[QGIS-Developer] How to use postProcessLayer in plugin processing

[René-Luc Dhont]

Hi Devs,

I would like to set the style an output layer in a script processing. My 
algorithm is added to QGIS Processing by a plugin.

I have tested this :

```

class loadNamedStylePostProcessor(QgsProcessingLayerPostProcessorInterface):

     def __init__(self, outputName, feedback):
         super().__init__()
         self.outputName = outputName
         feedback.pushInfo('loadNamedStylePostProcessor init')
         feedback.pushInfo(self.outputName)

     def postProcessLayer(layer, context, feedback):
         feedback.pushInfo('postProcessLayer')
         feedback.pushInfo(self.outputName)
         feedback.pushInfo(layer.name())


class testPostProcessLayerAlgorithm(QgsProcessingAlgorithm):

     OUTPUT = 'OUTPUT'
     INPUT = 'INPUT'

     def initAlgorithm(self, config):
         self.addParameter(
             QgsProcessingParameterFeatureSource(
                 self.INPUT,
                 self.tr('Input layer'),
                 [QgsProcessing.TypeVectorAnyGeometry]
             )
         )

         self.addParameter(
             QgsProcessingParameterFeatureSink(
                 self.OUTPUT,
                 self.tr('Output layer')
             )
         )

     def processAlgorithm(self, parameters, context, feedback):
         source = self.parameterAsSource(parameters, self.INPUT, context)
         (sink, dest_id) = self.parameterAsSink(parameters, self.OUTPUT,
                context, source.fields(), source.wkbType(), 
source.sourceCrs())

         total = 100.0 / source.featureCount() if source.featureCount() 
else 0
         features = source.getFeatures()

         for current, feature in enumerate(features):
             # Stop the algorithm if cancel button has been clicked
             if feedback.isCanceled():
                 break

             # Add a feature in the sink
             sink.addFeature(feature, QgsFeatureSink.FastInsert)

             # Update the progress bar
             feedback.setProgress(int(current * total))

         context.layerToLoadOnCompletionDetails(
             dest_id
         ).setPostProcessor(
             loadNamedStylePostProcessor(self.OUTPUT, feedback)
         )

         return {self.OUTPUT: dest_id}

     def name(self):
         return 'test_post_process_layer'

     def displayName(self):
         return self.tr('Test postProcessLayer')

     def group(self):
         return self.tr('Tests')

     def groupId(self):
         return 'tests'

     def tr(self, string):
         return QCoreApplication.translate('Processing', string)

     def createInstance(self):
         return self.__class__()
```

The postProcesser is well initialize, but I never have the messages form 
postProcessLayer.

Any one has already use it in some code ?

Regards,
René-Luc