Can't get positions of QGraphicsItems in scene

Question

I am trying to get the positions of the graphicsitems in the scene. But their QPointF value always remains (0,0).

I am painting when mouse-click event occurs. On debugging scene->items(), I get

(QGraphicsItem(this =0x22edff0, parent =0x0, pos =QPointF(0, 0) , z = 0 , flags = ( ) ) )

for each graphics item in scene but with different memory address.

This is my mainwindow.cpp code:

#include "mainwindow.h"
#include <QDebug>
MainWindow::MainWindow()
{
  scene = new QGraphicsScene;
  view = new QGraphicsView;
  view->setScene(scene);
  button = new QPushButton("Item");
  QGridLayout  *layout = new QGridLayout;
  layout->addWidget(button);
  layout->addWidget(view);
  setLayout(layout);
  connect(button, SIGNAL(clicked()), this, SLOT(createItem()));
}

void MainWindow::createItem()
{
  myEntity = new Item;
  scene->addItem(myEntity);
  count_items();
}

void MainWindow::count_items()
{
  qDebug() << scene->items().count();
  qDebug() << scene->items();
}

MainWindow::~MainWindow()
{}

This is my item.cpp code:

#include "item.h"
Item::Item()
{
  ClickFlag = true;
  PaintFlag = false;
} 
Item::~Item(){}

QRectF Item::boundingRect() const
{
  // outer most edges
  return QRectF(0,0,1450,1400);
}
void Item::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
  if(event->button()==Qt::LeftButton){
    if(ClickFlag){
        x = event->pos().x();
        y = event->pos().y();
        PaintFlag = true;
        ClickFlag = false;
       }
   }
}

void Item::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                              QWidget *widget)
{
  if(PaintFlag){
    QPen paintPen;
    paintPen.setWidth(4);
    pt.setX(x);
    pt.setY(y);
    painter->setPen(paintPen);
    painter->drawPoint(x,y);
    update();              
  }
}

I can't seem to find the position of these items correctly.

You never invoke `QGraphicsItem::setPos` on your items, which allow to set the position of your items in their belonging scene. Quite normal that their default position remains zero, even if your are painting at another point in the graphicsItem referential — epsilon, Aug 30 '14 at 12:20
I have looked into `setPos()` function but where do I use it? I could not figure it out. — Gurjot Bhatti, Aug 30 '14 at 15:21
You can read all about coordinate system in qgraphicsview framework [here](https://qt-project.org/doc/qt-4.7/graphicsview.html#id-a93690a1-04f1-42c1-8bdf-90819fca8982) — epsilon, Aug 30 '14 at 18:29

score 2 · Accepted Answer · answered Aug 30 '14 at 23:21

This task is supposed to be implemented in another way. For example:

Use QGraphicsScene::addEllipse to add small ellipse (which will look like a point) to the scene. Save the pointer to it in a class variable. The ellipse itself should be at the center, e.g. (-1, -1, 2, 2).
Reimplement QGraphicsScene::mousePressEvent, detect mouse clicks and call setPos for the ellipse item (or add new ellipse each time and immediately call setPos if you need multiple points).
Use QGraphicsItem::pos to get previously set positions.

Reimplementing QGraphicsItem::paint is usually an over-complication. Qt have plenty of item classes for all common needs. Just build your scene from geometric primitives, pixmaps, etc.

Can't get positions of QGraphicsItems in scene

1 Answers1