Problem with QGraphicsLineItem, it keeps "flashing"

frnklu20

Hi,
In my program i can connect lines between objects(QGraphicItem) in a diagramscene
First i drew the lines between the objects position(object->pos()), but i wanted to connect the lines in the center of the object. So the program draws a line using the position (object->pos()+p), P is a QPointF.

But now i have a problem, when i move the objects and the line keeps updating, it flashes. Like it follows the position
object->pos()+p but sometimes for some miliseconds it comeback to the object->pos()

I don't know if i'm being clear...
https://www.youtube.com/watch?v=MLcGfTGzFOw
here is a video that i recorded with my cellphone

Diagramscene.cpp

void DiagramScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
         ...            
case InsertLine:
            line = new QGraphicsLineItem(QLineF(mouseEvent->scenePos(),
                                        mouseEvent->scenePos()));
            line->setPen(QPen(myLineColor, 2));
            addItem(line);
            break;
         ...
}
void DiagramScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    if (myMode == InsertLine && line != 0) {
        QLineF newLine(line->line().p1(), mouseEvent->scenePos());
        line->setLine(newLine);
    } else if (myMode == MoveItem) {
        QGraphicsScene::mouseMoveEvent(mouseEvent);
    }
}

Arrow.cpp(that's the class that handles the line function)

void Arrow::updatePosition()
{
    QLineF line(mapFromItem(myStartItem,p1), mapFromItem(myEndItem,p2));//p1 and p2 here
    setLine(line);
    update();
}

void Arrow::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
          QWidget *)
{
    if (myStartItem->collidesWithItem(myEndItem))
        return;

    QPen myPen = pen();
    myPen.setColor(myColor);
    painter->setPen(myPen);
    painter->setBrush(myColor);
    QPointF start(myStartItem->pos()+p1);
    QPointF end(myEndItem->pos()+p2);//trid to add the points here, but nothing happens

    
    QLineF centerLine(myStartItem->pos(), myEndItem->pos());

    painter->drawLine(line());
    setLine(QLineF(start+p1,end+p2));
    if (isSelected()) {
        painter->setPen(QPen(myColor, 1, Qt::DashLine));
        QLineF myLine = line();
        myLine.translate(0, 4.0);
        painter->drawLine(myLine);
        myLine.translate(0,-8.0);
        painter->drawLine(myLine);
    }
}

i don't have any ideia how to fix it, can somebody help me please?

SGaist

Hi,

Are you sure you are not replacing the line on each move ?

frnklu20

how can i be certain?that's everything related to moving

SGaist

You can add a qDebug statement so you know where it's going.

frnklu20

I don't know why but if i use just

setLine(QLineF(myStartItem->pos(),myEndItem->pos()));

instead of

setLine(QLineF(myStartItem->pos()+p,myEndItem->pos()+p));
//p is abitrary point

it works just fine, doesn't flashes

and it makes no sense to me

Unsolved Problem with QGraphicsLineItem, it keeps "flashing"