Expression variables are more often than not static

python/core/qgsexpression.sip

@@ -574,6 +574,11 @@ Does this function use a geometry object.
  :rtype: bool
 %End
 
+        virtual bool isStatic( const QgsExpression::NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const;
+%Docstring
+ :rtype: bool
+%End
+
         virtual QSet<QString> referencedColumns( const QgsExpression::NodeFunction *node ) const;
 %Docstring
  Returns a set of field names which are required for this function.

src/core/qgsexpression.cpp

@@ -4212,9 +4212,29 @@ const QList<QgsExpression::Function *> &QgsExpression::Functions()
         << new StaticFunction( QStringLiteral( "layer_property" ), 2, fcnGetLayerProperty, QStringLiteral( "General" ) )
         << new StaticFunction( QStringLiteral( "raster_statistic" ), ParameterList() << Parameter( QStringLiteral( "layer" ) )
                                << Parameter( QStringLiteral( "band" ) )
-                               << Parameter( QStringLiteral( "statistic" ) ), fcnGetRasterBandStat, QStringLiteral( "General" ) )
-        << new StaticFunction( QStringLiteral( "var" ), 1, fcnGetVariable, QStringLiteral( "General" ) )
+                               << Parameter( QStringLiteral( "statistic" ) ), fcnGetRasterBandStat, QStringLiteral( "General" ) );
 
+    // **var** function
+    StaticFunction *varFunction =  new StaticFunction( QStringLiteral( "var" ), 1, fcnGetVariable, QStringLiteral( "General" ) );
+    varFunction->setIsStaticFunction(
+      []( const NodeFunction * node, QgsExpression * parent, const QgsExpressionContext * context )
+    {
+      if ( node->args()->count() > 0 )
+      {
+        Node *argNode = node->args()->at( 0 );
+
+        if ( !argNode->isStatic( parent, context ) )
+          return false;
+
+        if ( fcnGetVariable( QVariantList() << argNode->eval( parent, context ), context, parent ).isValid() )
+          return true;
+      }
+      return false;
+    }
+    );
+
+    sFunctions
+        << varFunction
         //return all attributes string for referencedColumns - this is caught by
         // QgsFeatureRequest::setSubsetOfAttributes and causes all attributes to be fetched by the
         // feature request
@@ -5590,10 +5610,7 @@ QgsExpression::Node *QgsExpression::NodeFunction::clone() const
 
 bool QgsExpression::NodeFunction::isStatic( QgsExpression *parent, const QgsExpressionContext *context ) const
 {
-  Q_UNUSED( parent )
-  Q_UNUSED( context )
-  // TODO some functions are static!
-  return false;
+  return Functions()[mFnIndex]->isStatic( this, parent, context );
 }
 
 bool QgsExpression::NodeFunction::validateParams( int fnIndex, QgsExpression::NodeList *args, QString &error )
@@ -6299,6 +6316,14 @@ bool QgsExpression::Function::usesGeometry( const QgsExpression::NodeFunction *n
   return true;
 }
 
+bool QgsExpression::Function::isStatic( const QgsExpression::NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const
+{
+  Q_UNUSED( parent )
+  Q_UNUSED( context )
+  Q_UNUSED( node )
+  return false;
+}
+
 QSet<QString> QgsExpression::Function::referencedColumns( const NodeFunction *node ) const
 {
   Q_UNUSED( node )
@@ -6310,7 +6335,15 @@ bool QgsExpression::Function::operator==( const QgsExpression::Function &other )
   return ( QString::compare( mName, other.mName, Qt::CaseInsensitive ) == 0 );
 }
 
