Enhancing Interaction: Handling mouseMoveEvent for a QGraphicsItem

Handling Context Menus and Mouse Events in Qt Graphics View

While working on a Qt application implementing a GraphicsView with two movable Square items, I encountered an intriguing issue. After interacting with a context menu associated with these Square items, any subsequent clicks on the QGraphicsView would unintentionally cause the item right-clicked previously to move to the location of the new click. This behavior was unexpected and certainly not what I wanted for a smooth user interaction. In this blog post, I will discuss how I addressed this undesired behavior by refining the event handling for the Square items.

Understanding the Problem

My implementation involved a GraphicsView containing multiple instances of a custom QGraphicsItem, specifically a class called Square. Each Square could be moved by dragging it around the scene. Additionally, I implemented a custom context menu for these squares, allowing specific actions to be triggered when interacting with them.

However, the issue arose after interacting with the context menu. If I opened the context menu on a Square and then clicked anywhere else in the scene, the square that originally spawned the menu would jump to the position where I clicked. This was clearly a problem related to how mouse events were being processed after the context menu was displayed.

Diagnosing the Issue

In my Square class, which inherits from QGraphicsItem, I implemented mouse event handlers (mousePressEvent, mouseMoveEvent, and mouseReleaseEvent) to manage the dragging of the square around the scene. After showing the context menu, it seemed that the click used to dismiss the menu was also triggering the mousePressEvent, causing the item to think it should move to the next location where the mouse was clicked.

Solving the Problem

To address this issue, I needed to ensure that the mouse press event that dismissed the context menu did not unintentionally trigger a move operation for the square. I achieved this by adding a flag to the Square class to keep track of whether the context menu had just been closed.

Here’s how I modified the Square and GraphicsView classes to incorporate this behavior:

Modifications in Square class:

class Square : public QObject, public QGraphicsItem {
    Q_OBJECT

public:
    explicit Square(QObject* parent = nullptr) : QObject(parent), QGraphicsItem(), ignoreNextPress(false) {}
    ~Square() {}

private:
    bool ignoreNextPress;

    QRectF boundingRect() const override { return {-3, -3, 6, 6}; }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override {
        painter->setPen(Qt::black);
        painter->setBrush(Qt::green);
        painter->drawRect(boundingRect());
        Q_UNUSED(option);
        Q_UNUSED(widget);
    }

    void mousePressEvent(QGraphicsSceneMouseEvent *event) override {
        if (ignoreNextPress) {
            ignoreNextPress = false;
            event->ignore();
        } else {
            // Normal processing here
        }
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override {
        if (!ignoreNextPress) {
            auto newPosition = mapToScene(event->pos());
            this->setPos(newPosition);
        }
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent* event) override {
        ignoreNextPress = false;
    }

    void setIgnoreNextPress(bool status) {
        ignoreNextPress = status;
    }
};

Modifications in GraphicsView::showContextMenu:

void GraphicsView::showContextMenu(const QPoint& pos) {
    // Existing code to setup context menu
    contextMenu.exec(mapToGlobal(pos));
    auto square = dynamic_cast<Square*>(item);
    if (square) {
        square->setIgnoreNextPress(true);
    }
}

Conclusion

By introducing a flag (ignoreNextPress) within the Square class, I effectively managed to control the behavior of mouse events following the interaction with the context menu. This solution required minimal additions to the code while adequately resolving the unintended move action caused by the context menu dismissal. This approach underscores the importance of precise control over event handling in graphical applications developed with frameworks like Qt, ensuring that user interactions remain intuitive and bug-free. This modification ensures that each component behaves as expected, providing a robust and user-friendly interface.