How to interactively resize a QGraphicsObject?

Question

I'm having trouble figuring out how to scale a custom sub classed QGraphicsObject in Qt. I have the code working so that the mouse cursor changes when hovering over the edges object's boundingRect(Left,Right,Top,Bottom). I have the overloaded mouse events working properly. And I have custom methods that are trying to change the boundingRect. But the object refuses to change size.

As a quick and simple test. I tried to change the size of the object in the paint() method using hard coded values. And what happens is that the object does change size. But the collisions are still using the old boundingRect size.

What seems to be tripping me up is that QGraphicsObject uses a 'const' boundingRect value. And I can't figure out how to deal with that.

ResizableObject::ResizableObject( QGraphicsItem *parent ): QGraphicsObject( parent ), mResizeLeft( false ), mResizeRight( false ), mResizeBottom( false ), mResizeTop( false )
{
    setAcceptHoverEvents( true );
    setFlags( ItemIsMovable | ItemIsSelectable );
}

QRectF ResizableObject::boundingRect() const
{
    return QRectF(0 , 0 , 100 , 100);
}

void ResizableObject::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    Q_UNUSED( option )
    Q_UNUSED( widget )
    painter->setRenderHint(QPainter::Antialiasing);

    //Determine whether to draw a solid, or dashed, outline border
    if (isSelected() )
    { 
        //Draw a dashed outlined rectangle
        painter->setPen( Qt::DashLine );
        painter->drawRect( boundingRect() );
    }
    else painter->drawRect(0,0,20,20); //Draw a normal solid outlined rectangle

    //No collisions are happening
    if(scene()->collidingItems(this).isEmpty())
    {
        //qDebug()<< "empty"; 
    }
    //Collisions are happening
    else
    {
        foreach(QGraphicsItem *item, collidingItems()) 
        {
            qDebug()<< item->pos();
        }
    }
}

The only examples I can find use Qwidget or QGraphicsRectItem. But those are slightly different and the constructors are not constants. Can anyone tell me how to change the size of these QGraphicsObjects with the mouse?

Answer 1

The paint method's only job is to paint. It can't be doing anything else - that includes attempting to change the object's size.

Instead of making the item itself resizable, let's see if we can have a more generic way of resizing any item, of any class. First, we may not wish all objects to be resizable. We need a way to figure which of them are. We also need a way to tell an item to resize - a QGraphicsItem doesn't offer a way to do that. In generic programming, one pattern used to provide such functionality is a traits class. This class implements the application-specific traits that will customize the generic resize behavior to our needs.

For this simple example, we allow any QGraphicsEllipseItems to be resizable. For your application, you can implement any logic you desire - without any subclassing. You can certainly create a base class for resizable items, and expose it via the traits, if you have many custom, resizable items. Even then, the traits class can still support items of other classes.

In all cases, the items to be resized must be selectable, and we exploit the selection mechanism to hook into the scene. This could be also done without using selections.

// https://github.com/KubaO/stackoverflown/tree/master/questions/graphics-resizable-32416527
#include <QtWidgets>

class SimpleTraits {
public:
    /// Determines whether an item is manually resizeable.
    static bool isGraphicsItemResizeable(QGraphicsItem * item) {
        return dynamic_cast<QGraphicsEllipseItem*>(item);
    }
    /// Gives the rectangle one can base the resize operations on for an item
    static QRectF rectFor(QGraphicsItem * item) {
        auto ellipse = dynamic_cast<QGraphicsEllipseItem*>(item);
        if (ellipse) return ellipse->rect();
        return QRectF();
    }
    /// Sets a new rectangle on the item
    static void setRectOn(QGraphicsItem * item, const QRectF & rect) {
        auto ellipse = dynamic_cast<QGraphicsEllipseItem*>(item);
        if (ellipse) return ellipse->setRect(rect);
    }
};

To implement a "wide rubber band" controller, we need a helper to tell what rectangle edges a given point intersects:

/// The set of edges intersecting a rectangle of given pen width
Qt::Edges edgesAt(const QPointF & p, const QRectF & r, qreal w) {
    Qt::Edges edges;
    auto hw = w / 2.0;
    if (QRectF(r.x()-hw, r.y()-hw, w, r.height()+w).contains(p)) edges |= Qt::LeftEdge;
    if (QRectF(r.x()+r.width()-hw, r.y()-hw, w, r.height()+w).contains(p)) edges |= Qt::RightEdge;
    if (QRectF(r.x()-hw, r.y()-hw, r.width()+w, w).contains(p)) edges |= Qt::TopEdge;
    if (QRectF(r.x()-hw, r.y()+r.height()-hw, r.width()+w, w).contains(p)) edges |= Qt::BottomEdge;
    return edges;
}

