QGIS 3.16 Python script: processAlgorithm return empty layer

Question

The script process well except the returned output. I want that the script displays the "output_layer" at the end of my script as a temporary layer in my QGIS project.
I understand that the output have to be a sink type.
Following this tutorial: http://www.qgistutorials.com/en/docs/3/processing_python_scripts.html, I try to add the features of the output_layer in sink at the end of the script.
def initAlgorithm(self, config=None):
     
    self.addParameter(QgsProcessingParameterVectorLayer(self.INPUT_vector,self.tr( 
    'couche parcelle'), 
    types=[QgsProcessing.TypeVectorPolygon],
    defaultValue=None)
    )
    
    self.addParameter(QgsProcessingParameterRasterLayer(self.INPUT_raster,self.tr(
    'raster classification'),
    defaultValue=None)
    )

self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT, self.tr('Output layer'), type=QgsProcessing.TypeVectorPolygon, createByDefault=True, defaultValue=None))

def processAlgorithm(self, parameters, context, feedback):       
    
    source = self.parameterAsSource(
        parameters,
        self.INPUT_vector,
        context
    )

(sink, dest_id) = self.parameterAsSink(
        parameters,
        self.OUTPUT,
        context,
        source.fields(),
        source.wkbType(),
        source.sourceCrs()
    )

[...]

# At the end of the script: Read output_layer and create output sink

for f in output_layer['OUTPUT'].getFeatures():
        sink.addFeature(f, QgsFeatureSink.FastInsert)
        feedback.pushInfo('feature: {}'.format(f.fields().names()))

return {self.OUTPUT: dest_id}

The displayed output layer contains the input layer fields instead of the output_layer fields. feedback.pushInfo displays correct info, "output_layer" is OK.
What is the correct code to have "output_layer" correctly returned and displayed at the end of the script?

Lalikinelo · Accepted Answer

I finaly find the solution:
The problem comes from the sink construction. line:
(sink, dest_id) = self.parameterAsSink(...)

I changed the source.fields() to output_layer['OUTPUT'].fields()
source.fields() was limiting the number of fields with the number of fields of the input layer.

Kadir Şahbaz · Answer

First, you have to add the following method to your processing algorithm class to be able to add  temporary layers to the project.
def flags(self):
    return super().flags() | QgsProcessingAlgorithm.FlagNoThreading

Then use this anywhere before return:
QgsProject.instance().addMapLayer(output_layer['OUTPUT'])

QGIS 3.16 Python script: processAlgorithm return empty layer

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