Tweak processing API to better handle dynamic properties

python/core/processing/qgsprocessingalgorithm.sip

@@ -872,7 +872,7 @@ class QgsProcessingFeatureBasedAlgorithm : QgsProcessingAlgorithm
  :rtype: QgsCoordinateReferenceSystem
 %End
 
-    virtual QgsFeature processFeature( const QgsFeature &feature, QgsProcessingFeedback *feedback ) = 0;
+    virtual QgsFeature 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

src/core/processing/qgsprocessingalgorithm.cpp

@@ -699,6 +699,13 @@ QVariantMap QgsProcessingFeatureBasedAlgorithm::processAlgorithm( const QVariant
   if ( !sink )
     return QVariantMap();
 
+  // prepare expression context for feature iteration
+  QgsExpressionContext prevContext = context.expressionContext();
+  QgsExpressionContext algContext = prevContext;
+
+  algContext.appendScopes( createExpressionContext( parameters, context, dynamic_cast< QgsProcessingFeatureSource * >( mSource.get() ) ).takeScopes() );
+  context.setExpressionContext( algContext );
+
   long count = mSource->featureCount();
 
   QgsFeature f;
@@ -713,7 +720,8 @@ QVariantMap QgsProcessingFeatureBasedAlgorithm::processAlgorithm( const QVariant
       break;
     }
 
-    QgsFeature transformed = processFeature( f, feedback );
+    context.expressionContext().setFeature( f );
+    QgsFeature transformed = processFeature( f, context, feedback );
     if ( transformed.isValid() )
       sink->addFeature( transformed, QgsFeatureSink::FastInsert );
 
@@ -723,6 +731,9 @@ QVariantMap QgsProcessingFeatureBasedAlgorithm::processAlgorithm( const QVariant
 
   mSource.reset();
 
+  // probably not necessary - context's aren't usually recycled, but can't hurt
+  context.setExpressionContext( prevContext );
+
   QVariantMap outputs;
   outputs.insert( QStringLiteral( "OUTPUT" ), dest );
   return outputs;

src/core/processing/qgsprocessingalgorithm.h

@@ -874,7 +874,7 @@ class CORE_EXPORT QgsProcessingFeatureBasedAlgorithm : public QgsProcessingAlgor
      * can break valid model execution - so use with extreme caution, and consider using
      * \a feedback to instead report non-fatal processing failures for features instead.
      */
-    virtual QgsFeature processFeature( const QgsFeature &feature, QgsProcessingFeedback *feedback ) = 0;
+    virtual QgsFeature processFeature( const QgsFeature &feature, QgsProcessingContext &context, QgsProcessingFeedback *feedback ) = 0;
 
     virtual QVariantMap processAlgorithm( const QVariantMap &parameters,
                                           QgsProcessingContext &context, QgsProcessingFeedback *feedback ) override;