[Development] QAbstractItemModel::setItemData behaviour

[Luca Beldi]

Hi everyone,
While trying to submit a patch to fix QStringListModel::setItemData https://codereview.qt-project.org/235730 we opened a much larger discussion on QAbstractItemModel, that expanded on the forum https://forum.qt.io/topic/93207/qabstractitemmodel-setitemdata-partial-success  but that I ultimately would like your opinion on:

At the moment, QAbstractItemModel::setItemData calls setData for each role until the first failure and returns false if that happens.
This opens up 2 issues:

*         QAbstractItemModel::setItemData returns false even when some but not all data was actually set without rolling back the changes

*         QAbstractItemModel::setItemData depends on the ordering of the roles

For the second point, an example that illustrates the problem is (QStringListModel::setItemData just uses the base class implementation):
QStringListModel::setItemData({{Qt::DisplayRole,QVariant("test")},{ Qt::DecorationRole,QVariant(QIcon())}) will return false but actually set the string to "test"
QStringListModel::setItemData({{Qt::EditRole,QVariant("test")},{ Qt::DecorationRole,QVariant(QIcon())}) will return false and do nothing on the model

My proposal is:

*         Remove the ordering-dependency  QAbstractItemModel::setItemData (i.e. sets all the roles it can and returns false if any of them failed)

*         Make setItemData in subclasses of QAbstractItemModel behave transaction-like (i.e. try to set all the roles, if any fails rollback the other changes and return false)

This however is technically a change of behaviour so I would like people way smarter than me to agree on the way forward.

Thanks,
Regards,
Luca Beldi (aka VRonin)

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