When to use the updateFeature() method?

Question

I am using QGIS 3.12 and I want to update the content of a field by storing the feature's X and Y coordinates. The following code works given that a point layer is in the layer panel. The function works when the layer.updateFeature(feat.id()) statement is blocked, and so I could not understand the use of this method?

layer = iface.activeLayer()
layer.startEditing()
all_features = layer.getFeatures()
for feat in all_features:
    geom = feat.geometry()
    x = geom.asPoint().x()
    y = geom.asPoint().y()
    
    new_x = {feat.fieldNameIndex('x'): x}
    layer.dataProvider().changeAttributeValues({feat.id(): new_x })
    
    new_y = {feat.fieldNameIndex('y'): y}
    layer.dataProvider().changeAttributeValues({feat.id(): new_y })
    
    #layer.updateFeature(feat.id()) #?
 
# Commit changes
layer.commitChanges()

Answer 1

You need to either use only the .dataProvider() (QgsVectorDataProvider) methods which call the underlying data and modify it or use the edit methods of QgsVectorLayer.

so, either :

layer = iface.activeLayer()
layer.startEditing()
all_features = layer.getFeatures()
for feat in all_features:
    geom = feat.geometry()
    x = geom.asPoint().x()
    y = geom.asPoint().y()
    
    feat.setAttribute('x', x)
    feat.setAttribute('y', y)
    
    layer.updateFeature(feat)
 
# Commit changes
layer.commitChanges()

or

layer = iface.activeLayer()
all_features = layer.getFeatures()
for feat in all_features:
    geom = feat.geometry()
    x = geom.asPoint().x()
    y = geom.asPoint().y()
    
    new_xy = {
        feat.fieldNameIndex('x'): x, 
        feat.fieldNameIndex('y'): y
    }
    layer.dataProvider().changeAttributeValues({feat.id(): new_xy})