[Qt-interest] QDate representation in QCombobox

[Anders Bakken]

On Thu, Jul 22, 2010 at 03:42:47AM +0100, Petric Frank wrote:
> Hello,
>
> i have a model (QSqlTableModel) set to an instance of QCombox (->setModel).
> This comboc box is configured to be not editable.
> >From the model i select a column which represents a date and passed it to the
> combo box instance. I've verified that the data returned from the model is of
> type QVariant::Date.
>
> For the QComboBox instance i set the delegate to my own one to so that i get
> my own representation of the date value.
> This works well for the items in the drop-down list, but not for the one in
> the selected field.
>
> Where i have to set the delegate to get my representation of the date in the
> selected field ?

It turns out it's a terrible ordeal.

The text on the combo box when not opened is not painted by a delegate.
It kinda should be but that's too late to change now.

This usually leaves you with the option of changing the style or
reimplementing paintEvent.

The former is usually cleaner but in this case I think I would go for
the latter for this reason.

The style option passed to the style for painting the text converts the
date into a string using QVariant::toString() which uses the rather
illusive under-the-hood methods of QVariant for its conversion.

It does turn out that you can find the QDate using the following
approach:

QVariant foo = styleOption->currentText;
QDate date = foo.toDate();

but that's kinda nasty.

Moreover the style is not passed a pointer to the combobox itself so
you'd have to hack horribly to get a hold of the actual model to find
the real value.

That being said the paintEvent of QComboBox is rather nice and short and
the parts of it that you don't want to change can easily be changed so I
would recommend this method.

I did throw together an example taking the first approach but I would
rather do something like this:

class MyCombo : ...
{
    void paintEvent(...)
    {
        QStylePainter painter(this);
        painter.setPen(palette().color(QPalette::Text));

        // draw the combobox frame, focusrect and selected etc.
        QStyleOptionComboBox opt;
        initStyleOption(&opt);
        /* All other lines are copied from qcombobox.cpp

        inside here you could do something along the lines of:

        opt.currentText = myConversionOfDateToString(currentIndex().data().toDate());

        */

        painter.drawComplexControl(QStyle::CC_ComboBox, opt);

        // draw the icon and text
        painter.drawControl(QStyle::CE_ComboBoxLabel, opt);
    }
};

I've attached the example I did write.

regards

--
Anders Bakken

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#include <QtGui>

class Style : public QPlastiqueStyle
{
    Q_OBJECT
public:
    void drawControl(ControlElement element, const QStyleOption *option,
                     QPainter *painter, const QWidget *widget) const
    {
        if (element == CE_ComboBoxLabel) {
            QDate date = QVariant(qstyleoption_cast<const QStyleOptionComboBox*>(option)->currentText).toDate();
            painter->drawText(option->rect, Qt::AlignCenter, QString::number(date.day()));
        } else {
            QPlastiqueStyle::drawControl(element, option, painter, widget);
        }
    }
};

class ItemDelegate : public QStyledItemDelegate
{
public:
    void paint(QPainter *painter,
               const QStyleOptionViewItem &option, const QModelIndex &index) const
    {
        painter->drawText(option.rect, Qt::AlignCenter, QString::number(index.data().toDate().day()));
    }
};
#include "main.moc"


int main(int argc, char **argv)
{
    QApplication a(argc, argv);
    QStandardItemModel model;
    for (int i=0; i<10; ++i) {
        QStandardItem *item = new QStandardItem;
        item->setData(QDate::currentDate().addDays(i), Qt::DisplayRole);
        model.setItem(i, 0, item);
    }
    ItemDelegate id;
    QComboBox box;
    box.setStyle(new Style);
    box.setItemDelegate(&id);
    box.setModel(&model);
    box.show();
    return a.exec();
}