Using QgsMapCanvas on a QGIS Processing script

Question

I am writing an QGIS Python processing script, the result of this script is a formatted PDF. In one of the steps, I need to set map canvas extent to my shapefile extent, but it is not working.

What is the difference on this context from a standalone script, for instance?

This is how I am trying to set map canvas extent:

# Reference to map Canvas
canvas = QgsMapCanvas()

# allow anti aliasing
canvas.enableAntiAliasing(flag=True)
        
# canvas color
canvas.setCanvasColor(Qt.white)
        
#  canvas extension
canvas.setExtent(fieldlayer.extent())
        
#  set canvas layer
canvas.setLayers([fieldlayer])

In this context, I can't start qgs application through the usual command lines because the script would crash:

#  Start qgs
os.environ["QT_QPA_PLATFORM"] = "offscreen"

QgsApplication.setPrefixPath("/usr", True)
qgs = QgsApplication([], False)
qgs.initQgis()

### qgs manipulation here ###

# Exit qgs
qgs.exitQgis()

The whole script can be found here:

https://1drv.ms/u/s!AqMcxdEHKstsmhpHooyFXagqS2-d

Answer 1

You don't need to start a new QGIS instance in Processing script. Maybe you should change geo_pdf_layout_print method to this:

def geo_pdf_layout_print(self, qgz_project, class_shp, qgz_final_path, feedback):

    project = QgsProject.instance() ### <--
    project.read(qgz_project)
    project.write()

    fieldlayer = QgsVectorLayer(class_shp, Path(class_shp).stem, "ogr")
    project.addMapLayer(fieldlayer, False)

    layertree = project.layerTreeRoot()
    layertree.insertChildNode(1, QgsLayerTreeLayer(fieldlayer))

    layoutmanager = project.layoutManager()
    layout = layoutmanager.layoutByName("0001_MG")

    mapqgis = [i for i in layout.items() if isinstance(i, QgsLayoutItemMap)][1]
    mapqgis.setRect(20, 20, 20, 20)        
    mapqgis.attemptMove(QgsLayoutPoint(17.64, 37.4, QgsUnitTypes.LayoutMillimeters))
    mapqgis.attemptResize(QgsLayoutSize(176.4, 190, QgsUnitTypes.LayoutMillimeters))
    mapqgis.zoomToExtent(fieldlayer.extent())

    feedback.pushInfo(f"\n\n\nEXT = {fieldlayer.extent()}\n\n\n")

    canvas = iface.mapCanvas() ### <--
    canvas.enableAntiAliasing(flag=True)        
    canvas.setCanvasColor(Qt.white)
    canvas.setExtent(fieldlayer.extent())
    canvas.setLayers([fieldlayer])
    project.write(qgz_final_path)

I have no chance to test it.