[processing] Allow QgsVectorLayerSelectedFeatureSource to

create expression context scopes

Allows algorithms which require the underlying layer scope
to operate correctly when running in selected features only
mode.

Fixes #17170

(cherry-picked from d5617b8)

Author: nyalldawson

python/core/qgsvectorlayerfeatureiterator.sip.in

@@ -153,7 +153,7 @@ Setup the simplification of geometries to fetch using the specified simplify met
 
 
 
-class QgsVectorLayerSelectedFeatureSource : QgsFeatureSource
+class QgsVectorLayerSelectedFeatureSource : QgsFeatureSource, QgsExpressionContextScopeGenerator
 {
 %Docstring
 QgsFeatureSource subclass for the selected features from a :py:class:`QgsVectorLayer`.
@@ -185,6 +185,8 @@ the QgsVectorLayerSelectedFeatureSource will not be reflected.
 
     virtual QString sourceName() const;
 
+    virtual QgsExpressionContextScope *createExpressionContextScope() const;
+
 
 
 };

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -1083,6 +1083,7 @@ QgsVectorLayerSelectedFeatureSource::QgsVectorLayerSelectedFeatureSource( QgsVec
   , mSelectedFeatureIds( layer->selectedFeatureIds() )
   , mWkbType( layer->wkbType() )
   , mName( layer->name() )
+  , mLayer( layer )
 {}
 
 QgsFeatureIterator QgsVectorLayerSelectedFeatureSource::getFeatures( const QgsFeatureRequest &request ) const
@@ -1127,3 +1128,11 @@ QString QgsVectorLayerSelectedFeatureSource::sourceName() const
 {
   return mName;
 }
+
+QgsExpressionContextScope *QgsVectorLayerSelectedFeatureSource::createExpressionContextScope() const
+{
+  if ( mLayer )
+    return mLayer->createExpressionContextScope();
+  else
+    return nullptr;
+}

src/core/qgsvectorlayerfeatureiterator.h

@@ -21,7 +21,9 @@
 #include "qgsfields.h"
 #include "qgscoordinatereferencesystem.h"
 #include "qgsfeaturesource.h"
+#include "qgsexpressioncontextscopegenerator.h"
 
+#include <QPointer>
 #include <QSet>
 #include <memory>
 
@@ -285,7 +287,7 @@ class CORE_EXPORT QgsVectorLayerFeatureIterator : public QgsAbstractFeatureItera
  * QgsFeatureSource subclass for the selected features from a QgsVectorLayer.
  * \since QGIS 3.0
  */
-class CORE_EXPORT QgsVectorLayerSelectedFeatureSource : public QgsFeatureSource
+class CORE_EXPORT QgsVectorLayerSelectedFeatureSource : public QgsFeatureSource, public QgsExpressionContextScopeGenerator
 {
   public:
 
@@ -302,6 +304,7 @@ class CORE_EXPORT QgsVectorLayerSelectedFeatureSource : public QgsFeatureSource
     QgsWkbTypes::Type wkbType() const override;
     long featureCount() const override;
     QString sourceName() const override;
+    QgsExpressionContextScope *createExpressionContextScope() const override;
 
 
   private:
@@ -311,6 +314,7 @@ class CORE_EXPORT QgsVectorLayerSelectedFeatureSource : public QgsFeatureSource
     QgsFeatureIds mSelectedFeatureIds;
     QgsWkbTypes::Type mWkbType = QgsWkbTypes::Unknown;
     QString mName;
+    QPointer< QgsVectorLayer > mLayer;
 
 };