QAbstractItemModel::dataChanged what would you prefer?

[VRonin]

Flavour question about QAbstractItemModel::dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles):

Say you have a model that changes, at the same time, the Qt::DisplayRole of the index(0,0) and Qt::FontRole of the index(1,0)

Would you prefer that the model emitted dataChanged(index(0,0),index(1,0),{Qt::DisplayRole,Qt::FontRole}); or would you rather have dataChanged(index(0,0),index(0,0),{Qt::DisplayRole}); dataChanged(index(1,0),index(1,0),{Qt::FontRole});?

[kshegunov]

Me, flavor 2, especially if Qt::UserRole isn't anything that should be updated in the GUI.

[VRonin]

@kshegunov said in QAbstractItemModel::dataChanged what would you prefer?:

if Qt::UserRole isn't anything that should be updated in the GUI.

You are right, that aspect is important, changed the question so that both roles matter for the UI

[SGaist]

Hi,

Also number two as it might optimise the number of calls to data.

[kshegunov]

I probably would stick to flavor 2, even though performance-wise shouldn't much matter. On the other hand for anything more "uncountable" (as 4, 5+ model indexes) then I'd switch to flavor 1.

[JKSH]

I'd generally prefer flavour 2 too. It's more "correct", since index(0, 0) hasn't changed its FontRole and index(1, 0) hasn't changed its DisplayRole.

In the end though, I'd also consider the code complexity and impact. If this was a custom model within my own app, I'd just choose the option that requires simpler code (which I think is Flavour 1).

[Christian Ehrlicher]

When you look at the default implementation you will see that the second one is better:
https://code.woboq.org/qt5/qtbase/src/widgets/itemviews/qabstractitemview.cpp.html#_ZN17QAbstractItemView11dataChangedERK11QModelIndexS2_RK7QVectorIiE

[J.Hilk]

I would go with the 2nd option, simply for the fact that it's much clearer, what is supposed to happen. Or rather what just happend.

[VRonin]

@JKSH said in QAbstractItemModel::dataChanged what would you prefer?:

I'd just choose the option that requires simpler code (which I think is Flavour 1)

it's actually flavour 2 in my case as I would have to check that the two indexes are one next to the other while I don't care using flavour 2

@Christian-Ehrlicher said in QAbstractItemModel::dataChanged what would you prefer?:

When you look at the default implementation you will see that the second one is better:

Ok, this is a show stopper. I did not realise sending topLeft!=bottomRight (often) triggers a repaint of the entire view. This makes flavour 2 strictly better

Thanks everyone for your insight