Qt Widgets: Understanding QGraphicsItem::setSelected()

Purpose

• This is crucial for interactive graphics applications where users need to select and manipulate items in the scene.
• When set to true, the item becomes visually highlighted, typically with a dashed border, indicating it's selected.
• Changes the selection state of a graphical item within a QGraphicsScene.

Usage

1. • Before using setSelected(), ensure the item's selection is enabled by setting the ItemIsSelectable flag using:

     item->setFlags(item->flags() | Qt::ItemIsSelectable);
     

2. Set Selection State
   

   • Call setSelected(bool selected) on the QGraphicsItem to change its selection state:

     item->setSelected(true);  // Select the item
     item->setSelected(false); // Deselect the item
     

Behavior

• When you set an item to selected, the following occurs:
  • The item's visual appearance might change to indicate selection (often a dashed border).
  • The item becomes part of the scene's selection, accessible through QGraphicsScene::selectedItems().
  • Events like mouse clicks and drags might be handled differently for selected items (e.g., used for dragging or editing).

Additional Considerations

• Multiple items can be selected simultaneously within a scene.
• You might need to implement custom selection handling in your QGraphicsItem subclass to respond to selection events (e.g., update appearance, handle dragging).
• setSelected() only affects the selection state within the scene. It doesn't necessarily change the item's internal state or behavior on its own.

Example

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

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

    // Create a scene and view
    QGraphicsScene scene;
    QGraphicsView view(&scene);

    // Create a rectangle item and enable selection
    QGraphicsRectItem *rect = new QGraphicsRectItem(0, 0, 100, 50);
    rect->setFlags(rect->flags() | Qt::ItemIsSelectable);
    scene.addItem(rect);

    // Connect a slot to handle item selection changes
    QObject::connect(&scene, &QGraphicsScene::selectionChanged, &view, [&view] {
        if (view.scene()->selectedItems().isEmpty()) {
            // No item selected
            qDebug() << "No item selected";
        } else {
            // Get the first selected item (adjust as needed)
            QGraphicsItem* selectedItem = view.scene()->selectedItems().first();
            qDebug() << "Item selected at" << selectedItem->pos();
        }
    });

    view.show();

    return app.exec();
}

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Selecting Multiple Items

This code shows how to iterate through selected items and print their properties:

#include <QGraphicsScene>
#include <QGraphicsItem>
#include <QDebug>

void handleSelectionChanged(const QList<QGraphicsItem*>& selectedItems) {
    qDebug() << "Selected items:";
    for (QGraphicsItem* item : selectedItems) {
        qDebug() << "  - Item type:" << item->type();
        qDebug() << "  - Position:" << item->pos();
    }
}

// ... (rest of your code)

// Connect the slot to selection changed signal
QObject::connect(&scene, &QGraphicsScene::selectionChanged, nullptr, Qt::QueuedConnection, qMaybeCast<void(*)(const QList<QGraphicsItem*>&)>(&handleSelectionChanged));

Custom Selection Handling

This code demonstrates subclassing QGraphicsItem and overriding the mousePressEvent handler to react differently based on the selection state:

class MyCustomItem : public QGraphicsItem {
    Q_OBJECT

public:
    MyCustomItem(const QRectF& rect) : QGraphicsItem(rect) {
        setFlags(flags() | Qt::ItemIsSelectable);
    }

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent* event) override {
        if (isSelected()) {
            // Handle selection-specific actions (e.g., drag, edit)
            qDebug() << "Selected item pressed at" << event->pos();
        } else {
            // Handle non-selected item press (e.g., single click actions)
            qDebug() << "Non-selected item pressed at" << event->pos();
        }
        QGraphicsItem::mousePressEvent(event);
    }
};

Programmatically Selecting All Items

This code shows how to select all items in the scene using QGraphicsScene::selectAll():

// ... (after adding items to the scene)

// Select all items
scene.selectAll();

Deselecting All Items

This code shows how to deselect all items in the scene using QGraphicsScene::clearSelection():

// ... (after items might be selected)

// Deselect all items
scene.clearSelection();

---

Custom Selection Handling

• This approach offers more granular control over item selection behavior tailored to your application's needs.
• This could involve maintaining a separate internal flag for selection and providing methods like setIsSelected(bool) to control it.
• Instead of relying solely on the visual selection state, you can implement your own selection logic within your QGraphicsItem subclass.

Using QGraphicsItem::setFlags()

• While not directly related to selection state, you can use QGraphicsItem::setFlags() to control certain aspects of item interaction that might simulate selection-like behavior.
  • For example, setting the ItemIsFocusable flag allows the item to receive keyboard focus, which can be used to visually highlight the focused item.
  • This approach doesn't involve the standard selection mechanism but can provide a focus-based indication for specific uses.

Using QGraphicsEffect (Qt 5.6 and later)

• While not a direct replacement for selection, it provides flexibility in how you visually represent item interaction.
• This could involve creating an effect that alters the item's appearance based on a custom selection flag or other criteria.
• In Qt versions 5.6 and above, you can leverage the QGraphicsEffect class to create custom visual effects for your items.

Choosing the Right Method

• For more specialized selection logic or alternative visual cues, consider custom selection handling, setFlags(), or QGraphicsEffect.
• If you need standard selection behavior with visual highlighting and event handling, QGraphicsItem::setSelected() is the recommended choice.
• The best approach depends on your application's requirements.

• It's generally recommended to leverage the built-in selection features whenever possible for consistency and maintainability.
• Keep in mind that deviating from the standard selection mechanism might require additional work to ensure proper interaction and user experience.