Allow adding sources to processing expression context

python/core/processing/qgsprocessingalgorithm.sip

@@ -326,10 +326,12 @@ class QgsProcessingAlgorithm
 %End
 
     QgsExpressionContext createExpressionContext( const QVariantMap &parameters,
-        QgsProcessingContext &context ) const;
+        QgsProcessingContext &context, QgsProcessingFeatureSource *source = 0 ) const;
 %Docstring
  Creates an expression context relating to the algorithm. This can be called by algorithms
  to create a new expression context ready for evaluating expressions within the algorithm.
+ Optionally, a ``source`` can be specified which will be used to populate the context if it
+ implements the QgsExpressionContextGenerator interface.
  :rtype: QgsExpressionContext
 %End
 

python/core/processing/qgsprocessingutils.sip

@@ -274,6 +274,12 @@ class QgsProcessingFeatureSource : QgsFeatureSource
     virtual QVariant maximumValue( int fieldIndex ) const;
 
 
+    QgsFeatureSource *source() const;
+%Docstring
+ Access the underlying original ``source``.
+ :rtype: QgsFeatureSource
+%End
+
 };
 
 

python/core/qgsexpressioncontext.sip

@@ -562,6 +562,7 @@ Constructor for QgsExpressionContext
 %End
 
 
+
     void setFeature( const QgsFeature &feature );
 %Docstring
  Convenience function for setting a feature for the context. The feature

src/core/processing/qgsprocessingalgorithm.cpp

@@ -124,10 +124,24 @@ QWidget *QgsProcessingAlgorithm::createCustomParametersWidget( QWidget * ) const
 }
 
 QgsExpressionContext QgsProcessingAlgorithm::createExpressionContext( const QVariantMap &parameters,
-    QgsProcessingContext &context ) const
+    QgsProcessingContext &context, QgsProcessingFeatureSource *source ) const
 {
   // start with context's expression context
   QgsExpressionContext c = context.expressionContext();
+
+  // If there's a source capable of generating a context scope, use it
+  if ( source )
+  {
+    QgsExpressionContextGenerator *generator = dynamic_cast<QgsExpressionContextGenerator *>( source->source() );
+    if ( generator )
+    {
+      const auto &scopes = generator->createExpressionContext().takeScopes();
+      for ( QgsExpressionContextScope *scope : scopes )
+        c << scope;
+    }
+  }
+  else
+
     if ( c.scopeCount() == 0 )
     {
       //empty scope, populate with initial scopes

src/core/processing/qgsprocessingalgorithm.h

@@ -339,9 +339,11 @@ class CORE_EXPORT QgsProcessingAlgorithm
     /**
      * Creates an expression context relating to the algorithm. This can be called by algorithms
      * to create a new expression context ready for evaluating expressions within the algorithm.
+     * Optionally, a \a source can be specified which will be used to populate the context if it
+     * implements the QgsExpressionContextGenerator interface.
      */
     QgsExpressionContext createExpressionContext( const QVariantMap &parameters,
-        QgsProcessingContext &context ) const;
+        QgsProcessingContext &context, QgsProcessingFeatureSource *source = nullptr ) const;
 
     /**
      * Checks whether the coordinate reference systems for the specified set of \a parameters

src/core/processing/qgsprocessingutils.cpp

@@ -695,3 +695,8 @@ QVariant QgsProcessingFeatureSource::maximumValue( int fieldIndex ) const
 {
   return mSource->maximumValue( fieldIndex );
 }
+
+QgsFeatureSource *QgsProcessingFeatureSource::source() const
+{
+  return mSource;
+}

src/core/processing/qgsprocessingutils.h

@@ -317,6 +317,11 @@ class CORE_EXPORT QgsProcessingFeatureSource : public QgsFeatureSource
     QVariant minimumValue( int fieldIndex ) const override;
     QVariant maximumValue( int fieldIndex ) const override;
 
+    /**
+     * Access the underlying original \a source.
+     */
+    QgsFeatureSource *source() const;
+
   private:
 
     QgsFeatureSource *mSource = nullptr;

src/core/qgsexpressioncontext.cpp

@@ -474,6 +474,13 @@ QgsExpressionContextScope *QgsExpressionContext::popScope()
   return nullptr;
 }
 
+QList<QgsExpressionContextScope *> QgsExpressionContext::takeScopes()
+{
+  QList<QgsExpressionContextScope *> stack = mStack;
+  mStack.clear();
+  return stack;
+}
+
 QgsExpressionContext &QgsExpressionContext::operator<<( QgsExpressionContextScope *scope )
 {
   mStack.append( scope );

src/core/qgsexpressioncontext.h

@@ -581,6 +581,16 @@ class CORE_EXPORT QgsExpressionContext
      */
     QgsExpressionContextScope *popScope();
 
+    /**
+     * Return all scopes from this context and remove them, leaving this context without
+     * any context.
+     * Ownership is transferred to the caller.
+     *
+     * \since QGIS 3.0
+     * \note Not available in Python
+     */
+    QList<QgsExpressionContextScope *> takeScopes() SIP_SKIP;
+
     /**
      * Appends a scope to the end of the context. This scope will override
      * any matching variables or functions provided by existing scopes within the