Then, we have a "rubberband" resize helper item that tracks the selection on its scene. Whenever the selection contains one item, and the item is resizable, the helper makes itself a child of the item to be resized, and becomes visible. It tracks the mouse drags on the active edges of the rubber band, and resizes the parent items accordingly.

template <typename Tr>
class ResizeHelperItem : public QGraphicsObject {
    QRectF m_rect;
    QPen m_pen;
    Qt::Edges m_edges;
    void newGeometry() {
        prepareGeometryChange();
        auto parentRect = Tr::rectFor(parentItem());
        m_rect.setTopLeft(mapFromParent(parentRect.topLeft()));
        m_rect.setBottomRight(mapFromParent(parentRect.bottomRight()));
        m_pen.setWidthF(std::min(m_rect.width(), m_rect.height()) * 0.1);
        m_pen.setJoinStyle(Qt::MiterJoin);
    }
public:
    ResizeHelperItem() {
        setAcceptedMouseButtons(Qt::LeftButton);
        m_pen.setColor(QColor(255, 0, 0, 128));
        m_pen.setStyle(Qt::SolidLine);
    }
    QRectF boundingRect() const Q_DECL_OVERRIDE {
        auto hWidth = m_pen.widthF()/2.0;
        return m_rect.adjusted(-hWidth, -hWidth, hWidth, hWidth);
    }
    void selectionChanged() {
        if (!scene()) { setVisible(false); return; }
        auto sel = scene()->selectedItems();
        if (sel.isEmpty() || sel.size() > 1) { setVisible(false); return; }
        auto item = sel.at(0);
        if (! Tr::isGraphicsItemResizeable(item)) { setVisible(false); return; }
        setParentItem(item);
        newGeometry();
        setVisible(true);
    }
    void paint(QPainter * p, const QStyleOptionGraphicsItem *, QWidget *) Q_DECL_OVERRIDE {
        p->setPen(m_pen);
        p->drawRect(m_rect);
    }
    void mousePressEvent(QGraphicsSceneMouseEvent * ev) Q_DECL_OVERRIDE {
        m_edges = edgesAt(ev->pos(), m_rect, m_pen.widthF());
        if (!m_edges) return;
        ev->accept();
    }
    void mouseMoveEvent(QGraphicsSceneMouseEvent * ev) Q_DECL_OVERRIDE {
        auto pos = mapToItem(parentItem(), ev->pos());
        auto rect = Tr::rectFor(parentItem());
        if (m_edges & Qt::LeftEdge) rect.setLeft(pos.x());
        if (m_edges & Qt::TopEdge) rect.setTop(pos.y());
        if (m_edges & Qt::RightEdge) rect.setRight(pos.x());
        if (m_edges & Qt::BottomEdge) rect.setBottom(pos.y());
        if (!!m_edges) {
            Tr::setRectOn(parentItem(), rect);
            newGeometry();
        }
    }
};

Finally, we create a simple scenario to try it all out:

int main(int argc, char ** argv) {
    QApplication app{argc, argv};
    QGraphicsScene scene;
    QGraphicsView view { &scene };
    typedef ResizeHelperItem<SimpleTraits> HelperItem;
    HelperItem helper;
    QObject::connect(&scene, &QGraphicsScene::selectionChanged, &helper, &HelperItem::selectionChanged);
    scene.addItem(&helper);
    auto item = scene.addEllipse(0, 0, 100, 100);
    item->setFlag(QGraphicsItem::ItemIsSelectable);
    view.setMinimumSize(400, 400);
    view.show();
    return app.exec();
}

Answer 2

I think you need to declare QRectF variable in somewhere and modify boundingRect() to simply return that one. And override mouse event handlers to modify that QRectF variable where you need (like mouseReleaseEvent() maybe,,)

virtual void    mouseMoveEvent(QGraphicsSceneMouseEvent * event)
virtual void    mousePressEvent(QGraphicsSceneMouseEvent * event)
virtual void    mouseReleaseEvent(QGraphicsSceneMouseEvent * event)

And don't forget to call prepareGeometryChange() before changing that value to invalidate item's boundingRect() cache.