Understanding QGraphicsItem::update() for Repainting in Qt Widgets

What it does

• QGraphicsItem::update() schedules a repaint request for the item, indicating that its appearance needs to be refreshed on the screen.
• In Qt's graphics framework, QGraphicsItem is the base class for items displayed in a QGraphicsScene.

When to use it

• Call update() whenever the visual representation of your QGraphicsItem changes due to:
  • Modification of its properties (e.g., position, size, color, shape)
  • Data updates that affect its appearance
  • Any internal logic that requires a redraw

What it doesn't do

• It doesn't directly perform the painting itself. The framework calls your item's paint() method to handle the actual drawing.
• It doesn't cause an immediate repaint. Qt processes paint requests efficiently within the event loop.

How it works

1. You call update() on your QGraphicsItem subclass.
2. Qt schedules a repaint request for the item, adding it to an internal queue.
3. When Qt enters the event loop and has time for painting, it processes the queued requests.
4. Qt calls your item's paint() method, passing a QPainter object for drawing.
5. You implement the paint() method to draw the item's desired visuals using the QPainter object. Qt provides various drawing primitives and functionalities within QPainter.

Example

#include <QtWidgets>

class MyItem : public QGraphicsItem {
public:
    MyItem(const QRectF& rect) : QGraphicsItem(rect) {}

    QRectF boundingRect() const override {
        return m_rect;
    }

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override {
        painter->setBrush(Qt::red);
        painter->drawRect(m_rect);
    }

private:
    QRectF m_rect;
};

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

    QGraphicsScene scene;
    MyItem* item = new MyItem(QRectF(0, 0, 100, 100));
    scene.addItem(item);

    QGraphicsView view(&scene);
    view.show();

    // ... (modify item properties or data that affect its appearance)

    item->update(); // Schedule repaint when the item needs to be redrawn

    return app.exec();
}

• Call update() whenever the item's appearance changes.
• Implement paint() to handle the actual drawing using QPainter.
• Override boundingRect() to provide the item's geometry information.
• Inherit from QGraphicsItem for custom items.

---

Update on Geometry Change

This example shows how to update an item when its size or position changes:

class ResizableRect : public QGraphicsItem {
public:
    ResizableRect(const QRectF& rect) : QGraphicsItem(rect) {}

    void setSize(const QSize& newSize) {
        prepareGeometryChange(); // Inform Qt about upcoming geometry change
        m_rect.setSize(newSize);
        update(); // Schedule repaint
    }

private:
    QRectF m_rect;

    QRectF boundingRect() const override {
        return m_rect;
    }

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override {
        painter->setBrush(Qt::blue);
        painter->drawRect(m_rect);
    }
};

// ... (usage in main function similar to previous example)

// Change size dynamically (e.g., in a slot connected to a user interaction)
connect(someButton, &QPushButton::clicked, [this, item]() {
    item->setSize(QSize(150, 150));
});

Update on Data Change

This example shows how to update an item based on external data changes:

class DataDisplayItem : public QGraphicsItem {
public:
    void setData(int value) {
        m_value = value;
        update(); // Schedule repaint based on new data
    }

private:
    int m_value;

    QRectF boundingRect() const override {
        return QRectF(0, 0, 100, 50); // Fixed size for this example
    }

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override {
        painter->drawText(boundingRect(), Qt::AlignCenter, QString::number(m_value));
    }
};

// ... (usage in main function)

DataDisplayItem* item = new DataDisplayItem();
scene.addItem(item);

// Update the item's data whenever needed
connect(someDataSource, &SomeDataSource::dataUpdated, [item](int newValue) {
    item->setData(newValue);
});

Update on User Interaction

This example demonstrates updating an item based on user interaction (mouse hover):

class InteractiveItem : public QGraphicsItem {
public:
    InteractiveItem(const QRectF& rect) : QGraphicsItem(rect) {}

private:
    void hoverEnterEvent(QGraphicsSceneHoverEvent* event) override {
        update(); // Schedule repaint on hover enter
    }

    void hoverLeaveEvent(QGraphicsSceneHoverEvent* event) override {
        update(); // Schedule repaint on hover leave
    }

    QRectF boundingRect() const override {
        return QRectF(0, 0, 50, 50); // Fixed size for this example
    }

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget) override {
        if (hasHoverEffect()) {
            painter->setBrush(Qt::green);
        } else {
            painter->setBrush(Qt::gray);
        }
        painter->drawRect(boundingRect());
    }
};

// ... (usage in main function)

InteractiveItem* item = new InteractiveItem(QRectF(0, 0, 50, 50));
scene.addItem(item);

---

Direct Painting in paint() Method

• Qt might decide to optimize repaints by batching them together, which wouldn't occur if you're constantly calling paint().
• This avoids the need for explicit update() calls, but it might lead to inefficiencies if repainting is triggered frequently based on minor changes.
• In simple cases, you can directly handle all drawing logic within the paint() method itself.

Using QTimer for Continuous Updates

• Within the slot, you can modify the item's properties or data, and Qt will automatically trigger a repaint when necessary due to the changes.
• Connect the timer's timeout signal to a slot that updates the item's data or state.
• For scenarios involving continuous updates (e.g., animations), consider using a QTimer.

Manual Repaint with QGraphicsScene::update()

• Use this cautiously as it bypasses Qt's internal optimization mechanisms for handling individual item updates.
• This method directly triggers a repaint request for the entire scene or a specific rectangle within it.
• If you have full control over the scene and need more granular control over repainting, you can use QGraphicsScene::update().

• Reserve QGraphicsScene::update() for scenarios where you need to force a repaint of the entire scene or a specific area, but be mindful of potential performance implications.
• Use QTimer for continuous updates where you want to control the update rate.
• If you have a simple paint() method that handles all drawing logic and repainting is infrequent, direct painting might be sufficient.
• For most cases, QGraphicsItem::update() is the recommended approach due to its efficiency and alignment with Qt's event loop.