Alternatives to Non-Existent QGraphicsScene::activePanel() Function

• QGraphicsScene: This class in Qt provides a surface for managing 2D graphical items like lines, rectangles, text, etc. It acts like a canvas where you can add and manipulate these visual elements.

• QGraphicsView::focusObject(): This function, belonging to QGraphicsView (the class that displays the scene), returns a pointer to the item that currently has focus within the view.

• QGraphicsItem::isActive(): This function exists within QGraphicsItem (which inherits from QObject). It checks if the item is currently under user interaction, like having focus or being clicked.

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Example 1: Checking for Active Item

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsRectItem>

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

  // Create a graphics scene
  QGraphicsScene scene;

  // Add a rectangle item to the scene
  QGraphicsRectItem *rectItem = new QGraphicsRectItem(0, 0, 100, 50);
  scene.addItem(rectItem);

  // Create a graphics view to display the scene
  QGraphicsView *view = new QGraphicsView(&scene);

  // Connect the scene's item selection changed signal to a slot
  QObject::connect(&scene, &QGraphicsScene::selectionChanged, [&](){
      // Check if an item is selected (active)
      if (scene.selectedItems().isEmpty()) {
          qDebug() << "No item is actively selected";
      } else {
          // Access the first selected item (assuming single selection)
          QGraphicsItem *selectedItem = scene.selectedItems().first();
          qDebug() << "Item with name " << selectedItem->objectName() << " is actively selected";
      }
  });

  view->show();

  return app.exec();
}

This code demonstrates how to connect to the selectionChanged signal of the scene. Whenever the selection changes (meaning an item becomes actively selected or deselected), the connected slot checks if any item is selected and optionally retrieves information about the selected item.

Example 2: Highlighting Active Item

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QPen>

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

  // Create a graphics scene
  QGraphicsScene scene;

  // Add a rectangle item to the scene
  QGraphicsRectItem *rectItem = new QGraphicsRectItem(0, 0, 100, 50);
  scene.addItem(rectItem);

  // Create a graphics view to display the scene
  QGraphicsView *view = new QGraphicsView(&scene);

  // Pen for highlighting the active item
  QPen activePen(Qt::red, 3);

  // Connect the scene's item selection changed signal to a slot
  QObject::connect(&scene, &QGraphicsScene::selectionChanged, [&](){
      // Iterate through all items
      foreach (QGraphicsItem *item, scene.items()) {
          if (scene.isSelected(item)) {
              item->setPen(activePen);  // Set pen for active item
          } else {
              item->setPen(QPen());  // Reset pen for non-active items
          }
      }
  });

  view->show();

  return app.exec();
}

This example builds upon the previous one. It introduces a pen to visually distinguish the actively selected item. Whenever the selection changes, the code iterates through all items in the scene and sets the pen based on whether the item is selected (active) or not.

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1. • Use QGraphicsScene::selectedItem() or iterate through QGraphicsScene::selectedItems() to get the actively selected item(s) within the scene.
   • Connect to signals like QGraphicsScene::selectionChanged to track selection changes and identify the newly active item.

2. Custom Implementation
   

   • If you require panel-like behavior, you can create custom item classes that inherit from QGraphicsItem and implement your logic for handling active/inactive states. You can manage these custom items within the scene and track their active states based on user interaction or other criteria.