[Qt-interest] Drag & Drop of hierachical items in a TreeView

[Daniel Price]

-----Original Message-----
From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Stephen Kelly
Sent: 27 November 2009 19:19
To: qt-interest at trolltech.com
Subject: Re: [Qt-interest] Drag & Drop of hierachical items in a TreeView

Daniel Price wrote:

> I'm trying to figure how to do drag and drop editing of hierarchical data
> within an QAbstractItemModel-derived adaptor within a QTreeView.
> 
> Single items with no children seem to work fine (I'm only interested in
> moving data, not copying it). However if I move an item with children, the
> children are lost and I'm not sure how I can preserve them as the
> dropMimeData() virtual only gives me the mime data and drop target.

> It is odd that the children are lost. How are you doing the actual move?

I sent an email to Nokia support and they provided an example - you have to cache the child items recursively using PersistantModelIndexes and then re-constitute the tree after the drop using insertRow() etc. It's a bit messy but it works.


If you are using Qt4.6, try the beginMoveRows/endMoveRows api. It should not 
be necessary to reconstruct the entire tree if you do that.

I'm using 4.5.2. Is this a new feature of 4.6? It preserves the children?

> 
> I can think of same ways around this but I don't know what the 'best
> practice' solution would be:
> 
> 1)      Encode the QModelIndex of the original item into the mime data
> (QDataStream) and use it to recursively iterate through and reconstruct
> the tree in the drop location (it seems that the source items are not
> removed until after dropMimeData() is called so the index should remain
> valid).
> 
> 2)      Cache the source model index somewhere when a drag is initiated
> (mimeData()?) and do the above.
> 
> Any suggestions? Has anyone got this working?

The way we do this in akonadi is by encoding a url of the item in the 
mimedata. The url can be used to get the internal data object (Collection or 
Item), and a move operation is kicked off. The move in the model is done 
when we get notified that the move is complete.

see dropMimeData here:
http://websvn.kde.org/trunk/KDE/kdepimlibs/akonadi/entitytreemodel.cpp?view=markup

and monitoredCollectionMoved here:
http://websvn.kde.org/trunk/KDE/kdepimlibs/akonadi/entitytreemodel_p.cpp?view=markup

All the best,

Steve.

Thanks for the tips.

> 
> 
> Daniel Price
> Software Engineer
> FXhome Limited
> 
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