[QGIS-Developer] Layer inserted into a layertree group using a processing post processor does not repaint on toggle

[Nyall Dawson]

On Tue, 23 Jul 2019 at 02:01, Tejas L <tejvan at live.com> wrote:
>
> Hello devs,
>
> I am using the QGIS postprocessing interface to add layers to a layer group. The layer group is created in processAlgorithm, while the indivdual layers are added from postprocessing classses.
>
> The layer group gets created as expected along with the child layers. However toggling the individual layers does not repaint the canvas and has no effect. Toggling the entire group does work though.
>
> The pseudocode is as follows:
>
> def processAlgorithm(...):
>     root = context.project().layerTreeRoot()
>     group = root.insertGroup(0, "myGroup")

Here's where your problem lies. You're creating the group in the
processAlgorithm step, which is run in a background thread. Apart from
being at severe risk of crashes, this also has the side-effect of
making the new group "live" in the background thread. As soon as your
algorithm finishes and that thread exits, it will stop processing
events and things will go... murky... with your group.

Solution: don't do ANYTHING like this in a background thread, and do
all this logic in the post-processor (which runs in the main thread).

Nyall


>
>     self.postp = MyPostProcessor(group)
>
>     layer = QgsVectorLayer(...)
>     if layer and self.context.willLoadLayerOnCompletion(layer.id()):
>        self.context.layerToLoadOnCompletionDetails(layer.id()).setPostProcessor(self.postp).
>
> MyPostProcessor is used to basically move the layer to the group.
>
> When the layer group is also created from within the postprocessing class, everthing works perfectly.
>
> What am I doing wrong?
>
> Regards,
> Tej
>
>
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