Using setData(Qt::EditRole, xxx) with QTableWidgetItem

[VRonin]

You are opening the pandora vase.

The solution is create a class:

class PeciseDoubleFactory : public QItemEditorFactory{
public:
PeciseDoubleFactory() = default;
virtual ~PeciseDoubleFactory() = default;
virtual QWidget* createEditor(int userType, QWidget *parent) const override{
if(userType == QVariant::Double){
        QDoubleSpinBox *sb = new QDoubleSpinBox(parent);
        sb->setFrame(false);
        sb->setMinimum(-DBL_MAX);
        sb->setMaximum(DBL_MAX);
        sb->setDecimals(6); // This is the maximum number of decimals you want
        return sb;
}
return QItemEditorFactory::(userType,parent);
}
};

then you have to set the delegate like this:

// PeciseDoubleFactory m_factory; is a private member
QStyledItemDelegate* preciseDoubleDelegate=new QStyledItemDelegate(ui.testTableWidget);
preciseDoubleDelegate->setItemEditorFactory(&m_factory);
ui.testTableWidget->setItemDelegate(preciseDoubleDelegate);

If you want to know what is happening here is the long story:

• QTableWidget uses the default QStyledItemDelegate
• QStyledItemDelegate, when you try to edit a value, will call the editor from the default QItemEditorFactory
• QItemEditorFactory if the value you are editing is a double will create a default QDoubleSpinBox
• QDoubleSpinBox has 2 decimals by default
• When editing is finished the QDoubleSpinBox::value is used to determine the new value that goes into the model and, as stated in QDoubleSpinBox documentation, it will round the returned value to the set number of decimals. Here is when you have a problem
• The delegate will then convert the double in the model to string

[diverger]

Thanks.

More further, it seems the entire widget use only one editor factory, what if I want to give different cells with different decimal count? Can I implement that base on your solution?

Sure, I can create dozens of QDoublespinbox, and give them needed decimal count and bind them to each cell, but that seems so ugly. That will be my last choice.

[VRonin]

This allows you to chose the number of decimals:

class PeciseDoubleFactory : public QItemEditorFactory{
private:
int m_decimals;
public:
void setDecimals(int val){
if(val<=0)
m_decimals=2;
else
m_decimals=val;
}
int decimals() const { return m_decimals;}
PeciseDoubleFactory() :m_decimals(2) {}
virtual ~PeciseDoubleFactory() = default;
virtual QWidget* createEditor(int userType, QWidget *parent) const override{
QWidget* result = QItemEditorFactory::(userType,parent);
if(userType == QMetaType::Double){
       Q_ASSERT(qobject_cast<QDoubleSpinBox*>(result));
        qobject_cast<QDoubleSpinBox*>(result)->setDecimals(m_decimals);
}
return result;
}
};

To have different decimals across the model just create different QStyledItemDelegate with different PeciseDoubleFactory and use setItemDelegateForColumn or setItemDelegateForRow

[diverger]

@VRonin Thanks for your help. Now I have another problem. Assume I double press one cell, and a spinbox appear and I change it's value to 2.00, but after the editing, the text showed in the cell change to "2", it abandoned the trailing zeros. How to make it preserve the trailing zeros?

[VRonin]

class DecimalsDelegate : public QStyledItemDelegate{
Q_OBJECT
Q_PROPERTY(unsigned int decimals READ decimals WRITE setDecimals NOTIFY decimalsChanged)
public:
explicit DecimalsDelegate(QObject* parent = nullptr)
:QStyledItemDelegate(parent)
,m_decimals(2U)
{}
virtual ~DecimalsDelegate() = default;
DecimalsDelegate(const DecimalsDelegate&) = delete;
DecimalsDelegate& operator=(const DecimalsDelegate&) = delete;
void setDecimals(unsigned int dec){
if(m_decimals!=dec){
m_decimals=dec;
emit decimalsChanged();
}
}
unsigned int decimals() const { return m_decimals;}
virtual QString displayText(const QVariant &value, const QLocale &locale) const override{
switch(value.type()){
case QMetaType::Double:
case QMetaType::Float:
return locale.toString(value.toDouble(),'f',m_decimals);
default:
return QStyledItemDelegate::displayText(value,locale);
}
}
signals:
void decimalsChanged();
private:
unsigned int m_decimals;
}

then use this class instead of the plain QStyledItemDelegate using setDecimals() to decide how many decimals you want to show

[diverger]

@VRonin Great. This almost achieve my goal. I can set the decimals by column or row now. But any way to set them by cell. Now I have to show nums with different decimals in one column :(.

BTW, anyway to know which cell the 'value' from in the displayText() function? If I can get the cell position from displayText(), then I can format the text with different decimals by the cell.

