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

[kimaidou]

More pythonesque Nathan, thanks !

2015-04-17 14:32 GMT+02:00 Nathan Woodrow <madmanwoo at gmail.com>:

> 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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>>>>
>>>
>>>
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