Fix $length in labels (fixes #19355)

Use the project expression scope to access project
parameters (ellipsoid and distance/area units)

Author: Hugo Mercier

python/core/auto_generated/qgscoordinatetransformcontext.sip.in

@@ -185,6 +185,7 @@ Write the context's state to application settings.
 
 
 
+
 /************************************************************************
  * This file has been generated automatically from                      *
  *                                                                      *

src/core/expression/qgsexpression.cpp

@@ -102,6 +102,7 @@ void QgsExpression::setExpression( const QString &expression )
   d->mRootNode = ::parseExpression( expression, d->mParserErrorString, d->mParserErrors );
   d->mEvalErrorString = QString();
   d->mExp = expression;
+  d->mIsPrepared = false;
 }
 
 QString QgsExpression::expression() const
@@ -313,13 +314,25 @@ bool QgsExpression::needsGeometry() const
   return d->mRootNode->needsGeometry();
 }
 
-void QgsExpression::initGeomCalculator()
+void QgsExpression::initGeomCalculator( const QgsExpressionContext *context )
 {
-  if ( d->mCalc )
-    return;
-
-  // Use planimetric as default
-  d->mCalc = std::shared_ptr<QgsDistanceArea>( new QgsDistanceArea() );
+  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() )
+  {
+    d->mCalc->setSourceCrs( crs, tContext );
+  }
 }
 
 void QgsExpression::detach()
@@ -361,6 +374,8 @@ bool QgsExpression::prepare( const QgsExpressionContext *context )
     return false;
   }
 
+  initGeomCalculator( context );
+  d->mIsPrepared = true;
   return d->mRootNode->prepare( this, context );
 }
 
@@ -385,6 +400,12 @@ QVariant QgsExpression::evaluate( const QgsExpressionContext *context )
     return QVariant();
   }
 
+  if ( ! d->mIsPrepared )
+  {
+    qWarning( "QgsExpression::evaluate() called on an expression not yet prepared !" );
+    if ( ! prepare( context ) )
+      return QVariant();
+  }
   return d->mRootNode->eval( this, context );
 }
 

src/core/expression/qgsexpression.h

@@ -619,7 +619,7 @@ class CORE_EXPORT QgsExpression
 #endif
 
   private:
-    void initGeomCalculator();
+    void initGeomCalculator( const QgsExpressionContext *context );
 
     struct HelpArg SIP_SKIP
     {

src/core/expression/qgsexpressioncontextutils.cpp

@@ -257,6 +257,10 @@ QgsExpressionContextScope *QgsExpressionContextUtils::projectScope( const QgsPro
   QgsCoordinateReferenceSystem projectCrs = project->crs();
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs" ), projectCrs.authid(), true, true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs_definition" ), projectCrs.toProj4(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_ellipsoid" ), project->ellipsoid(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_distance_units" ), QgsUnitTypes::toString( project->distanceUnits() ), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_area_units" ), QgsUnitTypes::toString( project->areaUnits() ), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "_project_transform_context" ), QVariant::fromValue<QgsCoordinateTransformContext>( project->transformContext() ), true, true ) );
 
   // metadata
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_author" ), project->metadata().author(), true, true ) );
@@ -335,6 +339,7 @@ QgsExpressionContextScope *QgsExpressionContextUtils::layerScope( const QgsMapLa
 
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_name" ), layer->name(), true, true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_id" ), layer->id(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "_layer_crs" ), QVariant::fromValue<QgsCoordinateReferenceSystem>( layer->crs() ), true, true ) );
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer" ), QVariant::fromValue<QgsWeakMapLayerPointer >( QgsWeakMapLayerPointer( const_cast<QgsMapLayer *>( layer ) ) ), true, true ) );
 
   const QgsVectorLayer *vLayer = qobject_cast< const QgsVectorLayer * >( layer );

src/core/qgscoordinatetransformcontext.h

@@ -22,6 +22,7 @@
 #include "qgis_sip.h"
 #include "qgsdatumtransform.h"
 
