Drag and drop from QToolBar to QGraphicsView is not working as expected

Question

I am working on a project using Qt C++ . I have a QToolBar which contains some QButtons (read as QAction) and a QGraphicsView (the whole bottom part). It looks like this.

Now, I want to drag a QButton (read as QAction) from the QToolBar into central window (which actually is QGraphicsView) of my application.

Till now I have written the following SLOT for corresponding QAction to enable dragging and dropping.

/* Send_Message is one of the actions */
void CTestBuilderApp::on_actionSend_Message_triggered()
{    
    QByteArray itemData;    
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);    
    emit selectionState(None); 

    dataStream << "Send Message";    

    QMimeData *mimeData = new QMimeData;    
    mimeData->setData("application/x-dnditemdata", itemData);    

    QDrag *drag = new QDrag(this);    
    drag->setMimeData(mimeData);    
    drag->setPixmap(ui->ToolBox->actions().at->icon().pixmap(48,48));    
    drag->exec(Qt::CopyAction, Qt::CopyAction);    
    qDebug()<<"drag complete";
} 

But, as this SLOT is being called only when I am clicking on the QAction, I am not getting the proper effect of Drag and Drop. When I am clicking the button, only then the dragging is starting. One click on the QAction and then another click on QGraphicsView is doing the job, but it's not what it's supposed to be for drag & drop. It's not starting on pressing and then dragging.

Is there any way out to make this code work? I am searching for some SLOT that get called as soon as I press on the QAction i.e. it does not wait for Mouse Release to work.

Answer 1

You should implement the QGraphicsView slots (so you need to subclass it):

void GraphicsView::dragEnterEvent(QDragEnterEvent *p_event)
{
    p_event->acceptProposedAction();
}

void GraphicsView::dragMoveEvent(QDragMoveEvent *p_event)
{
    p_event->acceptProposedAction();
}

void GraphicsView::dropEvent(QDropEvent *p_event)
{
    p_event->acceptProposedAction();
}

In your QGraphicsView constructor you have to accept the drops:

setAcceptDrops(true);

In all the slots you have to accept the action to allow the flow:

enter -> move -> drop