Re: Efficiency question

[John Cupitt ng-london org uk]

The Surprises wrote:
> I am developing a gtk app that uses motion events to capture mouse
> pointer location and continuously update a gtk label.  I have also
> implemented routines to drag various gnome canvas items around a canvas.
> I have found that calling the gtk_label_set_text routine to update the
> label significantly degrades performance of the app.  There is a serious
> lag during fast canvas item drag that disappears completely when I
> comment out the gtk_label_set_text call.  Without this line, the items
> drag completely smooth even at full screen zoom.  With the line, I get
> smooth scrolling up to a certain app size, any larger causes slowdown.
> 
> The label, along with the canvas, is packed into a table.  My thinking
> is that the label changing text is causing all the widgets within the
> table to trigger a size_allocate or something.  The bigger the app, the
> more expensive the size allocate.  I did notice that the label widget
> changes size depending on the text size.  I played around with various
> packing options (x expand, fill, etc, etc), but couldn't get any
> improvement.  Any ideas on how to fix this?

Hi, as an experiment you could try gtk_widget_set_usize(100,100) on the 
label. This would give it a useless but fixed size ... if your app still 
runs slowly, you'll be able to rule out resizing problems.

Another solution would be to add a timer to your motion callback, and 
limit label updates to no more than (eg.) 5 per second. That should be 
fast enough for the user and it'll take the CPU load right down.

John



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