Issue moving an QGraphicsItem in a custom QGraphicsView - c++

Question

I am having issues moving a QGraphicItem in a custom QGraphicView class. What I would like to be able to to do is select the item by a left mouse click and then move it to where I've done a right mouse click.
I stongly suspect that my problem is that QGraphicsItem::setPos() requires the coordinates to be in parent coordinates, and I'm unsure which for of QMouseEvent::*Pos() to use, and how to convert it to parent coordinates.
Screens shots of what is happening, versus what I what follow the code.
main.cpp: (simple main here, standard test harness)
#include "QtTest.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QtTest w;
w.show();
return a.exec();
}
QtTest.h: (This defines the main application window)
#pragma once
#include <QtWidgets/QMainWindow>
class QGraphicsView;
class QGraphicsScene;
class QGraphicsItem;
class QMouseEvent;
class QtTest : public QMainWindow
{
Q_OBJECT
public:
QtTest(QWidget *parent = Q_NULLPTR);
private:
QGraphicsView* m_gv;
QGraphicsScene* m_pScene;
void setupUI();
};
QtTest.cpp: (implementation of the main application window)
#include "QtTest.h"
#include "testGV.h"
#include <QVariant>
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsItem>
#include <QMainWindow>
#include <QMouseEvent>
#include <QWidget>
QtTest::QtTest(QWidget *parent): QMainWindow(parent)
{
setupUI();
}
void QtTest::setupUI()
{
QWidget *centralWidget;
if (objectName().isEmpty())
setObjectName("QtTestClass");
resize(600, 400);
centralWidget = new QWidget(this);
centralWidget->setObjectName("centralWidget");
m_gv = new testGV(centralWidget);
m_gv->setObjectName("graphicsView");
m_gv->setGeometry(QRect(100, 10, 441, 331));
setCentralWidget(centralWidget);
}
testGV.h: (definition of custom widget)
#pragma once
#include <QGraphicsView>
#include <QGraphicsItem>
class testGV : public QGraphicsView
{
Q_OBJECT
public:
testGV(QWidget* parent);
protected:
void mousePressEvent(QMouseEvent*);
private:
QGraphicsScene* m_pScene;
QGraphicsItem* m_pItem;
void createScene();
};
testGV.cpp: (implementation of custom widget)
#include "testGV.h"
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsItem>
#include <QMouseEvent>
testGV::testGV(QWidget* parent) : QGraphicsView(parent)
{
createScene();
}
void testGV::createScene()
{
m_pScene = new QGraphicsScene();
m_pScene->addRect(QRect(30, 30, 150, 150), QPen(Qt::black), QBrush(Qt::black, Qt::NoBrush));
QGraphicsEllipseItem* pTemp = m_pScene->addEllipse(QRect(0, 0, 15, 15), QPen(Qt::black), QBrush(Qt::red, Qt::SolidPattern));
pTemp->setFlag(QGraphicsItem::ItemIsMovable);
pTemp->setFlag(QGraphicsItem::ItemSendsGeometryChanges);
setScene(m_pScene);
}
void testGV::mousePressEvent(QMouseEvent* pEvent)
{
if (pEvent->button() == 1) // left button click
{
m_pItem = itemAt(pEvent->pos());
}
else if (pEvent->button() == 2) // right button click
{
m_pItem->setPos(pEvent->pos());
m_pScene->update();
}
}
The image on the left is the initial display, when I right click on the red dot and then click in the square at about where the black dot is I get the image on the right. What I'm after is the red dot moving to where I clicked.

Answer 1

The cause of the problem is that the QMouseEvent has the position information in the coordinates of the view but the item uses the coordinates of the scene. The solution in that case is to map the coordinates of the view to the coordinates of the scene:
void testGV::mousePressEvent(QMouseEvent* pEvent)
{
if (pEvent->button() == Qt::LeftButton)
m_pItem = itemAt(pEvent->pos());
else if (pEvent->button() == Qt::RightButton)
if(m_pItem)
m_pItem->setPos(mapToScene(pEvent->pos()));
}
But even so there is a problem, the position for your items is with respect to the topLeft so the displacement will have an error, if you want to avoid that deviation then you must consider the position of the left click:
void testGV::mousePressEvent(QMouseEvent* pEvent)
{
if (pEvent->button() == Qt::LeftButton){
m_pItem = itemAt(pEvent->pos());
m_pItem->setData(0, mapToScene(pEvent->pos()));
}
else if (pEvent->button() == Qt::RightButton)
if(m_pItem){
QPointF p = m_pItem->data(0).toPointF();
QPointF sp = mapToScene(pEvent->pos());
m_pItem->setPos(m_pItem->pos() + sp - p);
m_pItem->setData(0, sp);
}
}
Note: When a pointer is created it points to any memory location so that can cause problems so I recommend initializing it to nullptr and also checking if the pointer is valid:
testGV::testGV(QWidget* parent) : QGraphicsView(parent), m_pItem(nullptr)
{
createScene();
}