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

[kimaidou]

Hi all,

Improve version here:
https://gist.github.com/mdouchin/72a09b629a5557c1dc1c

with:

* "replayed" variable to avoid probable infinite loop, thanks to Hugo's
proposal

* you can now initialize the method with a python list of scales you want
to force canvas to:

fs =  fs = forcedScale( [25000, 50000, 100000 ] )

Cheers,
Michaël

2015-04-17 12:27 GMT+02:00 kimaidou <kimaidou at gmail.com>:

> 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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>>
>
>
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