"keyPressEvent" is (sometimes) not called after DragDrop

tomweb

Hello,
i want to drag items from a table, but only when i press ALT.
Without i want only easy selection.

My solution:
void Cxy::keyPressEvent(QKeyEvent *event)
{ if(event->key() == Qt::Key_Alt) { gotable->setDragEnabled(true); }
}
void Cxy::keyReleaseEvent(QKeyEvent *event)
{ if(event->key() == Qt::Key_Alt) { gotable->setDragEnabled(false); }
}

works, but i often loose "keyReleaseEvent" when i drop the content in another program.
My solution: a retriggered timer till the windows function "GetKeyState" says "no more ALT". Works perfect.

But i still have a problem with "keyPressEvent".
This works fine: (A) 1. press ALT, 2. drag 3. drop 4. release ALT
This also: (B) 1. press ALT 2. drag 3. release ALT 4. drop in another program (my program looses focus)
This also: (C) 1. press ALT 2. drag 3. release ALT 4. drop in my own program

BUT, in case (C) qt don't call "keyPressEvent" when i press ALT next time.
In this single case i have to press ALT 2 times, only the second time i get a "keyPressEvent"
I even can press other keys, they are detected(!), but when i press ALT after (C) (and even after pressing other keys), the first pressed ALT has no effect.
Seems that only the ALT key is locked once after (C).

Any idea?
I could use "GetKeyState" again, but then i have to poll it permanently and waste many cpu-time...

tomweb

I guess, when Drag is in progress, Qt does not provide key-events at all.

Seems that key-handling is not the the main strenght from Qt, so i wrote my own key-handler by using
original windows funktion SetWindowsHookEx(WH_KEYBOARD_LL, myLowLevelKeyboardProc, NULL, 0);
Not portable, of course, but it works perfect in all cases without the need to program timers etc.
But i am still open for better soulutions...

raven-worx

@tomweb
a way cleaner approach instead of toggling the drag-enabled property like you do, would be to override QAbstractItemView::startDrag() and check if the ALT key is pressed - using QApplication::keyboardModifiers().

Assuming Cxy is subclassing QAbstractItemView

void Cxy::startDrag(Qt::DropActions supportedActions)
{
    if( !(QApplication::keyboardModifiers() & Qt::AltModifier) )
         return;

    QModelIndexList selectedIndexes = this->getSelectedDraggableIndexes();  // need to be implemented by yourself
    if (selectedIndexes .count() > 0)
    {
        QMimeData *data = this->model()->mimeData(selectedIndexes);
        if ( !data )
            return;

        QDrag *drag = new QDrag(this);
        drag->setPixmap(...);
        drag->setMimeData(data);
        drag->setHotSpot(...);
        Qt::DropAction defaultDropAction = Qt::IgnoreAction;
        if (this->defaultDropAction() != Qt::IgnoreAction && (supportedActions & this->defaultDropAction()))
            defaultDropAction = this->defaultDropAction();
        else if (supportedActions & Qt::CopyAction && dragDropMode() != QAbstractItemView::InternalMove)
            defaultDropAction = Qt::CopyAction;
        drag->exec(supportedActions, defaultDropAction);
        ....
    }
}

tomweb

Hello @raven-worx ,
thank you very much for this hint.
First you confused me a little bit with "subclassing QAbstractItemView".
I didn't find how to subclass "QAbstractItemView".
In effect, i only had to extended "myQTableWidget"
in Header:
bool useAltKey; // DragDrop with ALT on/off
void startDrag(Qt::DropActions supportedActions);
and in implementation:
void myQTableWidget::startDrag(Qt::DropActions supportedActions)
{ if( useAltKey && (!(QApplication::keyboardModifiers() & Qt::AltModifier)) ) { return; }
QTableWidget::startDrag( supportedActions);
}
Works perfect, and it's even easier to turn on/off this feature. Again thank you.

raven-worx

@tomweb
indeed...this is easier. :)