QGraphicsItem::focusItem() and Qt Widgets: A Mismatch

A Mismatch in Concepts

There seems to be a misunderstanding.

Qt Widgets is a different module, focusing on traditional desktop-style user interfaces.
It's used to determine which item within a graphics scene currently has focus.
QGraphicsItem::focusItem() is primarily associated with the Qt Graphics View Framework, not Qt Widgets.

Focus in Qt Widgets

Key points about focus in Qt Widgets

QWidget::focusWidget(): Returns the widget that currently has focus.
QWidget::hasFocus(): Returns true if the widget has input focus.
QWidget::setFocus(): Sets focus to a specific widget.

Bridging the Gap: QGraphicsView and Qt Widgets

Identify the widget
Determine the Qt Widget that is related to or contains the graphics view.
Obtain the graphics scene
Access the graphics scene associated with the graphics view.
Determine focused item
Use QGraphicsItem::focusItem() on the graphics scene to get the focused item.
Interact with both
Based on the focused item, you can manipulate both the graphics item and the corresponding widget.

Example

#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsItem>
#include <QWidget>

// ...

void someFunction(QGraphicsView* view, QWidget* relatedWidget) {
    QGraphicsScene* scene = view->scene();
    QGraphicsItem* focusedItem = scene->focusItem();

    // Do something with focusedItem based on its type and properties

    // Interact with relatedWidget based on the focused item
    // ...
}

Custom focus implementation
For complex scenarios, you might need to implement custom focus handling.
Event handling
You might need to handle focus-related events like focusIn(), focusOut(), keyPressEvent(), and keyReleaseEvent() to manage focus behavior appropriately.
Focus policies
Both Qt Widgets and Qt Graphics View Framework have focus policies to control how focus behaves.

Remember
The specific implementation depends heavily on your application's structure and requirements.

For instance:

What kind of interactions do you expect between the graphics items and the widgets?
How is the graphics view integrated into your Qt Widgets application?
What are you trying to achieve with QGraphicsItem::focusItem()?

If you're aiming to interact with elements in a Qt Widgets application, you'll primarily use functions like QWidget::setFocus(), QWidget::hasFocus(), and QWidget::focusWidget().

A Hypothetical Example (For Clarification)

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

class MyGraphicsView : public QGraphicsView {
public:
    MyGraphicsView(QWidget *parent = nullptr) : QGraphicsView(parent) {
        setScene(new QGraphicsScene(this));

        // Create some items
        QGraphicsRectItem* rect1 = scene()->addRect(0, 0, 100, 100);
        QGraphicsRectItem* rect2 = scene()->addRect(150, 150, 100, 100);

        // Set focus to rect1
        rect1->setFocus();
    }

    void keyPressEvent(QKeyEvent *event) {
        if (event->key() == Qt::Key_Tab) {
            QGraphicsItem* focusedItem = scene()->focusItem();
            if (focusedItem) {
                // Logic to focus the next item
            }
        }
        QGraphicsView::keyPressEvent(event);
    }
};

Focusing on Qt Widgets

#include <QWidget>
#include <QPushButton>

class MyWidget : public QWidget {
public:
    MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
        QPushButton* button1 = new QPushButton("Button 1", this);
        QPushButton* button2 = new QPushButton("Button 2", this);

        // Layout the buttons
        // ...

        // Set focus to button1
        button1->setFocus();
    }
};

If you have a Qt Graphics View embedded in a Qt Widget and need to interact between the two, you might use signals and slots or direct access to the widgets and items. However, this is generally discouraged for maintainability and separation of concerns.

Please provide more details about your specific use case if you need further assistance.

Are you facing issues with focus behavior in either framework?
Do you need to update a widget's properties based on a graphics item's state?
Are you trying to focus a widget based on a selected graphics item?

Before we delve into alternatives, it's essential to clarify the specific use case for QGraphicsItem::focusItem(). Are you trying to:

Something else entirely?
Handle focus-related events in a graphics item?
Determine which item in a graphics scene has focus?

Alternatives for Determining Focused Item

If you're aiming to find the focused item in a graphics scene, here are some approaches:

Directly Query the Scene:

QGraphicsScene::focusItem(): This is the most direct equivalent to QGraphicsItem::focusItem() at the scene level. It returns the item with focus within the entire scene.

Iterate Over Items:

If you need to check focus for specific items or groups, iterate over the scene's items and use QGraphicsItem::hasFocus() on each item.

Custom Focus Management:

For complex scenarios, you might implement a custom focus management system using signals and slots or other mechanisms to track focused items.

Alternatives for Handling Focus-Related Events

If you're dealing with focus-related events in a graphics item, consider these options:

Override Focus Events:

QGraphicsItem::focusInEvent() and QGraphicsItem::focusOutEvent(): These virtual functions are called when the item gains or loses focus.

Use Key Press/Release Events:

QGraphicsItem::keyPressEvent() and QGraphicsItem::keyReleaseEvent(): While not exclusively for focus, these events can be used to handle keyboard input, which is often tied to focus.

Custom Event Handling:

For intricate focus behavior, create custom events or signals to manage focus-related actions.

// Directly query the scene
QGraphicsItem* focusedItem = scene->focusItem();

// Iterate over items
for (QGraphicsItem* item : scene->items()) {
    if (item->hasFocus()) {
        // Item has focus
    }
}

// Override focus events
void MyItem::focusInEvent(QFocusEvent* event) {
    // Handle focus gained
}

void MyItem::focusOutEvent(QFocusEvent* event) {
    // Handle focus lost
}

Remember
The best approach depends on your specific requirements and the complexity of your application.

Please provide more details about your use case if you need further guidance.