Alternatives for Double-Click Event Handling in Qt Graphics Scenes

• mouseDoubleClickEvent()
  This is a virtual function inherited from QGraphicsItem. It gets called whenever the user performs a double-click on the area occupied by the QGraphicsProxyWidget within the scene.

• QGraphicsProxyWidget
  This class acts as an intermediary between a regular Qt widget and a QGraphicsScene. It allows you to embed standard widgets within the scene, enabling them to interact with the graphics scene's coordinate system.

Functionality

By default, QGraphicsProxyWidget doesn't implement any specific behavior for double-clicks. Instead, it forwards the event (a QGraphicsSceneMouseEvent object) to the embedded widget using the sendWidgetMouseEvent function.

This essentially passes the double-click event information to the widget as if it were a regular double-click on the widget itself. The embedded widget's own mouseDoubleClickEvent() handler (if it has one) will then be responsible for handling the double-click logic.

Key Points

• You can override this function in a custom subclass of QGraphicsProxyWidget to implement specific double-click behavior within the graphics scene context, if needed.
• It acts as a bridge, forwarding the event to the embedded widget for processing.
• QGraphicsProxyWidget::mouseDoubleClickEvent() itself doesn't perform any custom handling of double-clicks.

---

#include <QGraphicsProxyWidget>
#include <QPushButton>
#include <QMessageBox>

class MyDoubleClickableWidget : public QGraphicsProxyWidget {
  Q_OBJECT

public:
  MyDoubleClickableWidget(QWidget *widget) : QGraphicsProxyWidget(widget) {}

protected:
  void mouseDoubleClickEvent(QGraphicsSceneMouseEvent *event) override {
    // Check if left mouse button was double-clicked
    if (event->button() == Qt::LeftButton) {
      // Emit a custom signal (optional)
      emit doubleClicked();

      // Show a message box (example action)
      QMessageBox::information(nullptr, "Double Click!",
                               "You double-clicked on the widget!");
    }

    // Forward event to embedded widget (optional)
    QGraphicsProxyWidget::mouseDoubleClickEvent(event);
  }

signals:
  void doubleClicked();
};

1. We create a class MyDoubleClickableWidget that inherits from QGraphicsProxyWidget.
2. In the constructor, we take a pointer to the widget we want to embed (widget) and pass it to the base class constructor.
3. We override the mouseDoubleClickEvent() function.
4. Inside the function, we check if the left mouse button was double-clicked (event->button() == Qt::LeftButton).
5. If so, we can perform custom actions here, such as:
   • Emitting a custom signal (doubleClicked()) for further processing.
   • Showing a message box (example action).
6. Optionally, you can call the base class implementation (QGraphicsProxyWidget::mouseDoubleClickEvent(event)) to forward the event to the embedded widget for its own handling.

1. Create an instance of MyDoubleClickableWidget with your desired widget (MyWidget *widget = new MyWidget;).
2. Add the widget to a graphics scene using scene->addWidget(myDoubleClickableWidget);.
3. Connect to the doubleClicked() signal (if needed) to perform further actions based on the double-click event.

---

Subclassing QWidget

• Embed the subclassed widget directly into the scene using QGraphicsScene::addWidget().
• This allows you to implement your double-click logic directly within the widget itself, without relying on the graphics scene event handling.
• Instead of using QGraphicsProxyWidget, you can directly subclass the target QWidget and override its mouseDoubleClickEvent() function.

Using Event Filters

• This approach provides more flexibility as you can handle events for multiple widgets or the entire scene at once.
• Within the filter, you can check for double-click events and perform your custom logic.
• The event filter function (eventFilter()) will receive all events targeted at the widget or scene, including mouse double-clicks.
• Install an event filter on the QGraphicsProxyWidget or the scene itself.

Custom Signals and Slots

• This approach allows separation of concerns, keeping the widget responsible for event detection and signaling, and another object for handling the double-click logic.
• Emit the signal from within the widget's existing mouse event handler functions (e.g., mousePressEvent() or a custom slot connected to scene mouse events).
• Connect this signal to a slot in another object that handles the double-click logic.
• Define a custom signal (e.g., doubleClicked()) within your QGraphicsProxyWidget class.

Choosing the Right Alternative

The best alternative depends on your specific needs:

• Decoupling event detection and handling
  Custom signals and slots can improve code organization.
• Centralized event handling for multiple widgets
  Event filters provide more flexibility.
• Simple double-click handling within the widget
  Subclassing QWidget might be sufficient.