How to make a QGraphicsItem's position dependent on another QGraphicsItem?

Question

I want to make one QGraphicsItem in a QGraphicsScene move (or change size) when another one moves. But, trying to access either QGraphicsItem from the QGraphicsScene object causes crashes.

Here's a dummy example. I want to automatically:

1. Resize BlueItem's width when I click and drag RedItem.

2. Move RedItem when I click and drag BlueItem.

The dummy code:

redItem.h (A pixmap item)

#include <QGraphicsPixmapItem>

class RedItem : public QGraphicsPixmapItem
{
public:
  RedItem();

protected:
  void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};

redItem.cpp

RedItem::RedItem()
{
  setPixmap(QPixmap(redPixMap));
  setFlags(ItemIsMovable);
  setCacheMode(DeviceCoordinateCache);
}

void RedItem::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
  setPos(event->scenePos(),0); //I can override this part just fine.
}

blueItem.h (A resizable rectangle item)

#include <QGraphicsRectItem>

class BlueItem : public QGraphicsRectItem
{
public:
  BlueItem();

protected:
  void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};

blueItem.cpp is similar to redItem.cpp

graphicsViewWidget.h (The GraphicsView)

#include <QGraphicsView>
#include "blueItem.h"
#include "redItem.h"


class GraphicsViewWidget : public QGraphicsView
{
  Q_OBJECT
public:
  explicit GraphicsViewWidget(QWidget *parent = 0);

protected:
  virtual void mouseMoveEvent(QMouseEvent *event);

private:
  RedItem *red;
  BlueItem *blue;
};

graphicsViewWidget.cpp

#include "graphicsViewWidget.h"
#include <QRectF>

void Qx::createItem(int frame)
{

    red = new RedItem;
    red->setPos(0, 0);
    scene()->addItem(red);


    blue = new BlueItem;
    blue->setPos(10, 10);
    scene()->addItem(blue);
}

void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event) //QGraphicsSceneMouseEvent
{
  QGraphicsView::mouseMoveEvent(event);

  //if(redItem moves)
  //  resize blueItem;

  //if(blueItem moves)
  //  move redItem;

}

Any help or advice would be greatly appreciated.

Answer 1

If you make item B a child of item A, then it will live in its coordinate space. This means all transformations - moving, scaling, rotating... everything you apply to the parent item will also be applied to all its children and their children and so on...