Refreshing values in attribute table automatically when geometry changes?

Question

I can't find a solution for auto-refreshing attribute table data.

For instance, how can the data related to features of a Shapefile (e.g., the area) be updated if the feature geometry is changed.

Answer 1

You can enable automatic updates of feature attributes with the following PyQGIS script.

In this example, say you have a polygon layer named myPolygons, which has, among others, fields named area and perimeter.

Copy and paste the code into the QGIS Python Console.

# Initialize required variables
myLayer = QgsMapLayerRegistry.instance().mapLayersByName( 'myPolygons' )[0]
areaField = myLayer.fieldNameIndex( 'area' )
perimeterField = myLayer.fieldNameIndex( 'perimeter' )

# Slot, updates field values
def updateFeatureAttrs( fId, geom=None ):
    f = myLayer.getFeatures( QgsFeatureRequest( fId ) ).next()    
    if not geom:
        geom = f.geometry() 
    myLayer.changeAttributeValue( fId, areaField, geom.area() )
    myLayer.changeAttributeValue( fId, perimeterField, geom.length() )

# Update feature attributes when new features are added or geometry changes
myLayer.featureAdded.connect( updateFeatureAttrs )
myLayer.geometryChanged.connect( updateFeatureAttrs )

Now, every time you add a new polygon to the myPolygons layer or modify any polygon from it, its corresponding attributes (area and perimeter) will be updated.

In another question, I posted an example with line features.