restrict movable area of qgraphicsitem

Question

Is there a way to restrict the area where a QGraphicsItem like QRect can be moved when setFlag(ItemIsMovable) is set?

I'm new to pyqt and trying to find a way to move an item with the mouse, and the restrict it to only vertically/horizontally.

Answer 1

If you want to keep a limited area you can reimplement the ItemChanged()

Declare:

#ifndef GRAPHIC_H
#define GRAPHIC_H
#include <QGraphicsRectItem>
class Graphic : public QGraphicsRectItem
{
public:
    Graphic(const QRectF & rect, QGraphicsItem * parent = 0);
protected:
    virtual QVariant    itemChange ( GraphicsItemChange change, const QVariant & value );
};

#endif // GRAPHIC_H

implementation : ItemSendsGeometryChanges flag is needed to capture the change in position of QGraphicsItem

#include "graphic.h"
#include <QGraphicsScene>

Graphic::Graphic(const QRectF & rect, QGraphicsItem * parent )
    :QGraphicsRectItem(rect,parent)
{
    setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemSendsGeometryChanges);
}

QVariant Graphic::itemChange ( GraphicsItemChange change, const QVariant & value )
{
    if (change == ItemPositionChange && scene()) {
        // value is the new position.
        QPointF newPos = value.toPointF();
        QRectF rect = scene()->sceneRect();
        if (!rect.contains(newPos)) {
            // Keep the item inside the scene rect.
            newPos.setX(qMin(rect.right(), qMax(newPos.x(), rect.left())));
            newPos.setY(qMin(rect.bottom(), qMax(newPos.y(), rect.top())));
            return newPos;
        }
    }
    return QGraphicsItem::itemChange(change, value);
}

Then we define the rectangle of the scene, in this case will be 300x300

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    QGraphicsView * view = new QGraphicsView(this);
    QGraphicsScene * scene = new QGraphicsScene(view);
    scene->setSceneRect(0,0,300,300);
    view->setScene(scene);
    setCentralWidget(view);
    resize(400,400);

    Graphic * graphic = new Graphic(QRectF(0,0,100,100));
    scene->addItem(graphic);
    graphic->setPos(150,150);

}

This is to keep the graph within an area, good luck

Answer 2

re implement the mouseMoveEvent(self,event) in the QGraphicScene like the following :

def mousePressEvent(self, event ):

    self.lastPoint = event.pos()

def mouseMoveEvent(self, point):

    if RestrictedHorizontaly: # boolean to trigger weather to restrict it horizontally 
        x = point.x()
        y = self.lastPoint.y()
        self.itemSelected.setPos(QtCore.QPointF(x,y))<br> # which is the QgraphicItem that you have or selected before

hope it helps