updating model columns based on view update

user4592357

i have a tree view with 5 column, i created a context menu with checkable items to show/hide columns.

i create the model with column names:

class TreeModel : public QAbstractItemModel
{
public:
     explicit TreeModel(QList<QVariant> lstColumnNames, QObject *parent = nullptr) noexcept
            : QAbstractItemModel(parent)
            , m_lstColumnNames(std::move(lstColumnNames))

private:
     	QList<QVariant> m_lstColumnNames;

the reason i do this is to get tree header names and model column count:

QVariant TreeModel::headerData(int section, Qt::Orientation orientation, int role /* = Qt::DisplayRole */) const
{
	return orientation == Qt::Horizontal && role == Qt::DisplayRole
		? m_lstColumnNames[section]
		: QAbstractItemModel::headerData(section, orientation, role);
}

int TreeModel::columnCount(const QModelIndex &parent /* = QModelIndex() */) const
{
	return parent.isValid()
		? static_cast<TreeNode *>(parent.internalPointer())->columnCount()
		: m_lstColumnNames.size();
}

now, the problem is that, when i hide a column from context menu (and have 4 columns instead of 5), this list in model is not changed, i.e. columnCount() still returns the old 5.

one option i think of is to have list of pairs, QList<QPair<QVariant, bool>> m_lstColumnNames;, where the bool indicates whether the column is hidden or shown. that means i will need to update the column states every time i hide/show column from gui. also, to return column count i'd need to run over all columns and collect only visible ones.

i'd like to know what else can i do? probably something better.
thanks

Christian Ehrlicher

@user4592357 said in updating model columns based on view update:

this list in model is not changed, i.e. columnCount() still returns the old 5.

That's absolutely correct - you only hide a column in the view, you don't change the model. Why should the model change it's columnCount at all?

user4592357

@Christian-Ehrlicher said in updating model columns based on view update:

Why should the model change it's columnCount at all?

the problem is that, since model column count is the same, data() is called for those hidden columns.

Christian Ehrlicher

@user4592357 Why is this a problem?

user4592357

@Christian-Ehrlicher
i have one column, which does some heavy calculation for data retrieval, so i wouldn't want extra work be done

Christian Ehrlicher

@user4592357 You should not do the calculation inside the data() function.

user4592357

@Christian-Ehrlicher
then where? if data() wants to retrieve that model index info, i need to provide the correct data, since i don't know whether column is hidden in gui or not

Christian Ehrlicher

@user4592357 said in updating model columns based on view update:

then where?

When you fill the data into the model for example. data() is called very often and should do as less work as possible.

JonB

@user4592357
In view of @Christian-Ehrlicher's comments, you might consider adding a QProxyModel which hides the computed column from the outside world, makes life simpler.

user4592357

@Christian-Ehrlicher
okay, now i add the value of that column in a method filling tree node, only if the column is to be shown, because the computation of that data is slow.
now, if the column is hidden, and i show it, it doesn't display the data (because the logic of filling the node was done somewhere else)

JonB

@user4592357
I don't understand. If you allow for allow for hidden columns becoming shown, you have to write the logic to ensure the data is available. If necessary, react to the column being hidden/shown by refreshing the model or changing it or whatever is required to fetch the necessary data.