[QT4]QTreeView, QAbstractItemModel and a MySQL database

[wysota]

The view doesn't do any caching by itself. It is always the models responsibility. If you want a "retrieve on demand" approach, you have to code it yourself in the model. For example have a tree which has items and each item can be marked as "complete" or "unknown" and when data from a child of a "unknown" item is fetched, just query the database and attach new elements to the tree before returning the result. You'll have to watch for the "columnCount()" and "rowCount()" methods, because if you don't implement them right, you'll end up with filling the whole tree at the beginning.

[Alessandro]

This is the response from the Trolltech QT support. The QAbstractProxyModel seems to be a good possibility to map between different model structures.




> Is there a way to tell the QTreeView to cache the data and only to
> requery it when the Model tells it to do so?

Not out of the box.

> If I need to create my own cache tree structure (wich I actually fear
> I need to do), is there something within QT that lets me create such a
> tree in a comfortable way? Something like the DOM tools but without
> the XML.

Not really, but using a combination of QMap<> and QVector<> might be
what you want, i.e. something like this

struct Node
{
QMap<QString, QVariant> values;
Node* parent;
QVector<Node*> children;
};

and then provide some accessor functions that add/remove children and
values or something similar. You could then use the QAbstractProxyModel
class to provide mapping from your real model (if you have an existing
one or if you're currently using one of the SQL models).

Hope this helps.

--
Jan Erik Hanssen
Trolltech AS, Oslo - http://www.trolltech.com

[KShots]

Thanks. For my case, I'll likely do something different, but that would work for the general case.