-QgsExpression::StaticFunction::StaticFunction( const QString &fnname, const QgsExpression::ParameterList &params, QgsExpression::FcnEval fcn, const QString &group, const QString &helpText, std::function < bool ( const QgsExpression::NodeFunction *node ) > usesGeometry, std::function < QSet<QString>( const QgsExpression::NodeFunction *node ) > referencedColumns, bool lazyEval, const QStringList &aliases, bool handlesNull )
+QgsExpression::StaticFunction::StaticFunction( const QString &fnname, const QgsExpression::ParameterList &params,
+    QgsExpression::FcnEval fcn,
+    const QString &group,
+    const QString &helpText,
+    std::function < bool ( const QgsExpression::NodeFunction *node ) > usesGeometry,
+    std::function < QSet<QString>( const QgsExpression::NodeFunction *node ) > referencedColumns,
+    bool lazyEval,
+    const QStringList &aliases,
+    bool handlesNull )
   : Function( fnname, params, group, helpText, lazyEval, handlesNull )
   , mFnc( fcn )
   , mAliases( aliases )
@@ -6336,6 +6369,19 @@ QSet<QString> QgsExpression::StaticFunction::referencedColumns( const NodeFuncti
     return mReferencedColumns;
 }
 
+bool QgsExpression::StaticFunction::isStatic( const NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const
+{
+  if ( mIsStaticFunc )
+    return mIsStaticFunc( node, parent, context );
+  else
+    return false;
+}
+
+void QgsExpression::StaticFunction::setIsStaticFunction( std::function<bool ( const NodeFunction *, QgsExpression *, const QgsExpressionContext * )> isStatic )
+{
+  mIsStaticFunc = isStatic;
+}
+
 QVariant QgsExpression::Node::eval( QgsExpression *parent, const QgsExpressionContext *context )
 {
   if ( mHasCachedValue )

src/core/qgsexpression.h

@@ -578,6 +578,8 @@ class CORE_EXPORT QgsExpression
          * Functions are non lazy default and will be given the node return value when called **/
         bool lazyEval() const { return mLazyEval; }
 
+        virtual bool isStatic( const QgsExpression::NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const;
+
         /**
          * Returns a set of field names which are required for this function.
          * May contain QgsFeatureRequest::AllAttributes to signal that all
@@ -705,6 +707,30 @@ class CORE_EXPORT QgsExpression
                         const QStringList &aliases = QStringList(),
                         bool handlesNull = false );
 
+
+        /**
+         * Static function for evaluation against a QgsExpressionContext, using a named list of parameter values.
+         *
+         * Lambda functions can be provided that will be called to determine if a geometry is used an which
+         * columns are referenced.
+         * This is only required if this cannot be determined by calling each parameter node's usesGeometry() or
+         * referencedColumns() method. For example, an aggregate expression requires the geometry and all columns
+         * if the parent variable is used.
+         * If a nullptr is passed as a node to these functions, they should stay on the safe side and return if they
+         * could potentially require a geometry or columns.
+         */
+        StaticFunction( const QString &fnname,
+                        const QgsExpression::ParameterList &params,
+                        FcnEval fcn,
+                        const QString &group,
+                        const QString &helpText,
+                        std::function < bool ( const QgsExpression::NodeFunction *node ) > usesGeometry,
+                        std::function < QSet<QString>( const QgsExpression::NodeFunction *node ) > referencedColumns,
+                        std::function < bool( const NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) > isStatic,
+                        bool lazyEval = false,
+                        const QStringList &aliases = QStringList(),
+                        bool handlesNull = false );
+
         /** Static function for evaluation against a QgsExpressionContext, using a named list of parameter values and list
          * of groups.
          */
@@ -742,12 +768,17 @@ class CORE_EXPORT QgsExpression
 
         virtual QSet<QString> referencedColumns( const QgsExpression::NodeFunction *node ) const override;
 
+        virtual bool isStatic( const NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) const override;
+
+        void setIsStaticFunction( std::function < bool( const NodeFunction *node, QgsExpression *parent, const QgsExpressionContext *context ) > isStatic );
+
       private:
         FcnEval mFnc;
         QStringList mAliases;
         bool mUsesGeometry;
         std::function < bool( const QgsExpression::NodeFunction *node ) > mUsesGeometryFunc;
         std::function < QSet<QString>( const QgsExpression::NodeFunction *node ) > mReferencedColumnsFunc;
+        std::function < bool( const NodeFunction *node,  QgsExpression *parent, const QgsExpressionContext *context ) > mIsStaticFunc;
         QSet<QString> mReferencedColumns;
     };
 #endif