How to override 'paint' function of custom delegate class to draw QSpinBox - c++

Question

I have added a custom delegator to the QTableView. When I double click on an item I see the editor widget which is a 'QSpinBox' and I am able to edit the value fine. This editor widget disappears once the focus is lost and I understand that. What I want is QSpinBox to be there all the time. Looking at the Qt example here I know I need to override the paint function of QAbstractItemDelegate class to draw the QSpinBox but I don't know how to that. In general, I want to know how any of the Qt widgets can be drawn inside a paint function.
For reference, I am having following test code:
#include <QtWidgets/QApplication>
#include <QtGui>
#include <QTableview>
#include <QLayout>
#include <QColor>
#include <QStyledItemDelegate>
#include <QSpinbox>
class SpinBoxDeligate : public QStyledItemDelegate {
public:
QWidget * createEditor(QWidget *parent,
const QStyleOptionViewItem &option,
const QModelIndex &index) const override {
auto w = new QSpinBox(parent);
w->setFrame(false);
w->setMinimum(0);
w->setMaximum(100);
return w;
}
void setEditorData(QWidget *editor, const QModelIndex &index) const override {
static_cast<QSpinBox*>(editor)->setValue(index.data(Qt::EditRole).toInt());
}
void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override {
model->setData(index, static_cast<QSpinBox*>(editor)->value(), Qt::EditRole);
}
void paint(QPainter *painter, const QStyleOptionViewItem &option,
const QModelIndex &index) const
{
// What to replace below line with to have a QSpinBox
QStyledItemDelegate::paint(painter, option, index);
}
};
int main(int argc, char** argv)
{
QApplication app(argc, argv);
QStandardItemModel model(3, 1);
for (int r = 0; r < 3; ++r)
{
auto text = QString("%0").arg(r);
QStandardItem* item = new QStandardItem(text);
item->setFlags(Qt::ItemIsUserCheckable
| Qt::ItemIsEnabled
| Qt::ItemIsEditable
);
item->setData(Qt::Unchecked, Qt::CheckStateRole);
item->setData(text, Qt::ToolTipRole);
item->setData(QSize(100, 30), Qt::SizeHintRole);
item->setData(QIcon(":/QtMVC/Desert.jpg"), Qt::DecorationRole);
model.setItem(r, 0, item);
}
QTableView* table = new QTableView();
table->setModel(&model);
table->setItemDelegate(new SpinBoxDeligate());
QWidget w;
QVBoxLayout* containerLayout = new QVBoxLayout();
w.setLayout(containerLayout);
containerLayout->addWidget(table);
w.show();
return app.exec();
}

Answer 1

A possible solution for your background problem is to use paint() and for that you could create a QSpinBox and use grab to take an image, but before that you should calculate the geometry so that it does not cover the QCheckBox, as you see it is a tedious job, another way is using QStyle but it is still much more code.
A simple solution is to keep the editor open with the openPersistentEditor() method.
int main(int argc, char** argv)
{
QApplication app(argc, argv);
QStandardItemModel model(3, 1);
QTableView* table = new QTableView();
table->setModel(&model);
table->setItemDelegate(new SpinBoxDeligate());
for (int r = 0; r < 3; ++r)
{
auto text = QString("%0").arg(r);
QStandardItem* item = new QStandardItem(text);
item->setFlags(Qt::ItemIsUserCheckable
| Qt::ItemIsEnabled
| Qt::ItemIsEditable
);
item->setData(Qt::Unchecked, Qt::CheckStateRole);
item->setData(text, Qt::ToolTipRole);
item->setData(QSize(100, 30), Qt::SizeHintRole);
item->setData(QIcon(":/QtMVC/Desert.jpg"), Qt::DecorationRole);
model.setItem(r, 0, item);
table->openPersistentEditor(model.indexFromItem(item));
}
QWidget w;
QVBoxLayout* containerLayout = new QVBoxLayout();
w.setLayout(containerLayout);
containerLayout->addWidget(table);
w.show();
return app.exec();
}

Answer 2

I can think of two possible solutions:
It is possible to insert widgets into table cells. I know this is not what you're trying to do but it can be a better solution for your problem. Check setIndexWidget.
If you really want to render QSpinBox, you should use the render method of QWidget. To try, in your paint method create a QSpinBox and call it's render method passing it the QPainter pointer. After you get it working this way, you can improve your design by possibly holding a 'template' QSpinBox instance in your QTableView and use it to render different QSpinBox values inside cells where required