Update QGraphicsLineItem correctly - c++

Question

I'm trying to draw a line using the mouseEvent functions from my QGraphicsScene, when i press the mouse button and move over the scene, the line begin from the top left [0,0] when it should start at the point where i pressed the mouse button, but when i release and do that again the line is drawn normally, what is the reason for this behavior and how to resolve it ?
Here is the full code:
Scene.h:
#ifndef SCENE_H
#define SCENE_H
#include <QGraphicsScene>
#include <QGraphicsLineItem>
#include <QGraphicsSceneMouseEvent>
class Scene : public QGraphicsScene
{
public:
Scene();
private:
QGraphicsLineItem* line;
QPointF startPoint;
protected:
void mousePressEvent(QGraphicsSceneMouseEvent *event) override;
void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;
};
#endif // SCENE_H
Scene.cpp:
#include "Scene.h"
Scene::Scene() : startPoint(0,0)
{
line = new QGraphicsLineItem();
this->addItem(line);
}
void Scene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
startPoint = event->scenePos();
}
void Scene::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
qreal x = event->scenePos().x();
qreal y = event->scenePos().y();
line->setLine(startPoint.x(),startPoint.y(),x,y);
}
Main:
#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include "Scene.h"
int main(int argc,char* argv[])
{
QApplication app(argc,argv);
Scene scene;
QGraphicsView view(&scene);
view.setMinimumSize(800,600);
view.setAlignment(Qt::AlignLeft|Qt::AlignTop);
view.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
view.show();
return app.exec();
}

Answer 1

The problem is caused because the QGraphicsScene object is not initialized with a bounding box. In the constructor use
Scene::Scene() : QGraphicsScene(0, 0, 400, 400), startPoint(0,0)
(for example), and it will work as expected.
If you do not do this, at the time of the first click the scene coordinates will relocate so that the starting coordinates of your line (say, (x0, y0)) will be the scene coordinates of the top-left corner. So if you move a mouse just a bit, e.g. to (x0+1,y0), the actual scene coordinates will become (2*x0+1,2*y0).
Also, it is good practice to call the parent class' event handlers at the end of your overloaded ones.
Finally, this can be helpful:
https://www.walletfox.com/course/qgraphicsitemruntimedrawing.php