How to drag & drop QAbstractGraphicsShapeItem in QGraphicsView

Comments

It seems that using this method to drag and drop the internal QRectF or QPolygonF for QGraphicsRectItem etc are not updated. What would be the best way to add it? (do it by hand in MouseMoveEvent?)

Change of polygon means change of shape. This article is dealing with change of position using mouse events, thus, shape modification is out of scope of this example. --Daniil Ivanov

OK. I was just wondering wether there was a well-known standard solution to keep the internal data in sync with the position. If there isn't I do it by hand in moveMouseEvent. It would have been nice since most QAbstractGraphicsShapeItems will be QGraphicsXXXItems (XXX=Rect,Polygon, etc)

There is no problem with keeping internal data in sync as Qt does it for you. If you have a question, please, ask it on discussion board, this is wrong place for this purpose. --Daniil Ivanov

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Article Metadata

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Compatibility

Platform(s): S60, Maemo 5

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Article

Keywords: QGraphicsView, drag and drop

Created: divanov (21 Mar 2011)

Last edited: hamishwillee (11 Oct 2012)

Problem

Implement drag and drop in QGraphicsView for QAbstractGraphicsShapeItem items.

Solution

Sublass QGraphicsScene and reimplement mousePressEvent, mouseMoveEvent and mouseReleaseEvent to capture all mouse events.

#ifndef MAIN_H
#define MAIN_H
 
#include <QtGui>
 
class DnDGraphicsScene : public QGraphicsScene
{
    Q_OBJECT
public:
    DnDGraphicsScene(QWidget *parent = 0);
 
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent);
 
private:
    QAbstractGraphicsShapeItem *dragged;
    QPen selected;
    QBrush selectedBrush;
    QPen normal;
    QBrush normalBrush;
    QPointF offset;
};
 
#endif // MAIN_H

Press event detects beginning of a drag and release end of a drag. In between we move item over the scene.

#include <QtGui>
 
#include "main.h"
 
DnDGraphicsScene::DnDGraphicsScene(QWidget *parent)
  : QGraphicsScene(parent), dragged(0)
{
    selected = QPen(Qt::red, 3);
    selectedBrush  = QBrush(Qt::yellow);
}
 
void DnDGraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    dragged = qgraphicsitem_cast<QAbstractGraphicsShapeItem*>(
          itemAt(mouseEvent->scenePos(), QTransform()));
    if (dragged) {
        offset = dragged->pos() - mouseEvent->scenePos();
        normal = dragged->pen();
        normalBrush = dragged->brush();
        dragged->setPen(selected);
        dragged->setBrush(selectedBrush);
    } else
        QGraphicsScene::mousePressEvent(mouseEvent);
}
 
void DnDGraphicsScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    if (dragged) {
        dragged->setPos(offset + mouseEvent->scenePos());
    } else
        QGraphicsScene::mouseMoveEvent(mouseEvent);
}
 
void DnDGraphicsScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    if (dragged) {
        dragged->setPen(normal);
        dragged->setBrush(normalBrush);
        dragged = 0;
    } else
        QGraphicsScene::mouseReleaseEvent(mouseEvent);
}

In main function we put all things together

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QPolygonF polygon1;
    polygon1 << QPointF(0.0, 0.0) << QPointF(25.0, 0.0) << QPointF(12.5, 25.0);
    QGraphicsPolygonItem *item1 = new QGraphicsPolygonItem(polygon1);
    item1->setBrush(Qt::green);
    item1->setTransformOriginPoint(12.5, 12.5);
    item1->setRotation(45.0);
    QPolygonF polygon2;
    polygon2 << QPointF(50.0, 50.0) << QPointF(75.0, 50.0) << QPointF(62.5, 75.0);
    QGraphicsPolygonItem *item2 = new QGraphicsPolygonItem(polygon2);
    item2->setBrush(Qt::blue);
    item2->setTransformOriginPoint(12.5, 12.5);
    item2->setRotation(-45.0);
    DnDGraphicsScene scene;
    scene.addItem(item1);
    scene.addItem(item2);
    QGraphicsView view(&scene);
    view.show();
 
    return a.exec();
}