QgsExpression: set setGeomCalculator precedence

setGeomCalculator, setDistanceUnits and setAreaUnits have now
 precedence over expression scopes.

Author: Hugo Mercier

python/core/auto_generated/expression/qgsexpression.sip.in

@@ -293,8 +293,12 @@ Returns calculator used for distance and area calculations
     void setGeomCalculator( const QgsDistanceArea *calc );
 %Docstring
 Sets the geometry calculator used for distance and area calculations in expressions.
-(used by $length, $area and $perimeter functions only). By default, no geometry
-calculator is set and all distance and area calculations are performed using simple
+(used by $length, $area and $perimeter functions only).
+If the geometry calculator is set to None (default), prepare() will read variables
+from the expression context ("project_ellipsoid", "_project_transform_context" and
+"_layer_crs") to build a geometry calculator.
+If these variables does not exist and if setGeomCalculator() is not called,
+all distance and area calculations are performed using simple
 Cartesian methods (ie no ellipsoidal calculations).
 
 :param calc: geometry calculator. Ownership is not transferred. Set to a None to force
@@ -321,6 +325,8 @@ Returns the desired distance units for calculations involving geomCalculator(),
     void setDistanceUnits( QgsUnitTypes::DistanceUnit unit );
 %Docstring
 Sets the desired distance units for calculations involving geomCalculator(), e.g., "$length" and "$perimeter".
+If distance units are set to QgsUnitTypes.DistanceUnknownUnit (default), prepare() will read
+variables from the expression context ("project_distance_units") to determine distance units.
 
 .. note::
 
@@ -351,6 +357,8 @@ Returns the desired areal units for calculations involving geomCalculator(), e.g
     void setAreaUnits( QgsUnitTypes::AreaUnit unit );
 %Docstring
 Sets the desired areal units for calculations involving geomCalculator(), e.g., "$area".
+If distance units are set to QgsUnitTypes.AreaUnknownUnit (default), prepare() will read
+variables from the expression context ("project_distance_units") to determine distance units.
 
 .. note::
 

src/core/expression/qgsexpression.cpp

@@ -316,22 +316,34 @@ bool QgsExpression::needsGeometry() const
 
 void QgsExpression::initGeomCalculator( const QgsExpressionContext *context )
 {
-  if ( ! d->mCalc )
-    d->mCalc = std::shared_ptr<QgsDistanceArea>( new QgsDistanceArea() );
-  QString ellipsoid = context->variable( "project_ellipsoid" ).toString();
-  QString distanceUnitsStr = context->variable( "project_distance_units" ).toString();
-  QString areaUnitsStr = context->variable( "project_area_units" ).toString();
-  QgsCoordinateReferenceSystem crs = context->variable( "_layer_crs" ).value<QgsCoordinateReferenceSystem>();
-  QgsCoordinateTransformContext tContext = context->variable( "_project_transform_context" ).value<QgsCoordinateTransformContext>();
-
-  d->mCalc->setEllipsoid( ellipsoid.isEmpty() ? GEO_NONE : ellipsoid );
-  if ( ! distanceUnitsStr.isEmpty() )
-    setDistanceUnits( QgsUnitTypes::stringToDistanceUnit( distanceUnitsStr ) );
-  if ( ! areaUnitsStr.isEmpty() )
-    setAreaUnits( QgsUnitTypes::stringToAreaUnit( areaUnitsStr ) );
-  if ( crs.isValid() )
+  // Set the geometry calculator from the context if it has not been set by setGeomCalculator()
+  if ( context && ! d->mCalc )
   {
-    d->mCalc->setSourceCrs( crs, tContext );
+    QString ellipsoid = context->variable( "project_ellipsoid" ).toString();
+    QgsCoordinateReferenceSystem crs = context->variable( "_layer_crs" ).value<QgsCoordinateReferenceSystem>();
+    QgsCoordinateTransformContext tContext = context->variable( "_project_transform_context" ).value<QgsCoordinateTransformContext>();
+    if ( crs.isValid() )
+    {
+      d->mCalc = std::shared_ptr<QgsDistanceArea>( new QgsDistanceArea() );
+      d->mCalc->setEllipsoid( ellipsoid.isEmpty() ? GEO_NONE : ellipsoid );
+      d->mCalc->setSourceCrs( crs, tContext );
+    }
+  }
+
+  // Set the distance units from the context if it has not been set by setDistanceUnits()
+  if ( context && distanceUnits() == QgsUnitTypes::DistanceUnknownUnit )
+  {
+    QString distanceUnitsStr = context->variable( "project_distance_units" ).toString();
+    if ( ! distanceUnitsStr.isEmpty() )
+      setDistanceUnits( QgsUnitTypes::stringToDistanceUnit( distanceUnitsStr ) );
+  }
+
+  // Set the area units from the context if it has not been set by setAreaUnits()
+  if ( context && areaUnits() == QgsUnitTypes::AreaUnknownUnit )
+  {
+    QString areaUnitsStr = context->variable( "project_area_units" ).toString();
+    if ( ! areaUnitsStr.isEmpty() )
+      setAreaUnits( QgsUnitTypes::stringToAreaUnit( areaUnitsStr ) );
   }
 }
 
@@ -403,8 +415,7 @@ QVariant QgsExpression::evaluate( const QgsExpressionContext *context )
   if ( ! d->mIsPrepared )
   {
     qWarning( "QgsExpression::evaluate() called on an expression not yet prepared !" );
-    if ( ! prepare( context ) )
-      return QVariant();
+    prepare( context );
   }
   return d->mRootNode->eval( this, context );
 }

src/core/expression/qgsexpression.h

@@ -379,8 +379,12 @@ class CORE_EXPORT QgsExpression
 
     /**
      * Sets the geometry calculator used for distance and area calculations in expressions.
-     * (used by $length, $area and $perimeter functions only). By default, no geometry
-     * calculator is set and all distance and area calculations are performed using simple
+     * (used by $length, $area and $perimeter functions only).
+     * If the geometry calculator is set to nullptr (default), prepare() will read variables
+     * from the expression context ("project_ellipsoid", "_project_transform_context" and
+     * "_layer_crs") to build a geometry calculator.
+     * If these variables does not exist and if setGeomCalculator() is not called,
+     * all distance and area calculations are performed using simple
      * Cartesian methods (ie no ellipsoidal calculations).
      * \param calc geometry calculator. Ownership is not transferred. Set to a nullptr to force
      * Cartesian calculations.
@@ -399,6 +403,8 @@ class CORE_EXPORT QgsExpression
 
     /**
      * Sets the desired distance units for calculations involving geomCalculator(), e.g., "$length" and "$perimeter".
+     * If distance units are set to QgsUnitTypes::DistanceUnknownUnit (default), prepare() will read
+     * variables from the expression context ("project_distance_units") to determine distance units.
      * \note distances are only converted when a geomCalculator() has been set
      * \see distanceUnits()
      * \see setAreaUnits()
@@ -417,6 +423,8 @@ class CORE_EXPORT QgsExpression
 
     /**
      * Sets the desired areal units for calculations involving geomCalculator(), e.g., "$area".
+     * If distance units are set to QgsUnitTypes::AreaUnknownUnit (default), prepare() will read
+     * variables from the expression context ("project_distance_units") to determine distance units.
      * \note areas are only converted when a geomCalculator() has been set
      * \see areaUnits()
      * \see setDistanceUnits()