QGraphicsView/QGraphicsScene/QGraphicsItem slow pos updating

[ben1996123]

I have a lot of items in a QGraphicsScene (about 10000), and I need to update the position of every one of them, but when I do this it can take over 1 minute, so how can I (if possible) make it faster?

Here's the position update code:

void Dialog::updateAllPositions(){
    int x,y,z,x2,y2;
    for(int aa=0;aa<size.x()*size.y();aa++){
        x=aa%size.x();
        y=aa/size.x();
        z=tiles[y][x];
        if(z==0) continue;
        x2=(z-1)%size.x();
        y2=(z-1)/size.x();
        scene->items()[size.x()*size.y()-1-z]->setPos(ssize*x+sgap*x-sgap+(ssize-fontMetric.width(QString::number(z)))/2, ssize*y+sgap*y-sgap+(ssize-fontMetric.height())/2);
        scene->items()[2*size.x()*size.y()-2-z]->setPos((x-x2)*ssize+(x-x2)*sgap, (y-y2)*ssize+(y-y2)*sgap);
    }
}

void Dialog::updateAllPositions(){
    int x,y,z,x2,y2;
    for(int aa=0;aa<size.x()*size.y();aa++){
        x=aa%size.x();
        y=aa/size.x();
        z=tiles[y][x];
        if(z==0) continue;
        x2=(z-1)%size.x();
        y2=(z-1)/size.x();
        scene->items()[size.x()*size.y()-1-z]->setPos(ssize*x+sgap*x-sgap+(ssize-fontMetric.width(QString::number(z)))/2, ssize*y+sgap*y-sgap+(ssize-fontMetric.height())/2);
        scene->items()[2*size.x()*size.y()-2-z]->setPos((x-x2)*ssize+(x-x2)*sgap, (y-y2)*ssize+(y-y2)*sgap);
    }
}

To copy to clipboard, switch view to plain text mode 

ssize and sgap are constants. Even if I disable viewport updating it still takes just as long. Any ideas?

[ben1996123]

fixed

for anyone who has the same problem, heres what I did (even though tbh I dont really understand completely how it works)

I made a QList<QGraphicsItem*> q and set it equal to scene->items(), then instead of doing scene->items()[stuff]->setPos, do q[stuff]->setPos and it works a lot faster (I dont really understand why it works because scene->items is a const function and I never had to tell the scene that its items had changed, just change them in q)

void Dialog::updateAllPositions(){
    int x,y,z,x2,y2;
    QList<QGraphicsItem*> q=scene->items();
    for(int aa=0;aa<size.x()*size.y();aa++){
        x=aa%size.x();
        y=aa/size.x();
        z=tiles[y][x];
        if(z==0) continue;
        x2=(z-1)%size.x();
        y2=(z-1)/size.x();
        q[size.x()*size.y()-1-z]->setPos(ssize*x+sgap*x-sgap+(ssize-fontMetric.width(QString::number(z)))/2, ssize*y+sgap*y-sgap+(ssize-fontMetric.height())/2);
        q[2*size.x()*size.y()-2-z]->setPos((x-x2)*ssize+(x-x2)*sgap, (y-y2)*ssize+(y-y2)*sgap);
    }
}

void Dialog::updateAllPositions(){
    int x,y,z,x2,y2;
    QList<QGraphicsItem*> q=scene->items();
    for(int aa=0;aa<size.x()*size.y();aa++){
        x=aa%size.x();
        y=aa/size.x();
        z=tiles[y][x];
        if(z==0) continue;
        x2=(z-1)%size.x();
        y2=(z-1)/size.x();
        q[size.x()*size.y()-1-z]->setPos(ssize*x+sgap*x-sgap+(ssize-fontMetric.width(QString::number(z)))/2, ssize*y+sgap*y-sgap+(ssize-fontMetric.height())/2);
        q[2*size.x()*size.y()-2-z]->setPos((x-x2)*ssize+(x-x2)*sgap, (y-y2)*ssize+(y-y2)*sgap);
    }
}

To copy to clipboard, switch view to plain text mode 

its faster because it doesnt have to call scene->items thousands of times (each time returning a huge qlist). with 20k items, it only took 0.15 seconds intead of 12 minutes

[spirit]

Well, QGraphicsView::items is declared with const, but it returns a list of pointer to non-const items. So, you are fine.

[ben1996123]

Well, QGraphicsView::items is declared with const, but it returns a list of pointer to non-const items. So, you are fine.

ah, that makes sense, thanks