Painter error in paintCell() function when subclassing QCalendarWidget

Question

I want to create calendar, that will mark several dates entered by user. So I have subclassed QCalendarWidget and reimplemented painCell function. Here is my simplified code:

MyCalendar::MyCalendar(QWidget *parent)
    : QCalendarWidget(parent)
{   
    painter = new QPainter(this);   
}
void MyCalendar::setHolidays(QDate date)
{
    paintCell(painter, rect(),date);        
}

void MyCalendar::paintCell(QPainter * painter, const QRect & rect, const QDate & date) const
{
    painter->setBrush(Qt::red);
    QCalendarWidget::paintCell(painter, rect, date);
}

I can't do it though, because when creating QPainter object i am getting this message: "QWidget::paintEngine: Should no longer be called QPainter::begin: Paint device returned engine == 0, type: 1"

When I am not setting painter parent, I get this error when trying to set brush: "QPainter::setBrush: Painter not active" I think, I am creating QPainter object in a wrong place. Anyone knows, how to resolve this?

I was using Qt wiki snippet: https://wiki.qt.io/How_to_create_a_custom_calender_widget

Answer 1

You should not draw directly since the paintCell method is called internally, it is appropriate to save the dates in a list, and if the date used by paintCell is contained in that list, draw it in a personalized way:

#ifndef MYCALENDAR_H
#define MYCALENDAR_H

#include <QCalendarWidget>
#include <QPainter>
class MyCalendar : public QCalendarWidget
{
public:
    MyCalendar(QWidget *parent=Q_NULLPTR):QCalendarWidget{parent}{

    }
    void addHoliday(const QDate &date){
        mDates<<date;
        updateCell(date);
    }
    void paintCell(QPainter * painter, const QRect & rect, const QDate & date) const{
        if(mDates.contains(date)){
            painter->save();
            painter->setBrush(Qt::red);
            painter->drawRect(rect);
            painter->drawText(rect, Qt::AlignCenter|Qt::TextSingleLine, QString::number(date.day()));
            painter->restore();
        }
        else
            QCalendarWidget::paintCell(painter, rect, date);

    }
private:
    QList<QDate> mDates;
};

#endif // MYCALENDAR_H