QGraphicsRectItem move with mouse. How to? - c++

Question

I have QGraphicsView, QGraphicsScene and QGraphicsRectItem.
QGraphicsRectItem in the QGraphicsScene and the last one in the QGraphicsView. I want to move QGraphicsRectItem with mouse by clicking on it only! But in my implementation it moves if I click on any position on my QGraphicsScene. Whether it is my QGraphicsRectItem or some other place. And the second issue. The item has been moved to the center of the scene. Clicking on it again it starts to move from the home location.
void Steer::mousePressEvent(QMouseEvent *click)
{
offset = click->pos();
}
void Steer::mouseMoveEvent(QMouseEvent *event)
{
if(event->buttons() & Qt::LeftButton)
{
p1->setPos(event->localPos() - offset); //p1 movable item
}
}
What do I do wrong?
UPDATE:
main.cpp
#include "widget.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Steer w;
w.show();
return a.exec();
}
widget.h
#ifndef STEER_H
#define STEER_H
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QMouseEvent>
#include <QPoint>
#include <QGraphicsRectItem>
class Steer : public QGraphicsView
{
Q_OBJECT
private:
QGraphicsScene *scene;
QGraphicsRectItem *p1;
QPoint offset;
public:
explicit Steer(QGraphicsView *parent = 0);
~Steer(){}
public slots:
void mousePressEvent(QMouseEvent * click);
void mouseMoveEvent(QMouseEvent * event);
};
#endif // STEER_H
widget.cpp
#include "widget.h"
#include <QBrush>
Steer::Steer(QGraphicsView *parent)
: QGraphicsView(parent)
{
scene = new QGraphicsScene;
p1 = new QGraphicsRectItem;
//add player
p1->setRect(760, 160, 10, 80);
//add scene
scene->setSceneRect(0, 0, 800, 400);
//add moveable item
scene->addItem(p1);
//set scene
this->setScene(scene);
this->show();
}
void Steer::mousePressEvent(QMouseEvent *click)
{
offset = click->pos();
}
void Steer::mouseMoveEvent(QMouseEvent *event)
{
if(event->buttons() & Qt::LeftButton)
{
p1->setPos(event->localPos() - offset);
}
}

Answer 1

I'd try a different approach that is a little easier to understand:
#include <QtWidgets>
class Steer : public QGraphicsView
{
public:
Steer()
{
scene = new QGraphicsScene;
p1 = new QGraphicsRectItem;
//add player
p1->setRect(0, 0, 10, 80);
p1->setX(760);
p1->setY(160);
//add scene
scene->setSceneRect(0, 0, 800, 400);
//add moveable item
scene->addItem(p1);
//set scene
this->setScene(scene);
this->show();
}
protected:
void mousePressEvent(QMouseEvent * click)
{
if (p1->contains(p1->mapFromScene(click->localPos()))) {
lastMousePos = click->pos();
} else {
lastMousePos = QPoint(-1, -1);
}
}
void mouseMoveEvent(QMouseEvent * event)
{
if(!(event->buttons() & Qt::LeftButton)) {
return;
}
if (lastMousePos == QPoint(-1, -1)) {
return;
}
p1->setPos(p1->pos() + (event->localPos() - lastMousePos));
lastMousePos = event->pos();
}
private:
QGraphicsScene *scene;
QGraphicsRectItem *p1;
QPoint lastMousePos;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Steer w;
w.show();
return a.exec();
}
There are a few things to point out here:
Don't use setRect() to set the position of a QGraphicsRectItem. It doesn't work the way you think it might. Always use setPos() to change the position of an item.
Rename offset to something more descriptive. I chose lastMousePos. Instead of just updating it once when the mouse is pressed, also update it whenever the mouse is moved. Then, it's simply a matter of getting the difference between the two points and adding that to the position of the item.
Check if the mouse is actually over the item before reacting to move events. If the mouse isn't over the item, you need some way of knowing that, hence the QPoint(-1, -1). You may want to use a separate boolean flag for this purpose. This solves the problem that you saw, where it was possible to click anywhere in the scene to get the item to move.
Also, note the mapFromScene() call: the contains() function works in local coordinates, so we must map the mouse position which is in scene coordinates before testing if it's over the item.
The event functions are not slots, they're virtual, protected functions.
You could also consider handling these events in the items themselves. You don't need to do it from within QGraphicsView, especially if you have more than one of these items that need to be dragged with the mouse.