Alternatives to QGraphicsItem::setActive() for Focus and Selection in Qt

Purpose

• Activated items typically receive keyboard focus and might have a visual indication of being selected.
• Controls the activation state of a QGraphicsItem within a QGraphicsScene.

Arguments

• bool active:
  • true: Activates the item if the scene is active.
  • false: Deactivates the item.

Behavior

• When setActive(false) is called:
  • The item's panel is deactivated regardless of the scene's activity or the item's presence in a scene.
• When setActive(true) is called:
  • If the item is already in an active scene, the item's panel (a concept related to internal handling) is activated.
  • If the item is not in an active scene:
    • The active parameter determines the panel's activation state when the scene becomes active or the item is added to a scene.
      • true: Panel is activated.
      • false: Panel remains deactivated.

Key Points

• setActive() works in conjunction with the scene's activity state.
• The concept of a "panel" is internal to Qt and might not be directly visible in your application's code.
• Activation primarily affects keyboard focus and potentially visual cues for selection.

Example

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

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

    // Create a scene and add a rectangle item
    QGraphicsScene scene;
    QGraphicsRectItem *item = new QGraphicsRectItem(0, 0, 100, 50);
    scene.addItem(item);

    // Initially deactivate the item
    item->setActive(false);

    // ... (other code)

    // When needed, activate the item
    item->setActive(true);

    // ...

    return app.exec();
}

In this example, the rectangle item is initially deactivated. When you want to give it keyboard focus and potentially make it visually selected, you'd call item->setActive(true).

Additional Considerations

• Consider using setFocus() for more granular control over keyboard focus within a scene.
• Specific behaviors might vary depending on the item type and your application's implementation.
• QGraphicsItem::isActive() can be used to query the current activation state of an item.

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Example 1: Deactivating All Items in a Scene

This code iterates through all items in a scene and deactivates them:

void deactivateAllItems(QGraphicsScene* scene) {
    foreach (QGraphicsItem* item, scene->items()) {
        item->setActive(false);
    }
}

You can call this function when you want to clear the selection or focus within the scene.

Example 2: Activating an Item When Clicked

This code connects the mousePressEvent signal of a QGraphicsItem to a slot that activates the item:

void MyGraphicsItem::mousePressEvent(QMouseEvent* event) {
    if (event->button() == Qt::LeftButton) {
        setActive(true);
        // Optionally, update the item's appearance to indicate selection
    }
    QGraphicsItem::mousePressEvent(event);
}

This allows the user to activate the item by clicking on it.

Example 3: Highlighting Active Items

This code demonstrates how to visually indicate the active item using a custom QGraphicsItem class:

class HighlightableItem : public QGraphicsRectItem {
    Q_OBJECT

public:
    HighlightableItem(const QRectF& rect) : QGraphicsRectItem(rect) {}

protected:
    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override {
        painter->setBrush(isActive() ? Qt::lightGray : Qt::white);
        painter->drawRect(boundingRect());
    }
};

In this example, the paint() method of the HighlightableItem class sets the brush color to light gray if the item is active, and white otherwise. This provides a visual cue for the user.

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setFocus()

• Example:
• Use this if you need to manage focus independently of scene activation or visual selection cues.
• Provides more granular control over keyboard focus within a scene.

// Activate and give focus to an item
item->setActive(true);
item->setFocus();

setSelected()

• Example:
• Use this if you want to visually highlight selected items without necessarily affecting keyboard focus.
• Primarily used for managing selection state, which might be visually indicated.

// Select an item (might involve visual change)
item->setSelected(true);

Custom Event Handling

• Example:
• This approach offers maximum flexibility for defining custom selection logic and visual cues.
• Implement custom event handlers (e.g., mousePressEvent, keyPressEvent) to manage focus and selection behavior tailored to your application.

void MyGraphicsItem::mousePressEvent(QMouseEvent* event) {
    if (event->button() == Qt::LeftButton) {
        // Implement custom selection logic based on conditions,
        // potentially using setFocus() or setSelected()
    }
    QGraphicsItem::mousePressEvent(event);
}

• If you require more control over focus or selection behavior, consider using setFocus(), setSelected(), or custom event handling based on your specific requirements.
• If you need both keyboard focus and visual selection cues that follow Qt's conventions, setActive() might be a good initial choice.