Re: TreeView: Multiple selection inefficient?

["Andrew E. Makeev" <andrew solvo ru>]

mkay wrote:

> Hello,
>
> I am programming a gtkmm widget, and I stumbled over a problem which 
> leads to a large performance hit in my application.  I was told to 
> repost the problem here, because it wouldn't be a gtkmm specific 
> problem, but due to the underlying Gtk+ mechanisms.
> Disclaimer:  I haven't worked with Gtk+ before, since I'm not a C 
> programmer, but only with gtkmm.  So if some of the structure names 
> differ from those in Gtk+, my excuses.
>
>
> I'll try to outline the problem:
>
> If I set a TreeView to allow multiple selections, the signal handling 
> is as follows:
> If one or more rows are selected, a signal_selection_changed signal is 
> emitted.  In the signal handler, another callback function is called 
> for every row that is selected.  This happens internally, I only have 
> to call selected_foreach_iter with a functor (a sigc::slot) to which 
> the callback function is bound.
>
> Now here is the problem:
> Each time the selection changes, this callback function is called for 
> /every/ row in the selection again. This is also the case if I have 
> e.g. 5 rows selected and select one more row (the last 5 didn't 
> change); then the function is called again on all 6 rows!
> The problem is, that if you select the rows incrementally, e.g. by 
> selecting the first row in the TreeView and holding shift while going 
> down in the list to select more rows, this ends up in a massive bunch 
> of function calls.
>
> How massive it is, is easily computed (I'll take the next paragraph 
> from my last post in the gtkmm mailing list):
>
> Consider a TreeView with 1000 entries.  Now select the first entry, 
> hold shift and press the down-key to expand the selection downwards 
> till you reach the bottom.
> After hitting 'down' for the n-th time, the signal handler has been 
> called (n * (n+1)) / 2 times (1 + 2 + 3 + ... + n).  That means, after 
> selecting 1000 entries this way, the signal handler has been called 
> (1000 * 1001)/2 times, that is 500500 times.  In words: Over half a 
> /million/ times!
>
> In my widget, I'm displaying directory contents in a TreeView. On my 
> Debian GNU/Linux  installation, the directory /usr/bin for example has 
> 1779 entries.  I'll leave the calculation for you this time :)
>
>
> Am I missing something? Otherwise, this mechanism should be eventually 
> rethought, because the performance hit is pretty bad.
>
> Any comments are greatly appreciated.

I think, it is not GTK+ neither GTKMM issue.

You have to delay signal handler just until user keep send events to 
your application.
Set timer callback where you will call "selection_changed" handler, and 
reset it after each user event that changes selection.

Regards,
-andrew