[processing] Tweak api for QgsProcessingFeatureBasedAlgorithm

Instead of returning a single QgsFeature from processFeature, we now
return a list of features.

This allows feature based algorithms which return multiple features
per input feature, e.g. "explode" type algorithms which split a
single input feature into multiple output features.

python/core/processing/qgsprocessingalgorithm.sip.in

@@ -855,17 +855,20 @@ Returns the source's coordinate reference system. This will only return a valid
 called from a subclasses' processFeature() implementation.
 %End
 
-    virtual QgsFeature processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) = 0;
+    virtual QgsFeatureList processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) = 0;
 %Docstring
 Processes an individual input ``feature`` from the source. Algorithms should implement their
 logic in this method for performing the algorithm's operation (e.g. replacing the feature's
 geometry with the centroid of the original feature geometry for a 'centroid' type
 algorithm).
 
-Implementations should return the modified feature. Returning an invalid feature (e.g.
-a default constructed :py:class:`QgsFeature`) will indicate that this feature should be 'skipped',
-and will not be added to the algorithm's output. Subclasses can use this approach to
-filter the incoming features as desired.
+Implementations should return a list containing the modified feature. Returning an empty an list
+will indicate that this feature should be 'skipped', and will not be added to the algorithm's output.
+Subclasses can use this approach to filter the incoming features as desired.
+
+Additionally, multiple features can be returned for a single input feature. Each returned feature
+will be added to the algorithm's output. This allows for "explode" type algorithms where a single
+input feature results in multiple output features.
 
 The provided ``feedback`` object can be used to push messages to the log and for giving feedback
 to users. Note that handling of progress reports and algorithm cancelation is handled by

python/plugins/processing/algs/qgis/AddTableField.py

@@ -97,4 +97,4 @@ def processFeature(self, feature, context, feedback):
         attributes = feature.attributes()
         attributes.append(None)
         feature.setAttributes(attributes)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/DeleteColumn.py

@@ -88,4 +88,4 @@ def processFeature(self, feature, context, feedback):
         for index in self.field_indices:
             del attributes[index]
         feature.setAttributes(attributes)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/DeleteHoles.py

@@ -72,4 +72,4 @@ def prepareAlgorithm(self, parameters, context, feedback):
     def processFeature(self, feature, context, feedback):
         if feature.hasGeometry():
             feature.setGeometry(feature.geometry().removeInteriorRings(self.min_area))
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/DensifyGeometries.py

@@ -75,4 +75,4 @@ def processFeature(self, feature, context, feedback):
         if feature.hasGeometry():
             new_geometry = feature.geometry().densifyByCount(self.vertices)
             feature.setGeometry(new_geometry)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/DensifyGeometriesInterval.py

@@ -74,4 +74,4 @@ def processFeature(self, feature, context, feedback):
         if feature.hasGeometry():
             new_geometry = feature.geometry().densifyByDistance(float(interval))
             feature.setGeometry(new_geometry)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/ExtendLines.py

@@ -80,4 +80,4 @@ def processFeature(self, feature, context, feedback):
 
             feature.setGeometry(output_geometry)
 
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/FieldsMapper.py

@@ -168,4 +168,4 @@ def processFeature(self, feature, context, feedback):
             attributes.append(value)
         feature.setAttributes(attributes)
         self._row_number += 1
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/GeometryByExpression.py

@@ -118,4 +118,4 @@ def processFeature(self, feature, context, feedback):
                 raise QgsProcessingException(
                     self.tr('{} is not a geometry').format(value))
             feature.setGeometry(value)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/LinesToPolygons.py

@@ -88,7 +88,7 @@ def processFeature(self, feature, context, feedback):
             feature.setGeometry(QgsGeometry(self.convertToPolygons(feature.geometry())))
             if feature.geometry().isEmpty():
                 feedback.reportError(self.tr("One or more line ignored due to geometry not having a minimum of three vertices."))
-        return feature
+        return [feature]
 
     def convertWkbToPolygons(self, wkb):
         multi_wkb = None

python/plugins/processing/algs/qgis/OffsetLine.py

@@ -113,4 +113,4 @@ def processFeature(self, feature, context, feedback):
 
             feature.setGeometry(output_geometry)
 
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/Orthogonalize.py

@@ -91,4 +91,4 @@ def processFeature(self, feature, context, feedback):
                     self.tr('Error orthogonalizing geometry'))
 
             feature.setGeometry(output_geometry)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/PointOnSurface.py

@@ -75,4 +75,4 @@ def processFeature(self, feature, context, feedback):
                 raise QgsProcessingException(self.tr('Error calculating point on surface: `{error_message}`'.format(error_message=output_geometry.error())))
 
             feature.setGeometry(output_geometry)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/PolygonsToLines.py

