[Qt-qml] emit data on a QAbstracListModel does not update the QML ListView.

[Bartosh Wroblevksy]

Thanks Steve for answering my questions. This really helps.

> From: steveire at gmail.com
> To: qt-qml at trolltech.com
> Date: Tue, 14 Sep 2010 20:32:37 +0200
> Subject: Re: [Qt-qml] emit data on a QAbstracListModel does not update the	QML ListView.
> 
> Bartosh Wroblevksy wrote:
> 
> > 
> > Hello Steve,
> > Thank you for filing the BR.  I have a few questions.
> > 
> > 1) What's the number of the bug?
> 
> http://bugreports.qt.nokia.com/browse/QTBUG-13628
> 
> > 
> > 2) I have trouble seeing how in this case, moving rows and changing data
> > is not equivalent.  In fact,unless my morning coffee was not caffeinated.
> > If I write
> > 
> >     m_elements[1] ="x";   emit dataChanged(index(1,0), index(1,0));
> >  Should I not expect to see the QML listview change?
> 
> Yes, you are right. Indeed that is another bug. What you are doing 
> WorksFromMe, but I don't have a plugin for the model like you do, but I 
> create the QDeclarativeMainView in an application and use 
> setContextProperty(myModel). Maybe if loaded through a plugin qml doesn't 
> handle that signal somehow?
> 
> It's worth another bug report.
> 
> > 3) beginResetModel() and endResetModel() is clearly not what I want. My
> > application flickers horribly. In fact I hadinitially tried an emit
> > modelReset() which also made my application flick horribly. I was hoping
> > to avoid the flickerwith an emit dataChanged. Until this bug gets fixed,
> > is there any better alternative?
> 
> I don't think so, sorry.
> 
> > 4) You mention >*plus* updating
> > persistent indexes in between.>Sorry, can you elaborate on this? 
> 
> doMove()
> {
>   layoutAboutToBeChanged();
>   // do the move
>   m_items.move(0, 1);
>   
>   QModelIndexList pers = persistentIndexList();
>   foreach(const QModelIndex &idx, pers) {
>     if ( idx.row() == 0 ) {
>       const QModelIndex newIdx = createIndex(1, 0);
>       changePersistentIndex(idx, newIdx);
>     }
>     if ( idx.row() == 1 ) {
>       const QModelIndex newIdx = createIndex(0, 0);
>       changePersistentIndex(idx, newIdx);
>     }
>   }
>   layoutChanged();
> }
> 
> I hardcoded the 0 and 1 for clarity, but you can imagine that if you're 
> moving an arbitrary range of items an arbitrary distance the implementation 
> gets difficult. Avoid doing it yourself where possible and use beginMoveRows 
> etc instead. 
> 
> If you want to see an implementation implementing a move in a tree using 
> just the layout change stuff see ModelMoveLayoutChangeCommand::emitPreSignal 
> and ModelMoveLayoutChangeCommand::emitPostSignal here:
> 
> http://websvn.kde.org/trunk/KDE/kdelibs/kdeui/tests/proxymodeltestsuite/dynamictreemodel.cpp?view=markup
> 
> Compare to ModelMoveCommand::emitPreSignal and 
> ModelMoveCommand::emitPostSignal
> 
> Updating those is needed for selections, tree expansion proxy models etc to 
> work properly and not crash.
> 
> > 5) You
> > wrote
> >> + beginMoveRows(QModelIndex(), 0, 0, QModelIndex(), 2);>
> >> m_elements.move(0,1);> + endMoveRows();
> > Since the destination child is 1, I would have expected to write
> >> + beginMoveRows(QModelIndex(), 0, 0, QModelIndex(), 1);>
> >> m_elements.move(0,1);> + endMoveRows();
> > but then again, my coffee may be decaffeinated.
> 
> It's common for people to think that. I improved the docs with diagrams for 
> 4.7:
> 
> http://doc.trolltech.com/main-snapshot/qabstractitemmodel.html#beginMoveRows
> 
> It's consistent with other model api where you identify the 'gap' you want 
> to move the item to. Item at position 0 can't be moved to gap 0 or 1 because 
> it is already in between them. It can be moved to position 2 though. 
> Unfortunately the QList::move API is not the same. It's also easier to 
> implement dropMimeData like this.
> 
> All the best,
> 
> Steve.
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