[QGIS-Developer] Event woes in QGIS

[Bo Victor Thomsen]

Hi Lars -

I hazard a guess, that you want to execute some action after a /project/ 
has finished loading.

You might have a look on how to execute macro's after open, save or 
close events for the project:

In the project properties, you can enable macro's and write some python 
code that will be stored in the project:

Example: (Open an attribute window with data from layer 'kommune" after 
the project has finished loading)

|#--- macro start --- from qgis.core import QgsProject from qgis.utils 
import iface def openProject(): layers = 
QgsProject.instance().mapLayersByName('kommune') layer = layers[0] 
iface.showAttributeTable(layer) def saveProject(): pass def 
closeProject(): pass||#--- macro end ---|

Den 21-08-2019 kl. 14:15 skrev Lars I. Nielsen, LIFA A/S:
>
> Hi bold developers of QGIS.
>
> I’ve recently begun digging into utilizing the event (signal) model of 
> QGIS (3.4).
>
> And I’ve encountered a number of frustrations, as the events don’t 
> work the way they’re supposed to, at least according to the available 
> documentation (https://qgis.org/pyqgis/master/core/index.html).
>
> E.g., the event “layerLoaded” on the project instance with the 
> description “/Emitted when a layer from a projects (sic) was read/”, 
> which to me indicates, that the loading is done (due to the use of 
> “loaded” and “was”).
>
> But the event is emitted before the loading of the layer is done, and 
> you are therefore unable to find the layer it concerns.
>
> Another example is the event “metadataChanged” on a loaded layer, 
> which again to me indicates a past/done action (due to the use of 
> “changed”). But again, it’s emitted before the change actually takes 
> place, and the changed metadata isn’t available.
>
> If the two events are indeed preload events, their names should 
> instead be something like “layerLoading” and “metadataChanging”, or 
> even “layerPreLoad” and “metadataPreChange”.
>
> Is there a guide somewhere to tell how the event system is envisioned, 
> and which events are emitted pre-action and which are emitted 
> post-action ?
>
> Or must I (hopefully not) take a deep look into the QGIS source code 
> itself for this kind of information ?
>
> In general, pre-action events are of limited use, except when it comes 
> to deletions and destructions. For loading and changing stuff, 
> post-action events are almost always what you need.
>
> Med venlig hilsen
>
> Lars I. Nielsen, LIFA A/S
> GIS-konsulent, FME Certified Professional
> Geoinformatik
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-- 
Med venlig hilsen / Kind regards

Bo Victor Thomsen

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