

In this example:

• The main function creates the scene, widget, and view, setting up the visual elements.
• MyScene handles mouse events to track the currently selected widget during dragging.
• The mouseMoveEvent overrides Qt's default behavior and triggers a repaint after movement for visual feedback.
• MyWidget inherits from QGraphicsWidget and sets the ItemIsMovable flag to allow dragging.

---

1. Overriding QGraphicsItem::itemChange
   

   • This event is emitted whenever a property of the QGraphicsItem (including its position) changes.
   • You can override the itemChange method in your custom QGraphicsWidget class to detect movement:

   class MyWidget : public QGraphicsWidget {
   public:
       // ... other widget-related code
   
       bool itemChange(GraphicsItemChange change, const QVariant &value) override {
           if (change == QGraphicsItem::ItemPositionChange) {
               // Access the new position using value.toPoint()
               QPointF newPos = value.toPoint();
               // Update your data model or perform other actions based on the new position
               // ...
               return true;
           }
           return QGraphicsWidget::itemChange(change, value);
       }
   };
   

   Here, you check if the change is QGraphicsItem::ItemPositionChange and then extract the new position using value.toPoint(). This allows you to react to the movement without relying on the specific QGraphicsSceneMoveEvent class.

2. Using QGraphicsItem::pos()
   

   • You can directly access the current position of the QGraphicsItem using the pos() method at any point in your code. This can be used within event handlers or other parts of your application logic:

   void MyWidget::handleAction() {
       QPointF currentPos = pos();
       // Use currentPos for any calculations or updates based on the widget's position
       // ...
   }
   

   This approach provides direct access to the position without relying on events, but it might require more manual checking for updates in your code.

3. • While less efficient, you can set up a timer that periodically checks the position of the widget using pos(). This might be useful for scenarios where you need to react to position changes at specific intervals.

The choice of method depends on your specific needs:

• Timers are less efficient but could be suitable for specific use cases.
• pos() offers direct access but requires more manual checking.
• itemChange is a good balance of flexibility and event-driven approach.