Model View alter display text

[JonB]

I have a model (QStandardItemModel-derived) and a view (QTableView-derived). I wish to alter the displayed text for certain rows which hold a numeric to show as percentage, e.g. 30 -> 30%. (The principle here would apply to any other transformation, e.g. $30 or (30) for accountancy negative numbers, and is not uncommon in tables displaying, say, numeric values)

I used to accomplish this by overriding data(const QModelIndex &index, int role = Qt::DisplayRole) const having it go essentially:

if (role == Qt::DisplayRole)
    if (index.row() == percentRow)
    {
        int val = Base::data(index, role).toInt();
        return QString("%1%").arg(val);
    }

i.e. making it return the string I wanted instead of the numeric. This was easy and I could do it in 30 seconds.

I was told in no uncertain terms by experts here that this was wicked, I must leave the data type returned as-was for all sorts of reasons, not convert to string here. I should use QStyledItemDelegate for this. Fair enough.

I come to do that now and was assuming I could use QString QStyledItemDelegate::displayText(const QVariant &value, const QLocale &locale) const, which would not have been too bad. However I now see that does not convey the QModelIndex, so I cannot select by row/column.

Which means I seem to be left with overriding void QStyledItemDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const, and doing quite a bit of work there (converting to string and dealing with painters).

Is this really what I have to do to alter the string displayed for a number? Seems crazy to me to be going down into painters :( There isn't anything I can do via the QStyleOptionViewItem &option, is there? Nor can I get a displayText() override to know about the QModelIndex, can I?

[VRonin]

initStyleOption is also virtual so you can do the chage directly there with minimal effort.

class ListviewDelegate : public QStyledItemDelegate{
    Q_OBJECT
    Q_DISABLE_COPY_MOVE(ListviewDelegate)
public:
    explicit ListviewDelegate(QObject* parent = nullptr)
        : QStyledItemDelegate(parent)
    {}
protected:
    void initStyleOption(QStyleOptionViewItem *option, const QModelIndex &index) const override{
        QStyledItemDelegate::initStyleOption(option,index);
        if (index.row() == percentRow)
            option->text += QChar('%');
    }
};

if you need to do anything more fancy to the text you can just use option->text = doSomethingFancy(displayText(index.data(Qt::DisplayRole), option->locale));

[Bonnie]

I'm not a expert but returning a QString looks fine to me. :)

[JonB]

@Bonnie
I wish! I got beyond shouted out (nicely) by expert people here saying I should be boiled in oil :) Not quite, but they were very firm on "don't change the return type of Qt::DisplayRole, it isn't intended for this at all, and the only time it should differ from Qt::EditRole is as an example where you are doing a spreadsheet and you have the display role showing a calculated result number while the edit role is the text of a formula". A quite different type of situation. For output like this the way to do is through a item delegate, only. Model code must not concern with output format. I posted an explicit question about just this and gracefully accepted their answers, and promised I would adhere to it next time I wrote code... :)

Lemme see if I can find it:

[mrjj]

Hi
Its not that much painter code to append %

void ListviewDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option,
                             const QModelIndex &index) const
{


    QStyleOptionViewItem nonConstOption(option);
    initStyleOption(&nonConstOption, index);
    int val = 66;// get from model
    nonConstOption.text = QString("%1%").arg(val);
    QApplication::style()->drawControl(QStyle::CE_ItemViewItem, &nonConstOption, painter, nullptr);
}

[JonB]

@mrjj
Thank you for supplying this. Looks to me like some damn hairy code to append a % to a number :)

I need to look through the docs for this tomorrow to understand what is going on. Since I won't want to interfere with whatever paint() does or does not do for all other cases, I guess I'll want something like:

if (index.row() == percentRow)
{
    QVariant v = data(index);
    if (v.somethingAboutTypeIsNumber())
    {
        ( your code, using  QString("%1%").arg(v);, but I don't think that accepts a QVariant )
        return;
    }
}
QStyledItemDelegate::paint(painter, option, index);

Good grief, this is getting hard for one darned % character...! :(

[VRonin]

initStyleOption is also virtual so you can do the chage directly there with minimal effort.

class ListviewDelegate : public QStyledItemDelegate{
    Q_OBJECT
    Q_DISABLE_COPY_MOVE(ListviewDelegate)
public:
    explicit ListviewDelegate(QObject* parent = nullptr)
        : QStyledItemDelegate(parent)
    {}
protected:
    void initStyleOption(QStyleOptionViewItem *option, const QModelIndex &index) const override{
        QStyledItemDelegate::initStyleOption(option,index);
        if (index.row() == percentRow)
            option->text += QChar('%');
    }
};

if you need to do anything more fancy to the text you can just use option->text = doSomethingFancy(displayText(index.data(Qt::DisplayRole), option->locale));

[JonB]

@VRonin
Thank you, this looks much more like a reasonable approach! No offence to @mrjj's :) I will go investigate this now and report back....

...and I am pleased to report this works as desired, and in a reasonable code fashion. The secret was indeed that initStyleOption() is virtual.