Cant get "mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)" to work.

[ademmler]

Hi there,

in a bigger project I try too create a dialog for editing whitepoint and greybalance.
In order to trace and detect my issue I have extracted the minimal code.

What do I have: A dialog with 2 GroupBoxes. Each has a GraphicsView and his own scene.
After painting a background I add an ellipse-item and set:

QGraphicsEllipseItem *ellipse = new QGraphicsEllipseItem (250 -5, 250 - 5, 10, 10);    
ellipse->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemIsFocusable | QGraphicsItem::ItemSendsScenePositionChanges | QGraphicsItem::ItemSendsGeometryChanges);
scene->addItem(ellipse);

I do this, because I aim to grab and move this circle - which works.
Now I try to react on the movement and position changes. In my minimal.h .

void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent);

In my minimal.cpp

void WpEditor::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    mPoint = event->pos();
    qDebug() << "Mouse move " << mPoint;
    QGraphicsScene::mouseMoveEvent(event);
}

My issue: Nothing happens. What I am doing wrong here.

thx and regards

PS: If you need the full example pls let me know where to send it. I still can't upload here.

[JonB]

@ademmler
You mean your method is not hit? Start by appending override in the .h file to be sure.

WpEditor is indeed a QGraphicsScene and not a QGraphicsView, right?

In the event I think you will want to use event->scenePos(); rather than event->pos();.

[mchinand]

It might be easier to sub-class QGraphicsObject and then make connections to its x/yChanged() signals.

[mchinand]

@ademmler said in Cant get "mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)" to work.:

void WpEditor::mouseMoveEvent(QGraphicsSceneMouseEvent *event)

What is your WpEditor class a sub-class of?

[ademmler]

@JonB said in Cant get "mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)" to work.:

@ademmler

Dear JonB - thx for your feedback.

You mean your method is not hit? Start by appending override in the .h file to be sure.

When I do so I get:
error: non-virtual member function marked 'override' hides virtual member function
Honestly I have never understood when I can override a QT function and when I can't.

WpEditoris indeed aQGraphicsSceneand not aQGraphicsView`, right?

It is a QGrahicsview (in an UI file) which I attache a scene to. In the constructor I do:
... some stuff
sceneWp = new QGraphicsScene(0, 0, 500, 500);
viewWp->setScene(sceneWp);
... some more stuff

In the event I think you will want to use event->scenePos(); rather than event->pos();.

Yes this is correct - my fault !

[jsulm]

@ademmler said in Cant get "mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)" to work.:

error: non-virtual member function marked 'override' hides virtual member function

Show the code. You're not correctly overriding.

[JonB]

@ademmler said in Cant get "mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)" to work.:

error: non-virtual member function marked 'override' hides virtual member function
Honestly I have never understood when I can override a QT function and when I can't.

This is totally a C++ issue, nothing special about Qt. And it is really easy to tell:

• If a method is marked virtual you can override it. And Qt docs will always show virtual for such methods.
• If a method is not marked virtual you cannot override it.

"Simplez" :) override is not mandatory, but I suggest you should always put it in if you are intending to override a method. Because you get an error then if the method, or its signature, are not virtual/overridable, and you will know. And in this case the error message tells you exactly that.

So you must either sub-class your QGraphicsScene and override its void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent) (which I have never tried) or you need to override the QGraphicsView::mouseMoveEvent(QMouseEvent *event) in your WpEditor. In this case, confirm that you will keep the left mouse pressed down while you do drag-move, else you will need to call QGraphicsView::setMouseTracking(true) .

Meanwhile, I noticed your original QGraphicsScene::mouseMoveEvent(event); in WpEditor::mouseMoveEvent(), your attempt to call the base class implementation. I took you at your word, but you need QGraphicsView::mouseMoveEvent(event); instead!

[ademmler]

@JonB thx again JonB.

Either I subclass QGraphicsScene - as you wrote before or using an event filter might help. I found this thread about it.
https://stackoverflow.com/questions/51151806/use-a-qgraphicsscenemouseevent-in-a-class-that-inherits-from-qgraphicsview

bool DisplayImage::eventFilter(QObject *watched, QEvent *event)
{
    if(watched == scene){
        // press event
        QGraphicsSceneMouseEvent *mouseSceneEvent;
        if(event->type() == QEvent::GraphicsSceneMousePress){
            mouseSceneEvent = static_cast<QGraphicsSceneMouseEvent *>(event);
            qDebug()<<mouseSceneEvent->scenePos()<<mouseSceneEvent->lastScenePos();
           // your logic here
        }
        // move event
        else if (event->type() == QEvent::GraphicsSceneMouseMove) {
            mouseSceneEvent = static_cast<QGraphicsSceneMouseEvent *>(event);
            qDebug()<<mouseSceneEvent->scenePos()<<mouseSceneEvent->lastScenePos();
            // your logic here
        }
        // release event
        else if (event->type() == QEvent::GraphicsSceneMouseRelease) {
            mouseSceneEvent = static_cast<QGraphicsSceneMouseEvent *>(event);
            qDebug()<<mouseSceneEvent->scenePos()<<mouseSceneEvent->lastScenePos();
            // your logic here
        }
    }
    return QGraphicsView::eventFilter(watched, event);
}

[JonB]

@ademmler
Yes, you can always do sub-classing/events via eventFilter() if that is what you want. It can avoid need for sub-classing in some cases, but the code is/may be "messier" then overriding the methods you need. As you please.

[mchinand]

What are you actually trying to do? I had suggested using a QGraphicsObject and its signals because I thought you were moving around the ellipse you create to change two parameters that affect other things in your application/UI and its position would reflect the current value of those parameters. If this is the case, even if you do this by watching mouse events in the scene, you will want to use changes in the ellipse object's position to make your parameter changes.

[ademmler]

@mchinand

hi you are right - I want to change parameters according to x/y changes.
I will have a look at you solution too! And come back of course.