[VRonin]

There are different approaches here. Do you need a deterministic number of decimals or do you need as many decimals as the number has?
e.g.
data: 12.34500

do you have a 4 digits fixed so 12.3450 or as little digits as necessary to represent the number in full 12.345?

[diverger]

@VRonin I set the data with Qt::EditRole, it's double. When I set the data, the needed decimal count is determined, too. If the data is 12.345, and I need 4 decimal, it should displayed as '12.3450'. If I only need 2 decimal, then it should displayed as '12.35'.

Them problem now is, I want some cells show 4, some cells show 3, and some cells show 2, etc.

[VRonin]

Just use a role

class DecimalsDelegate : public QStyledItemDelegate{
Q_OBJECT
public:
enum { NumOfDecimalsRole = Qt::UserRole + 888 }; /* 888 is just a random number, you can use whatever you want if it conflicts */
explicit DecimalsDelegate(QObject* parent = nullptr)
:QStyledItemDelegate(parent)
{}
virtual ~DecimalsDelegate() = default;
DecimalsDelegate(const DecimalsDelegate&) = delete;
DecimalsDelegate& operator=(const DecimalsDelegate&) = delete;
virtual void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const override
{
    Q_ASSERT(index.isValid());

    QStyleOptionViewItem opt = option;
    initStyleOption(&opt, index);
const QVariant numOfDecimals = index.data(NumOfDecimalsRole );
    if (numOfDecimals.isValid() && !numOfDecimals.isNull() && numOfDecimals.canConvert(QMetaType::Int)  && ( opt.features & QStyleOptionViewItem::HasDisplay) ) {
const QVariant value = index.data(Qt::DisplayRole);
if(value.canConvert(QMetaType::Double))
        opt.text = opt.locale.toString(value.toDouble(),'f',numOfDecimals.toInt());
    }
    const QWidget *widget = option.widget;
    const QStyle* const style = widget ? widget->style() : QApplication::style();
    style->drawControl(QStyle::CE_ItemViewItem, &opt, painter, widget);
}

}

now after you set the Qt::EditRole just set NumOfDecimalsRole to the number of decimals you need

Edit: fixed copy/paste error

[diverger]

@VRonin Great, it work.

I have another idea. Because I can't know the current index of the model in the displayText function. So, I can't change the decimal count by the 'value' 's 'index'. But in 'setEditorData' and 'setModelData', and 'createEditor', I can know the 'index'. So, when I set the widget item, I use '

item->setData(Qt::EditRole, QString::number(ratio_error * 1000, 'f', 2));
ui.testTableWidget->setItem(row, 7, item);

And my derived class is as below:

virtual QWidget *createEditor(QWidget *parent, QStyleOptionViewItem const &option, QModelIndex const &index) const override
	{
		if (index.data(Qt::EditRole).isValid() && ((index.column() == 6) || (index.column() == 7)))
		{
			QDoubleSpinBox *spinbox = new QDoubleSpinBox(parent);
			QTableWidget *table = qobject_cast<QTableWidget *>(this->parent());

			if (index.column() == 6)
			{
				spinbox->setDecimals(2);
			}
			else if (index.column() == 7)
			{
				spinbox->setDecimals(3);
			}
			
			return spinbox;
		}
		else
		{
			return QStyledItemDelegate::createEditor(parent, option, index);
		}
	}

	virtual void iStyledItemDelegate::updateEditorGeometry(QWidget *editor,	QStyleOptionViewItem const &option,
		QModelIndex const &index) const override
	{
		editor->setGeometry(option.rect);
	}

	virtual void iStyledItemDelegate::setEditorData(QWidget *editor, QModelIndex const &index) const override
	{
		if (index.data(Qt::EditRole).isValid() && ((index.column() == 6) || (index.column() == 7)))
		{
			double value = index.model()->data(index, Qt::EditRole).toString().toDouble();
			QDoubleSpinBox *spinBox = qobject_cast<QDoubleSpinBox*>(editor);
			spinBox->setValue(value);
		}
		else
		{
			QStyledItemDelegate::setEditorData(editor, index);
		}
	}

	virtual void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override
	{
		if (index.data(Qt::EditRole).isValid() && ((index.column() == 6) || (index.column() == 7)))
		{
			QDoubleSpinBox *spin = qobject_cast<QDoubleSpinBox *>(editor);
			spin->interpretText();
			double value = spin->value();
			int decimals = spin->decimals();
			QString text = QString::number(value, 'f', decimals);
			model->setData(index, text, Qt::EditRole);
		}
		else
		{
			QStyledItemDelegate::setModelData(editor, model, index);
		}
	}

For the column 7 (the last column), it works as expected, but for column 6, when I double click the cell, the value goes to "0.00", and I debuged it, find for column 6, the invoke sequence is:

createEditor -> setEditorData -> setModelData, when invoke the 'setData' in this function, it invoke 'setEditorData' once more. It's right?

