qt - Snapping to grid doesn't work -

i trying snapping on grid such whatever draw should take gridpoints , no other points. have made grid in cadgraphicsscene.cpp , made different class snapping. grid made follows:

cadgraphicscene.cpp

void cadgraphicsscene::drawbackground(qpainter *painter, const qrectf &rect) {     const int gridsize = 50;     const int realleft = static_cast<int>(std::floor(rect.left()));     const int realright = static_cast<int>(std::ceil(rect.right()));     const int realtop = static_cast<int>(std::floor(rect.top()));     const int realbottom = static_cast<int>(std::ceil(rect.bottom()));      // draw grid.     const int firstleftgridline = realleft - (realleft % gridsize);     const int firsttopgridline = realtop - (realtop % gridsize);     qvarlengtharray<qline, 100> lines;      (qreal x = firstleftgridline; x <= realright; x += gridsize)         lines.append(qline(x, realtop, x, realbottom));     (qreal y = firsttopgridline; y <= realbottom; y += gridsize)         lines.append(qline(realleft, y, realright, y));      painter->setpen(qpen(qcolor(220, 220, 220), 0.0));     painter->drawlines(lines.data(), lines.size());      // draw axes.     painter->setpen(qpen(qt::lightgray, 0.0));     painter->drawline(0, realtop, 0, realbottom);     painter->drawline(realleft, 0, realright, 0); } 

my snap class looks follows:

snap.cpp

#include "snap.h" #include <qapplication>      snap::snap(const qrect& rect, qgraphicsitem* parent,                qgraphicsscene* scene):     qgraphicsrectitem(qrectf())     {         setflags(qgraphicsitem::itemisselectable |                 qgraphicsitem::itemismovable |                 qgraphicsitem::itemsendsgeometrychanges);     }      void snap::mousepressevent(qgraphicsscenemouseevent *event){         offset = pos() - computetopleftgridpoint(pos());         qgraphicsrectitem::mousepressevent(event);     }      qvariant snap::itemchange(graphicsitemchange change,     const qvariant &value)     {         if (change == itempositionchange && scene()) {             qpointf newpos = value.topointf();             if(qapplication::mousebuttons() == qt::leftbutton &&                 qobject_cast<cadgraphicsscene*> (scene())){                     qpointf closestpoint = computetopleftgridpoint(newpos);                     return closestpoint+=offset;                 }             else                 return newpos;         }         else             return qgraphicsitem::itemchange(change, value);     }      qpointf snap::computetopleftgridpoint(const qpointf& pointp){        cadgraphicsscene* customscene = qobject_cast<cadgraphicsscene*> (scene());         int gridsize = customscene->getgridsize();         qreal xv = floor(pointp.x()/gridsize)*gridsize;         qreal yv = floor(pointp.y()/gridsize)*gridsize;         return qpointf(xv, yv);     } 

snap.h

#ifndef snap_h #define snap_h  #include <qgraphicsrectitem> #include "cadgraphicsscene.h"  class snap : public qgraphicsrectitem { public:     snap(const qrect& rect, qgraphicsitem* parent,          qgraphicsscene* scene); protected:     void mousepressevent(qgraphicsscenemouseevent *event);     qvariant itemchange(graphicsitemchange change,     const qvariant &value); private:     qpointf offset;     qpointf computetopleftgridpoint(const qpointf &pointp); };  #endif // snap_h 

but nothing happened, no snapping done. can please me in above?

one of problems pass qrect() qgraphicsrectitem constructor in initialization list of snap class. means have 0 width , height. instead pass same qrect object pass snap constructor:

snap::snap(const qrect& rect, qgraphicsitem* parent, qgraphicsscene* scene) : qgraphicsrectitem(rect) 

you don't seem use parent , scene arguments might leave them out:

snap::snap(const qrect& rect) :     qgraphicsrectitem(rect) 

or if plan use parent can set default value 0 in declaration:

snap(const qrect& rect, qgraphicsitem* parent = 0); 

then pass them both base class constructor:

snap::snap(const qrect& rect, qgraphicsitem* parent) : qgraphicsrectitem(rect, parent) 

snap.h

#ifndef snap_h #define snap_h  #include <qgraphicsrectitem>  class snap : public qgraphicsrectitem { public:     snap(const qrect &rect, qgraphicsitem *parent = 0); protected:     void mousepressevent(qgraphicsscenemouseevent *event);     qvariant itemchange(graphicsitemchange change,     const qvariant &value); private:     qpointf offset;     qpointf computetopleftgridpoint(const qpointf &pointp); };  #endif // snap_h 

snap.cpp

#include "snap.h" #include <qdebug> #include <qmath.h>  snap::snap(const qrect& rect, qgraphicsitem* parent) :     qgraphicsrectitem(rect, parent) {     setflags(qgraphicsitem::itemisselectable |              qgraphicsitem::itemismovable |              qgraphicsitem::itemsendsgeometrychanges); }  void snap::mousepressevent(qgraphicsscenemouseevent *event){     offset = pos() - computetopleftgridpoint(pos());     qgraphicsrectitem::mousepressevent(event); }  qvariant snap::itemchange(graphicsitemchange change,                           const qvariant &value) {     qdebug()<<"inside itemchange";     if (change == itempositionchange && scene())     {         qpointf newpos = value.topointf();         qpointf closestpoint = computetopleftgridpoint(newpos);         return closestpoint+=offset;     }     else         return qgraphicsitem::itemchange(change, value); }  qpointf snap::computetopleftgridpoint(const qpointf& pointp){     int gridsize = 100;     qreal xv = qfloor(pointp.x()/gridsize)*gridsize;     qreal yv = qfloor(pointp.y()/gridsize)*gridsize;     return qpointf(xv, yv); } 

mainwindow.cpp

#include "mainwindow.h" #include "ui_mainwindow.h"  #include <qgraphicsview> #include <qlayout> #include "snap.h"  mainwindow::mainwindow(qwidget *parent) :     qmainwindow(parent),     ui(new ui::mainwindow) {     ui->setupui(this);     centralwidget()->setlayout(new qvboxlayout);     qgraphicsview *view = new qgraphicsview(this);     centralwidget()->layout()->addwidget(view);     snap *snap = new snap(qrect(0,0,100,100));     view->setscene(new qgraphicsscene);     view->scene()->additem(snap); }  mainwindow::~mainwindow() {     delete ui; }