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I want to create a custom list using QListView and so I had to extend QListView, QItemDelegate and QAbstractListModel and then implement the specific methods, along with QAbstractItemModel::data(const QModelIndex & index, int role = Qt::DisplayRole) const. It displays correctly on the screen at first glance, but the problem occurs after populating the list model.,The function data(index,role) is called 4-5 times per item model with different roles (some of them undefined roles/out of range/probably random). And it occurs not only after the initialization of the list model! When I hover a list element, the view calls data(index,role) with the correct index and role but right afterwards it is called again with an unexpected role value.

It seems an awkward behavior. I couldn't find the source of the strange calls. I put logs in every method of my custom classes to be sure that I'm not calling by mistake the data(index,role) method with wrong values.

Does anyone have some ideas where to look at or why this strange calls occur?

EDIT The source of the "strange" calls is in:

QSize CDelegate::sizeHint(const QStyleOptionViewItem & option, const QModelIndex & index) const
{
        return QSize(QItemDelegate::sizeHint(option, index));
}

somehow when calling QItemDelegate::sizeHint() it triggers data() with different role values.

In my case I have defined role values starting from 0(0,1,2,3). According to @Jens those values are reserved. Changing the starting value of my custom roles solved my problem.

Ispas Claudiu
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  • What are the mentioned "undefined roles"? What are their values? – vahancho Nov 25 '15 at 09:21
  • I have defined 4 roles(0,1,2,3), but function data() calls 6 consecutive times per item with the following roles: 13,1,6,0,6,10 – Ispas Claudiu Nov 25 '15 at 09:24
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    Custom roles start at 0x100, 0..3 are roles defined by Qt. See the list of roles in qnamespace.h. 13 for example ist SizeHintRole. – Jens Nov 25 '15 at 09:30
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    @IspasClaudiu, everything looks correct. Qt needs several types of data to correctly render an item. In your case they are: SizeHintRole, DecorationRole, FontRole, DisplayRole, FontRole, CheckStateRole. – vahancho Nov 25 '15 at 09:39
  • @Jens. indeed, changing my custom role start value it solved my problem, but still I don't know why the data(index,role) function calls with those values specified above. Please consider adding an answer to mark the problem as solved – Ispas Claudiu Nov 25 '15 at 09:40
  • @vahancho thank you:). It may be correct but in my case if I define the role values starting from 0 when data() calls it may generate unexpected behavior. I changed the role start value and seems that it's working. – Ispas Claudiu Nov 25 '15 at 09:44

1 Answers1

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Custom roles start at 0x100, 0..3 are roles defined by Qt. See the list of roles in qnamespace.h. 13 for example ist SizeHintRole.

The different role calls stem from the interns of QListView, which is trying to figure out, how the cells should be displayed. Example: If you want to change the font of a row, you simply add the switch for FontRole to your data() implementation and pass back a bold font whenever something is important and should be displayed in bold.

Jens
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