+#include <QMetaType>
 #include <QExplicitlySharedDataPointer>
 class QgsCoordinateReferenceSystem;
 class QgsReadWriteContext;
@@ -275,6 +276,8 @@ class CORE_EXPORT QgsCoordinateTransformContext
 
 };
 
+Q_DECLARE_METATYPE( QgsCoordinateTransformContext )
+
 #endif // QGSCOORDINATETRANSFORMCONTEXT_H
 
 

src/core/qgsexpressionprivate.h

@@ -70,6 +70,9 @@ class QgsExpressionPrivate
     std::shared_ptr<QgsDistanceArea> mCalc;
     QgsUnitTypes::DistanceUnit mDistanceUnit = QgsUnitTypes::DistanceUnknownUnit;
     QgsUnitTypes::AreaUnit mAreaUnit = QgsUnitTypes::AreaUnknownUnit;
+
+    //! Whether prepare() has been called before evaluate()
+    bool mIsPrepared = false;
 };
 ///@endcond
 

tests/src/core/testqgslayoutitem.cpp

@@ -1421,6 +1421,7 @@ void TestQgsLayoutItem::itemVariablesFunction()
   map->setId( QStringLiteral( "Map_id" ) );
 
   c = l.createExpressionContext();
+  e.prepare( &c );
   r = e.evaluate( &c );
   QGSCOMPARENEAR( r.toDouble(), 184764103, 100 );
 

tests/src/core/testqgsmapsettings.cpp

@@ -261,12 +261,15 @@ void TestQgsMapSettings::testIsLayerVisible()
   QCOMPARE( r.toBool(), false );
 
   QgsProject::instance()->removeMapLayer( vlA );
+  e.prepare( &context );
   r = e.evaluate( &context );
   QCOMPARE( r.toBool(), false ); // layer is deleted
+  e2.prepare( &context );
   r = e2.evaluate( &context );
   QCOMPARE( r.toBool(), true ); // layer still exists
 
   QgsProject::instance()->removeMapLayer( vlB );
+  e2.prepare( &context );
   r = e2.evaluate( &context );
   QCOMPARE( r.toBool(), false ); // layer is deleted
 

tests/src/python/test_qgspallabeling_tests.py

@@ -23,7 +23,8 @@
 from qgis.PyQt.QtCore import Qt, QPointF, QSizeF
 from qgis.PyQt.QtGui import QFont
 
-from qgis.core import QgsLabelingEngineSettings, QgsPalLayerSettings, QgsUnitTypes, QgsTextBackgroundSettings
+from qgis.core import QgsLabelingEngineSettings, QgsPalLayerSettings, QgsUnitTypes, QgsTextBackgroundSettings, QgsProject, QgsExpressionContextUtils, QgsExpressionContext
+from qgis.core import QgsCoordinateReferenceSystem
 
 from utilities import svgSymbolsPath
 
@@ -308,6 +309,24 @@ def test_curved_placement_below(self):
         self.lyr.placementFlags = QgsPalLayerSettings.BelowLine | QgsPalLayerSettings.MapOrientation
         self.checkTest()
 
+    def test_length_expression(self):
+        # compare length using the ellipsoid in kms and the planimetric distance in meters
+        self.lyr.fieldName = "round($length,5) || ' - ' || round(length($geometry),2)"
+        self.lyr.isExpression = True
+
+        QgsProject.instance().setCrs(QgsCoordinateReferenceSystem("EPSG:32613"))
+        QgsProject.instance().setEllipsoid("WGS84")
+        QgsProject.instance().setDistanceUnits(QgsUnitTypes.DistanceKilometers)
+
+        ctxt = QgsExpressionContext()
+        ctxt.appendScope(QgsExpressionContextUtils.projectScope(QgsProject.instance()))
+        ctxt.appendScope(QgsExpressionContextUtils.layerScope(self.layer))
+        self._TestMapSettings.setExpressionContext(ctxt)
+
+        self.lyr.placement = QgsPalLayerSettings.Curved
+        self.lyr.placementFlags = QgsPalLayerSettings.AboveLine | QgsPalLayerSettings.MapOrientation
+        self.checkTest()
+
 # noinspection PyPep8Naming