drag and drop detect source of drag fails

sandro4912

I have a class Room which holds connections to three neighbor Rooms.

class Room : public QGraphicsWidget
{
    Q_OBJECT
//....

private:
    static constexpr auto mCountOfNeigbours = 3;

    std::array<Room *, mCountOfNeigbours> mNeigbours;

No i want to drag and drop the player from one room to another. But only to the neighbour Rooms.

So i start a drag like this:

void Room::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    if (QLineF(event->screenPos(), event->buttonDownScreenPos(Qt::LeftButton))
        .length() < QApplication::startDragDistance()) {
        return;
    }
    
    setPlayer(false);

    auto drag = new QDrag(event->widget());
    auto mime = new QMimeData;
    drag->setMimeData(mime);

    mime->setImageData(playerDraggedImage());

    drag->setPixmap(QPixmap::fromImage(playerDraggedImage()));
    drag->setHotSpot(QPoint(0, 0));

    auto result = drag->exec();

    if (result == Qt::IgnoreAction) {
        setPlayer(true);
    }

    setCursor(Qt::OpenHandCursor);
}

The dragEnterEvent and the dropEvent which should get executed on a neigbour look like this:

void Room::dragEnterEvent(QGraphicsSceneDragDropEvent *event)
{
    if ((event->mimeData()->imageData() == playerDraggedImage()) &&
        isNeighbour(event)) {
            event->setAccepted(true);

    }
    else {
        event->setAccepted(false);
    }
}

void Room::dropEvent(QGraphicsSceneDragDropEvent *event)
{
    if ((event->mimeData()->imageData() == playerDraggedImage()) &&
            isNeighbour(event)) {
         setPlayer(true);
    }
    update();
}

With a helper which checks if the room is a neigbour of the source room were the drag started:

bool Room::isNeighbour(QGraphicsSceneDragDropEvent *event) const
{
    auto room = qobject_cast<Room *>(event->source());

    if (room == nullptr) {
        return false;
    }

    for(const auto &neighbour : mNeigbours) {

        if (room == neighbour) {
            return true;
        }
    }
    return false;
}

So far so good but it is not working. If i try to find the sender of the drag with

auto room = qobject_cast<Room *>(event->source());

The result is always zero.

However i can see were i make the drag:

auto drag = new QDrag(event->widget());

Here event->widget() has a address, Then why it arrives with nullptr on the target?

jsulm

@sandro4912 said in drag and drop detect source of drag fails:

Here event->widget() has a address

But is this widget a Room*?
qobject_cast<Room >(event->source()); will return nullptr if event->source() is not a Room

SGaist

Hi,

Beside @jsulm good remark, you are not accepting the event or the proposed action in your re-implementation of dropEvent.

sandro4912

The Rooms get put into a QGraphicsScene but i would have expect that QGraphicsSceneMouseEvent *event->widget() is a Room in Room::mouseMoveEvent

I added the accept like this:

void Room::dropEvent(QGraphicsSceneDragDropEvent *event)
{
    if ((event->mimeData()->imageData() == playerDraggedImage()) &&
            isNeighbour(event)) {
         setPlayer(true);
         update();
         event->setAccepted(true);
    }
    else {
        event->setAccepted(false);
    }
}

Is it maybe impossible like i think? Do i have to sublass QMimeData and pass the pointer of the object were the drag was created?

Christian Ehrlicher

@sandro4912 said in drag and drop detect source of drag fails:

QGraphicsSceneMouseEvent *event->widget()

So did you actually check it?

Returns the widget where the event originated,

I would not expect that a mouse move event's originator is a room object.

sandro4912

@sandro4912 said in drag and drop detect source of drag fails:

Is it maybe impossible like i think? Do i have to sublass QMimeData and pass the pointer of the object were the drag was created?

Yes i checked it and it is not a `Room. I guess i thought it wrongt. So to shorten my question.

How can you tell in a drop event were the drag was started from?

Making a custom QMimeData type and passing the Room pointer ?

SGaist

That's one way yes.

sandro4912

I solved the issue the pragmatic way:

class RoomMimeData : public QMimeData
{
    Q_OBJECT
public:
    void setRoom(Room *room);
    Room *room() const;

private:
    Room *mRoom;
};

void Room::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
//...
    auto drag = new QDrag(event->widget());
    auto mime = new RoomMimeData;
    mime->setRoom(this);
    drag->setMimeData(mime);

//..
}

void Room::dropEvent(QGraphicsSceneDragDropEvent *event)
{
    const auto roomData = qobject_cast<const RoomMimeData *>(event->mimeData());
    if(roomData && isNeighbour(roomData->room()) &&
            roomData->imageData() == playerDraggedImage()) {
         enter();
         update();
         event->setAccepted(true);
    }
    else {
        event->setAccepted(false);
    }
}