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

[Nathan Woodrow]

def __init__( self, *predefinedScales ):
and you can do this

forcedScale(25000, 50000, 100000)

On Fri, 17 Apr 2015 at 22:30 kimaidou <kimaidou at gmail.com> wrote:

> 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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>>> Qgis-developer at lists.osgeo.org
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>>>
>>
>>
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