QGraphicsObject "Click-jumping" when I want to move the item, and a snap algorithm is present.

enjoysmath

Bug demo video:

Relevant code:

#include "object.h"
#include "diagramscene.h"
#include <QFontMetrics>
#include <QPointF>
#include <QGraphicsSceneMouseEvent>

Object::Object(const QString& text)
    : Node(text)
{
    setFlags(ItemIsSelectable | ItemIsMovable); // | ItemSendsGeometryChanges);
    auto rect = label->boundingRect();
    label->setPos(-rect.center());
}

//void Object::mousePressEvent(QGraphicsSceneMouseEvent *event) {
//    offset = event->pos() - computeTopLeftGridPoint(pos());
//    Node::mousePressEvent(event);
//}

//void Object::mouseMoveEvent(QGraphicsSceneMouseEvent *event) {
//    Node::mouseMoveEvent(event);
//}

void Object::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    QPointF newPos = event->scenePos();
    QPointF closestPoint = computeTopLeftGridPoint(newPos);
    setPos(closestPoint);
}

//QVariant Object::itemChange(GraphicsItemChange change, const QVariant &value)
//{
////    if (change == ItemPositionChange && scene())
////    {
////        QPointF newPos = value.toPointF();
////        QPointF closestPoint = computeTopLeftGridPoint(newPos);
////        return closestPoint += offset;
////    }
////    else
//    return Node::itemChange(change, value);
//}

QPointF Object::computeTopLeftGridPoint(const QPointF& pointP)
{
    int gridSize = dynamic_cast<DiagramScene*>(scene())->gridSize();
    qreal xV = qFloor(pointP.x()/gridSize)*gridSize;
    qreal yV = qFloor(pointP.y()/gridSize)*gridSize;
    return QPointF(xV, yV);
}

I'm trying to adapt the canonical itemChange-based grid snapping technique to the more UX-friendly way of having the item follow exactly where the cursor is at until you release it, then it snaps. But as you can see I'm failing. It's not clear to me what is causing this growing jump as seen in the video. I've tried about 20 different things to correct it in code. The original itemChange code works, however! It's a mystery.

So for instance I've already tried adding in offset that was set on mouse press, but which the itemChange would add in. Only resulted in the same bug or worse.

The default QGraphicsScene move behavior is used - I'm not overriding it and manually setting positions, etc.

enjoysmath

Solution code: =)

#include "object.h"
#include "diagramscene.h"
#include <QFontMetrics>
#include <QPointF>
#include <QGraphicsSceneMouseEvent>

Object::Object(const QString& text)
    : Node(text)
{
    setFlags(ItemIsSelectable | ItemIsMovable); // | ItemSendsGeometryChanges);
    auto rect = label->boundingRect();
    label->setPos(-rect.center());
}

//void Object::mousePressEvent(QGraphicsSceneMouseEvent *event) {
//    offset = event->pos() - computeTopLeftGridPoint(pos());
//    Node::mousePressEvent(event);
//}

void Object::mouseMoveEvent(QGraphicsSceneMouseEvent *event) {
    setPos(event->scenePos());
}

void Object::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    QPointF newPos = pos();
    QPointF closestPoint = closestGridPoint(newPos);
    setPos(closestPoint);
}

//QVariant Object::itemChange(GraphicsItemChange change, const QVariant &value)
//{
////    if (change == ItemPositionChange && scene())
////    {
////        QPointF newPos = value.toPointF();
////        QPointF closestPoint = computeTopLeftGridPoint(newPos);
////        return closestPoint += offset;
////    }
////    else
//    return Node::itemChange(change, value);
//}

QPointF Object::closestGridPoint(const QPointF& pointP)
{
    int gridSize = dynamic_cast<DiagramScene*>(scene())->gridSize();
    qreal xV = qRound(pointP.x()/gridSize)*gridSize;    // BUGFIX: use qRound instead of qFloor here
    qreal yV = qRound(pointP.y()/gridSize)*gridSize;
    return QPointF(xV, yV);
}