@@ -79,7 +79,7 @@ def outputWkbType(self, input_wkb_type):
     def processFeature(self, feature, context, feedback):
         if feature.hasGeometry():
             feature.setGeometry(QgsGeometry(self.convertToLines(feature.geometry())))
-        return feature
+        return [feature]
 
     def convertWkbToLines(self, wkb):
         multi_wkb = None

python/plugins/processing/algs/qgis/ReverseLineDirection.py

@@ -67,4 +67,4 @@ def processFeature(self, feature, context, feedback):
             outGeom = QgsGeometry(reversedLine)
 
             feature.setGeometry(outGeom)
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/SetMValue.py

@@ -100,4 +100,4 @@ def processFeature(self, feature, context, feedback):
 
             feature.setGeometry(QgsGeometry(new_geom))
 
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/SetZValue.py

@@ -101,4 +101,4 @@ def processFeature(self, feature, context, feedback):
 
             feature.setGeometry(QgsGeometry(new_geom))
 
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/SingleSidedBuffer.py

@@ -122,4 +122,4 @@ def processFeature(self, feature, context, feedback):
 
             feature.setGeometry(output_geometry)
 
-        return feature
+        return [feature]

python/plugins/processing/algs/qgis/TextToFloat.py

@@ -85,4 +85,4 @@ def processFeature(self, feature, context, feedback):
                 feature[self.field_idx] = float(value)
         except:
             feature[self.field_idx] = None
-        return feature
+        return [feature]

src/3d/processing/qgsalgorithmtessellate.cpp

@@ -74,7 +74,7 @@ QgsTessellateAlgorithm *QgsTessellateAlgorithm::createInstance() const
   return new QgsTessellateAlgorithm();
 }
 
-QgsFeature QgsTessellateAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsTessellateAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
   QgsFeature f = feature;
   if ( f.hasGeometry() )
@@ -105,7 +105,7 @@ QgsFeature QgsTessellateAlgorithm::processFeature( const QgsFeature &feature, Qg
       f.setGeometry( g );
     }
   }
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/3d/processing/qgsalgorithmtessellate.h

@@ -47,7 +47,7 @@ class QgsTessellateAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QgsProcessing::SourceType outputLayerType() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override;
 
-    QgsFeature processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
 };
 

src/analysis/processing/qgsalgorithmaddincrementalfield.cpp

@@ -95,7 +95,7 @@ bool QgsAddIncrementalFieldAlgorithm::prepareAlgorithm( const QVariantMap &param
   return true;
 }
 
-QgsFeature QgsAddIncrementalFieldAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsAddIncrementalFieldAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
   if ( !mGroupedFieldNames.empty() && mGroupedFields.empty() )
   {
@@ -127,7 +127,7 @@ QgsFeature QgsAddIncrementalFieldAlgorithm::processFeature( const QgsFeature &fe
     mGroupedValues[ groupAttributes ] = value;
   }
   f.setAttributes( attributes );
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmaddincrementalfield.h

@@ -50,7 +50,7 @@ class QgsAddIncrementalFieldAlgorithm : public QgsProcessingFeatureBasedAlgorith
     QgsFields outputFields( const QgsFields &inputFields ) const override;
 
     bool prepareAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
   private:
 

src/analysis/processing/qgsalgorithmassignprojection.cpp

@@ -73,9 +73,9 @@ bool QgsAssignProjectionAlgorithm::prepareAlgorithm( const QVariantMap &paramete
   return true;
 }
 
-QgsFeature QgsAssignProjectionAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsAssignProjectionAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
-  return feature;
+  return QgsFeatureList() << feature;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmassignprojection.h

@@ -49,7 +49,7 @@ class QgsAssignProjectionAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
 
     bool prepareAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
   private:
 

src/analysis/processing/qgsalgorithmboundary.cpp

@@ -95,7 +95,7 @@ QgsWkbTypes::Type QgsBoundaryAlgorithm::outputWkbType( QgsWkbTypes::Type inputWk
   return outputWkb;
 }
 
-QgsFeature QgsBoundaryAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback *feedback )
+QgsFeatureList QgsBoundaryAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback *feedback )
 {
   QgsFeature outFeature = feature;
 
@@ -113,7 +113,7 @@ QgsFeature QgsBoundaryAlgorithm::processFeature( const QgsFeature &feature, QgsP
       outFeature.setGeometry( outputGeometry );
     }
   }
-  return outFeature;
+  return QgsFeatureList() << outFeature;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmboundary.h

@@ -47,7 +47,7 @@ class QgsBoundaryAlgorithm : public QgsProcessingFeatureBasedAlgorithm
 
     QString outputName() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 };
 
 ///@endcond PRIVATE

src/analysis/processing/qgsalgorithmboundingbox.cpp

@@ -71,7 +71,7 @@ QgsFields QgsBoundingBoxAlgorithm::outputFields( const QgsFields &inputFields )
   return fields;
 }
 
