[Qgis-developer] Listener for Atlas Change Page

[Shawn Tse]

Hi, thanks so much for the help. Right now the AtlasListener is throwing up a

NameError: name 'QObject' is not defined

I looked up the API and when instantiating the AtlasListener, I can
pass a QObject into the constructor. What QObject should I put in
there?

Thanks

On Thu, Dec 15, 2016 at 7:35 PM, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> 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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