Qt tip & Trick: Masking Widgets

Have you ever tried to create an overlay on top of some QWidgets to make them not acessible, but still visible to the user? I’m not saying about setEnabled(false), but to create a semitransparent – mask on top of some widgets with something inside, a modal dialog or so.

I was yesterday at #qt trying to figure out how to do that, aseigo had already told me that it was possible  by using a QGraphicsView as the mask widget with a transparent background, so I did this:

[snip]

view->scene()->setSceneRect( w->rect() );
view->setFixedSize( w->size() );
view->setFrameStyle( QFrame::NoFrame );
view->setHorizontalScrollBarPolicy( Qt::ScrollBarAlwaysOff );
view->setVerticalScrollBarPolicy( Qt::ScrollBarAlwaysOff );
view->setBackgroundRole( QPalette::Window );
view->setBackgroundBrush(QBrush(QColor(122,122,122,122)));
view->setAutoFillBackground( true );

[/sniṕ]

this was doing almost, but not quite, not what I wanted. It correctly paced the QGraphicsView on top of the widgets, no scalling, but it was opaque. then Leo Franchi came to the rescue.

QPalette p = view->palette();
p.setColor( QPalette::Window, Qt::transparent );
view->setPalette( p );

that way, the QGraphicsView was alpha-transparent on top of my widgets, and a nice mask-effect was given.

but beware that this approach does not handle resizing since the item does not belong to any QLayout, not a problem with Qt, since you can manage to make that happen using a eventFilter:

bool KWelcomeWidget::eventFilter( QObject* obj, QEvent* event ){
if( event->type() == QEvent::Resize ) {
QWidget *parentWidget = static_cast<QWidget*>(obj);
QGraphicsView *view = static_cast<QGraphicsView*>(d_ptr->m_part->widget());
view->scene()->setSceneRect( parentWidget->rect() );
view->setFixedSize( parentWidget->size() );
setFixedSize(parentWidget->size());
return false;
}
return QObject::eventFilter( obj, event );
}