QCombobox not using custom delegate to render current item - qt

Question

I am using a custom delegate to render rich text in a QComboBox. It is used to allow users to select a color to plot a variable on a graph. It works for items in the drop down menu, but not the selected item. Any help would be appreciated.
Here is the code for the delegate I am using:
class CustomDelegate : public QStyledItemDelegate
{
public:
CustomDelegate();
protected:
void paint ( QPainter * painter, const QStyleOptionViewItem & option, const QModelIndex & index ) const;
QSize sizeHint ( const QStyleOptionViewItem & option, const QModelIndex & index ) const;
};
void CustomDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const{
QStyleOptionViewItemV4 optionV4 = option;
initStyleOption(&optionV4, index);
QStyle *style = optionV4.widget? optionV4.widget->style() : QApplication::style();
QTextDocument doc;
doc.setHtml(optionV4.text);
/// Painting item without text
optionV4.text = QString();
style->drawControl(QStyle::CE_ItemViewItem, &optionV4, painter);
QAbstractTextDocumentLayout::PaintContext ctx;
// Highlighting text if item is selected
if (optionV4.state & QStyle::State_Selected)
ctx.palette.setColor(QPalette::Text, optionV4.palette.color(QPalette::Active, QPalette::HighlightedText));
QRect textRect = style->subElementRect(QStyle::SE_ItemViewItemText, &optionV4);
painter->save();
painter->translate(textRect.topLeft());
painter->setClipRect(textRect.translated(-textRect.topLeft()));
doc.documentLayout()->draw(painter, ctx);
painter->restore();
}
QSize CustomDelegate::sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const{
QStyleOptionViewItemV4 optionV4 = option;
initStyleOption(&optionV4, index);
QTextDocument doc;
doc.setHtml(optionV4.text);
doc.setTextWidth(optionV4.rect.width());
return QSize(doc.idealWidth(), doc.size().height());
}
I set the options and delegate for the combo box using code similar, I removed some of the options to reduce the size of the code:
ui->SelectColor->clear();
ui->SelectColor->addItem("Select Color");
ui->SelectColor->addItem("<font color='blue'>Blue</font>");
ui->SelectColor->addItem("<font color='darkBlue'>Dark Blue</font>");
ui->SelectColor->addItem("<font color='red'>Red</font>");
ui->SelectColor->addItem("Dark Yellow");
ui->SelectColor->addItem("<font color='magenta'>Dark Magenta</font>");
ui->SelectColor->addItem("White");
ui->SelectColor->setItemDelegate(new CustomDelegate);
Some of the options are just the names because they don't look good when rendered.

Answer 1

Your custom styled item delegate is applied to items that are part of QComboBox (i.e. ones in QComboBox popup), not for the current item which is represented. The easiest way to achieve what you want would be to introduce your custom class inheriting QComboBox, and then override void QWidget::paintEvent(QPaintEvent *event) method by applying changes you would like to introduce, e.g. set some colored text. Another way (in case you can not introduce another class inheriting QComboBox for some reason) would be to introduce an event filter that does something just after the QPaintEvent. However, usage of event filters might be tricky, and I would advise you to just introduce some another class, and then override paint event.
Now, in case you want to show current item in the same way as an option in the combo box list popup, you could do the following (code is not complete, and you should apply it to your needs yourself):
virtual void paintEvent(QPaintEvent* e) override
{
// QComboBox::paintEvent(e); - this will leave just a rectangle in which you can perform your custom drawings.
// Will make your option colored at least.
QPainter p(this);
QTextDocument doc;
doc.setHtml(this->currentText());
doc.drawContents(&p, rect());
}
Finally, it is worth to mention that QComboBox painting is pretty complex, and you might still need to rewrite pretty everything that Qt already has done for you, so it would be possible to apply other styles you might want not to lose. In order to do that, you should take a look into the source code of this class. You can do this in the Code Browser by Woboq for C & C++ (QComboBox). Hopefully, this clarifies the problem for you more, and now you know what to do in order to achieve your goal.