[diverger]

@VRonin Finally, I found the reason, I forgot set the minimum and maximum of the spinbox, it seem it's default minimum value is "0", and I just need show a minus float number in column 6. So it is "rounded" to zero.

Can you check my code below, if it's a 'right' usage case?

class iStyledItemDelegate : public QStyledItemDelegate
{
public:
	explicit iStyledItemDelegate(QObject *parent = nullptr)
		: QStyledItemDelegate(parent)
	{}

	virtual ~iStyledItemDelegate() = default;

	virtual QWidget *createEditor(QWidget *parent, QStyleOptionViewItem const &option, QModelIndex const &index) const override
	{
		if ((index.column() == 6) || (index.column() == 7))
		{
			QDoubleSpinBox *spinbox = new QDoubleSpinBox(parent);
			spinbox->installEventFilter(const_cast<iStyledItemDelegate*>(this));
			
			QTableWidget *table = qobject_cast<QTableWidget *>(this->parent());
			spinbox->setFrame(false);
			spinbox->setMinimum(-DBL_MAX);
			spinbox->setMaximum(DBL_MAX);

			if (index.column() == 6)
			{
				spinbox->setDecimals(2);
			}
			else if (index.column() == 7)
			{
				spinbox->setDecimals(3);
			}
		
			return spinbox;
		}
		else
		{
			return QStyledItemDelegate::createEditor(parent, option, index);
		}
	}

	virtual void updateEditorGeometry(QWidget *editor,	QStyleOptionViewItem const &option,
		QModelIndex const &index) const override
	{
		editor->setGeometry(option.rect);
	}

	virtual void setEditorData(QWidget *editor, QModelIndex const &index) const override
	{
		QDoubleSpinBox *spinBox = qobject_cast<QDoubleSpinBox*>(editor);
		
		if (spinBox && ((index.column() == 6) || (index.column() == 7)))
		{
			double value = index.data(Qt::EditRole).toString().toDouble();
			spinBox->setValue(value);
		}
		else
		{
			QStyledItemDelegate::setEditorData(editor, index);
		}
	}

	virtual void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override
	{
		QDoubleSpinBox *spin = qobject_cast<QDoubleSpinBox *>(editor);
	
		if (spin && ((index.column() == 6) || (index.column() == 7)))
		{
			spin->interpretText();
			model->setData(index, spin->text(), Qt::EditRole);
		}
		else
		{
			QStyledItemDelegate::setModelData(editor, model, index);
		}
	}
};

[VRonin]

@diverger said in Using setData(Qt::EditRole, xxx) with QTableWidgetItem:

if it's a 'right' usage case

The delegate should never depend on the model or view structure so if (index.column() == is never the right usage case

[diverger]

@VRonin Yes, I know it's not 'good' for 're-use'. But I can't find a method to set delegate to individual cells. In my case, take for example column 6. Some rows may need show 3, some rows may need show 2 decimals, depending on the column 2's contents. So I make this class dedicated to this. Any suggestions to implement my goal but no depending to 'index' in the derived class?

[VRonin]

https://forum.qt.io/topic/71692/using-setdata-qt-editrole-xxx-with-qtablewidgetitem/10

Then set the NumOfDecimalsRole in a slot connected to the dataChanged of the model

Btw, looking at the code above it looks like you need 2 decimals in column 6 and 3 decimals in column 7 and this can be achieved with 2 delegates set on 2 columns with setItemDelegeateForColumn

P.S.

model->setData(index, spin->text(), Qt::EditRole);

this will set a string, not a number so the decimals in a string are meaningless

[diverger]

@VRonin :)，

1. In my previous example, I simplified my usage case.
2. I use strings instead of numbers, because the displayText function won't abandon the trailing zeros of the number strings. I think if I use 'model->setData(index, spin->value(), Qt::EditRole);' , then the displayText() will format the numbers for me, and I can't access 'index' in it, so it will always abandon the trailing zeros.
3. So, my solution is actually avoid using numbers, but use text, and when create editor, I format the text to number, when user finish the editing, I format the values back to text and store it in the model.

[VRonin]

This is a display problem so it should not be handled at the model level. as shown above, the solution is to reimplement paint() in the delegate

[diverger]

@VRonin You are right. Thanks again. :)

[Teddy van Jerry]

It turns out to be a very good method which I seeks for a really long time. But now I use Qt 6.0.0, so the last return statement does not work well. So it needs to be changed into:

return QItemEditorFactory::createEditor(userType, parent);

And it is done!