[Qgis-developer] Force canvas to predefined scales : help needed to improve a script

[kimaidou]

Hi Hugo,

Thanks for the anwser. I used some "print scale" and "print targetScale" in
my script to check the behavious, and I did not have any "infinite loop" or
strange behaviour with it. It seems the related QGIS code is armed against
this risks.

This python script was only a prototype, and I will surely have a look at
the cpp classes and propose a PR to add this option in QGIS core.

Michaël

2015-04-17 11:26 GMT+02:00 Hugo Mercier <hugo.mercier at oslandia.com>:

> Hi,
>
> Le 17/04/2015 10:46, kimaidou a écrit :
> > Hi QGIS !
> >
> > I would like to be able in QGIS to force the canvas scales to some
> > predefined ones. Since we now have project scales (and QGIS pre-defined
> > scales) in the scale selector, we could use them, and add a simple
> > checkbox near the combobox "Stick to predefined scales".
> >
> > The aim is to force the canvas to render only at these scales. For
> > example, if I use the rectangle zoom tool, and I should go toe 1/56003,
> > I would instead land on 1/50000 , the closest pre-defined scale.
>
> That would be nice to have it in the core. I already had to do something
> similar with a plugin.
>
> >
> > I made a little python script as a proof of concept to illustrate it :
> > http://paste.debian.net/167247/
> >
> > In this script, I hard coded the scales, but oviously we should get them
> > from the project properties.
> >
> > I have a question regarding my script. Since I use the
> > QgsMapCanvas::scaleChanged signal, I assume the rendering is done twice
> > ? Once the "normal" way, and once again after I use the
> > QgsMapCanvas::zoomScale method after calculating the new target scale.
> >
> > Any idea for improving it ? I assume I would have to disconnect one (not
> > found yet) signal/slot, then set the scale and reconnect this
> > signal/slot afterwards ?
>
> QObject::blockSignals might be of help here. But in this case, you don't
> want to block all signals, only yours.
>
> I guess your solution would work.
> You can also use a flag in your callback, something like :
>
>     def setScale( self, scale ):
>         if self.reentrant:
>                 return
>
>         targetScale = min(
>             self.predefinedScales,
>             key=lambda x:abs(x-scale)
>         )
>         self.reentrant = True
>         self.mc.zoomScale( targetScale )
>         self.reentrant = False
>
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