QTreeView and Qt::ItemIsAutoTristate

webzoid

I've spent a while now, faffing with the QTreeView and the Qt::ItemIsAutoTristate flag to try and get parent/child tree items to check/uncheck/partially check where applicable.

Can anyone point me towards a working solution for this, please?

I have a mostly-working QAbstractItemModel implementation (code base is too large to post) however, the final piece to the puzzle is getting these checkboxes to work in a tri-state fashion.

Things I have done so far in the custom QAbstractItemModel class:

ORed in the Qt::ItemIsAutoTristate flag together with the Qt::ItemIsUserCheckable flag in the flags function
Overridden the hasChildren function, returning true/false where applicable
Overridden the data function and return the correct Qt::CheckState value

I've now run out of ideas. Googling doesn't prove to be particularly useful either.

Any help would be mahoosively appreciated! Thanks.

VRonin

a few questions:

what version of Qt are you using?
Are you subclassing the model just to implement this or do you have other needs?
When your custom model emits dataChanged does it (or could it) pass the 3rd argument?

webzoid

@VRonin thanks for replying.

I'm currently running Qt 5.10.1 however there is no reason why I cannot change to the latest.

I am subclassing the model to satisfy other needs.

At the moment, dataChanged only emits two arguments however I see no reason why I could not emit a third argument. What would the 3rd argument be useful for?

Christian Ehrlicher

So please provide a simple testcase - a simple data() and setData() function with only the checkbox stuff will help to see what's going wrong. It does work for me.

webzoid

@Christian-Ehrlicher Are you using a QTreeView or a QTreeWidget?

Since I posted this question, I did a bit of digging around the qtreeview.cpp source files and there was little mention of anything checkbox or tristate related so I'm actually wondering whether this auto-tristate feature is supported out-the-box.

I'll try and post some code shortly...

Christian Ehrlicher

I don't see why QTreeView should somehow mention the QCheckBox at all. The only this QTreeView has to do (and which is done in QAbstractItemView) is to properly pas the Qt::CheckState role to the delegate and handle the click.

webzoid

@Christian-Ehrlicher sorry, maybe I should have said that there was no mention of CheckState rather than checkbox as I appreciate that the QTreeView would not need any reference to this.

webzoid

Ok, so I've taken the editabletreemodel example project and I've made the following changes to support checkboxes:

treeitem.h:

Added a Qt::CheckState checked field to the private members
Changed the data function signature to QVariant data(int column, int role) const
Changed the setData function signature to bool setData(int column, const QVariant &value, int role)

treeitem.cpp:

Altered the data and setData functions as follows:

QVariant TreeItem::data(int column, int role) const
{
	if (role == Qt::CheckStateRole) {
		if (column == 0) {
			return checked;
		}
		else {
			return QVariant();
		}
	}

    return itemData.value(column);
}

bool TreeItem::setData(int column, const QVariant &value, int role)
{
    if (column < 0 || column >= itemData.size())
        return false;

	if ((column == 0) && (role == Qt::CheckStateRole)) {
		checked = (Qt::CheckState)value.toUInt();
		return true;
	}

    itemData[column] = value;
    return true;
}

treemodel.cpp:

Updated all references to data or setData, adding in the role parameter except in setupModelData where Qt::EditRole is passed as the 3rd argument
Altered the flags, data and setData functions as follows:

Qt::ItemFlags TreeModel::flags(const QModelIndex &index) const
{
    if (!index.isValid())
        return 0;

	return Qt::ItemIsEditable | QAbstractItemModel::flags(index) | Qt::ItemIsUserCheckable | Qt::ItemIsAutoTristate;
}

QVariant TreeModel::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
        return QVariant();

	if (role != Qt::DisplayRole && role != Qt::EditRole && role != Qt::CheckStateRole)
        return QVariant();

    TreeItem *item = getItem(index);

	return item->data(index.column(), role);
}

bool TreeModel::setData(const QModelIndex &index, const QVariant &value, int role)
{
	if (role != Qt::EditRole && role != Qt::CheckStateRole)
        return false;

    TreeItem *item = getItem(index);
	bool result = item->setData(index.column(), value, role);

    if (result)
        emit dataChanged(index, index, {role});

    return result;
}

When I run the example, I can happily check/uncheck items in the QTreeView however, the tristate-ness is not propagated.

Christian Ehrlicher

A checkbox can either be checked or unchecked. A tri-state state can only happen programatically e.g. when the children are checked and unchecked therefore the default delegate only supports checked/unchecked. The tri-state has to be calculated by the model. See for example the implementation in QTreeWidget: https://code.woboq.org/qt5/qtbase/src/widgets/itemviews/qtreewidget.cpp.html#1840

webzoid

@Christian-Ehrlicher Ah ok, so it must be implemented programmatically.