[Qgis-developer] Listener for Atlas Change Page

[Nyall Dawson]

On 14 December 2016 at 13:42, Shawn Tse <shawntse at gmail.com> wrote:
> Thanks, that looks like exactly the kind of function that I'm looking
> for. I'm using QGIS 2.18.0, I'm wondering, is there any way to use
> this function without having to select a menu item in the the plugin?
>
> I'm not sure how to get started, because most of the existing plugins
> seem to be activated by a menu item or a toolbar button in the main
> editor instead of via the atlas interface.
>
> Thank you and I really appreciate the help!

Hi Shawn,

Have a read up on how signals and slots work in PyQt. Basically you
can listen out for the signal and get it to trigger a function in your
python class. Untested psuedocode below:

class AtlasListener(QObject):
   def featureChanged(self, feature):
      print('new feature: {}'.format(feature.id())

  def setComposition(self, composition):
      composition.atlasComposition().featureChanged.connect(self.featureChanged)

l=AtlasListener()
l.setComposition(iface.activeComposers()[0].composition())


Nyall


>
> On Mon, Dec 12, 2016 at 11:02 PM, Nyall Dawson <nyall.dawson at gmail.com> wrote:
>>
>>
>> On 13 Dec 2016 4:44 PM, "Shawn Tse" <shawntse at gmail.com> wrote:
>>
>> Hi all,
>>
>> I'm interested in being able to make graphs that update depending on
>> what the active feature in the atlas is. I know some python and a
>> little bit of the QGIS API, but I'm curious - is there some sort of
>> listener function for the atlas that gets called whenever the Preview
>> Atlas/Prev Feature/Next Feature/First Feature/Last Feature buttons are
>> pressed?
>>
>>
>> Try QgsAtlasComposition::featureChanged. See
>> https://qgis.org/api/classQgsAtlasComposition.html#ab1a15fdb6d20ebb1254ebb6714254fac
>>
>> Nyall
>>
>>
>> What I eventually want to do is to manipulate the node items that were
>> created in a recent updated version of QGIS, and use them to make a
>> line graph, gridlines, etc.
>>
>> I'm hoping to draw the data directly from the shapefiles themselves or
>> from a .csv file.
>>
>> I originally tried to use an expression in a text box to do this, but
>> it seems that expressions are unable to use custom functions to
>> manipulate the composer. So I'm hoping that there's some sort of
>> Listener method.
>>
>> Thanks.
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