[Qt-interest] Proxy models

[Kishore]

On Monday 30 Aug 2010 5:07:43 pm Stephen Kelly wrote:
> Kishore wrote:
> > I have been working with proxy models lately and am intrigued by their
> > complexity.
> > 
> > In short, it seems that most implementations of proxies use a QMap that
> > maps beween QPersistantModelIndex's of the source and the proxy. Somehow
> > this seems very inefficient o me. Memory consumption would go very high
> > when the number of indexes increase. It is not uncommon for say a mail
> > application to have several thousands of indexes and maybe 2-3 proxies
> > between the view and the actual data.
> 
> That's what we do in proxies used kmail, kaddressbook etc, execpt that we
> use two QHashes, not QMap. Additionally, you don't have to map every index,
> only the parent indexes, so it's not a huge problem given that in a mail
> application you'll probably have only a few hundred mail folders (which are
> the parents).

In a mail application, every mail that has a reply is a parent. Right?

> We use far more than 4 or 5 proxies too, because you can have
> multiple tabs with different data and each tab has at least two proxies. 12
> at runtime is probably not uncommon. On mobile we don't have tabs but we
> have more proxies there for navigational reasons. A running kmail-mobile
> has about 18, though many are often empty or contain only a small amount
> of data, or do trivial fast mapping.
> 
> At any rate, they are not the cause of slowdown.

Interesting. And as Bo writes, it is not much memory overhead too!

> QSortFilterProxyModel doesn't use persistent indexes from the source model,
> which I thought was interesting and annoying because there's more work to
> do to keep them updated manually.

Well, indexes will need to be updated by someone right? If it's not the proxy, 
it is the source.

> > Is this design really the best way considering speed and memory? I have
> > not really seen any other MVC classes for comparison...
> 
> Note that you can create mappings lazily. If you have a tree and the view
> has not expanded everything it won't ask for everything. You can create
> mappings in your rowCount or hasChildren implementation and only maintain
> them then.

Tree models are far more complex than tables and lists when implementing 
proxies! :) It seems quite clearly that maintaining persistent indexes is 
really the most effective solution here.

> I also don't really know of any comparable MVC APIs, but I had a short look
> at gtkTreeView, but I don't think that has any proxying concept. I'm sure
> .NET has some MVC stuff now too, but I haven't looked at it. If you find
> any let us know.
> 
> All the best,
> 
> Steve.
> 
> > I am getting more and more interested in this and am curious to see some
> > other MVC architectures! :)
-- 
Cheers!
Kishore