-QgsFeature QgsBoundingBoxAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsBoundingBoxAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
   QgsFeature f = feature;
   if ( f.hasGeometry() )
@@ -95,7 +95,7 @@ QgsFeature QgsBoundingBoxAlgorithm::processFeature( const QgsFeature &feature, Q
           << QVariant();
     f.setAttributes( attrs );
   }
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmboundingbox.h

@@ -46,7 +46,7 @@ class QgsBoundingBoxAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type ) const override { return QgsWkbTypes::Polygon; }
     QgsFields outputFields( const QgsFields &inputFields ) const override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
 };
 

src/analysis/processing/qgsalgorithmcentroid.cpp

@@ -66,7 +66,7 @@ QgsCentroidAlgorithm *QgsCentroidAlgorithm::createInstance() const
   return new QgsCentroidAlgorithm();
 }
 
-QgsFeature QgsCentroidAlgorithm::processFeature( const QgsFeature &f, QgsProcessingContext &, QgsProcessingFeedback *feedback )
+QgsFeatureList QgsCentroidAlgorithm::processFeature( const QgsFeature &f, QgsProcessingContext &, QgsProcessingFeedback *feedback )
 {
   QgsFeature feature = f;
   if ( feature.hasGeometry() )
@@ -77,7 +77,7 @@ QgsFeature QgsCentroidAlgorithm::processFeature( const QgsFeature &f, QgsProcess
       feedback->pushInfo( QObject::tr( "Error calculating centroid for feature %1" ).arg( feature.id() ) );
     }
   }
-  return feature;
+  return QgsFeatureList() << feature;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmcentroid.h

@@ -49,7 +49,7 @@ class QgsCentroidAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QgsProcessing::SourceType outputLayerType() const override { return QgsProcessing::TypeVectorPoint; }
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override { Q_UNUSED( inputWkbType ); return QgsWkbTypes::Point; }
 
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 };
 
 ///@endcond PRIVATE

src/analysis/processing/qgsalgorithmconvexhull.cpp

@@ -69,7 +69,7 @@ QgsFields QgsConvexHullAlgorithm::outputFields( const QgsFields &inputFields ) c
   return fields;
 }
 
-QgsFeature QgsConvexHullAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback *feedback )
+QgsFeatureList QgsConvexHullAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback *feedback )
 {
   QgsFeature f = feature;
   if ( f.hasGeometry() )
@@ -93,7 +93,7 @@ QgsFeature QgsConvexHullAlgorithm::processFeature( const QgsFeature &feature, Qg
       f.setAttributes( attrs );
     }
   }
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmconvexhull.h

@@ -47,7 +47,7 @@ class QgsConvexHullAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type ) const override { return QgsWkbTypes::Polygon; }
     QgsFields outputFields( const QgsFields &inputFields ) const override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
 };
 

src/analysis/processing/qgsalgorithmdropgeometry.cpp

@@ -74,11 +74,11 @@ QgsFeatureRequest QgsDropGeometryAlgorithm::request() const
   return QgsFeatureRequest().setFlags( QgsFeatureRequest::NoGeometry );
 }
 
-QgsFeature QgsDropGeometryAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsDropGeometryAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
   QgsFeature f = feature;
   f.clearGeometry();
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmdropgeometry.h

@@ -48,7 +48,7 @@ class QgsDropGeometryAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override;
     QgsFeatureRequest request() const override;
-    QgsFeature processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 };
 
 ///@endcond PRIVATE

src/analysis/processing/qgsalgorithmdropmzvalues.cpp

@@ -82,7 +82,7 @@ bool QgsDropMZValuesAlgorithm::prepareAlgorithm( const QVariantMap &parameters,
   return true;
 }
 
-QgsFeature QgsDropMZValuesAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
+QgsFeatureList QgsDropMZValuesAlgorithm::processFeature( const QgsFeature &feature, QgsProcessingContext &, QgsProcessingFeedback * )
 {
   QgsFeature f = feature;
   if ( f.hasGeometry() )
@@ -95,7 +95,7 @@ QgsFeature QgsDropMZValuesAlgorithm::processFeature( const QgsFeature &feature,
     f.setGeometry( QgsGeometry( newGeom.release() ) );
   }
 
-  return f;
+  return QgsFeatureList() << f;
 }
 
 ///@endcond

src/analysis/processing/qgsalgorithmdropmzvalues.h

@@ -48,7 +48,7 @@ class QgsDropMZValuesAlgorithm : public QgsProcessingFeatureBasedAlgorithm
     QString outputName() const override;
     QgsWkbTypes::Type outputWkbType( QgsWkbTypes::Type inputWkbType ) const override;
     bool prepareAlgorithm( const QVariantMap &parameters, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
-    QgsFeature processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
+    QgsFeatureList processFeature( const QgsFeature &feature,  QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;
 
   private: