Lazily initialize QgsDistanceArea in expressions

nyalldawson · nyalldawson · commit e9bc48324937 · 2020-05-09T05:17:30.000+10:00
This is barely ever used (only when the $perimeter, $length or $area
functions are used in an expression), and is expensive to construct.

We've been merrily creating them EVERY time we prepare an expression
resulting in a lot of wasted effort. By lazily creating it only
if needed we save a bunch of wasted processing and ultimately get
MUCH faster expression preparation.

(especially noticable when animating vector layers using data defined
symbol settings or categorized/graduated/rule based renderers)
diff --git a/src/core/expression/qgsexpression.cpp b/src/core/expression/qgsexpression.cpp
@@ -272,15 +272,11 @@ void QgsExpression::initGeomCalculator( const QgsExpressionContext *context )
   // Set the geometry calculator from the context if it has not been set by setGeomCalculator()
   if ( context && ! d->mCalc )
   {
-    QString ellipsoid = context->variable( QStringLiteral( "project_ellipsoid" ) ).toString();
-    QgsCoordinateReferenceSystem crs = context->variable( QStringLiteral( "_layer_crs" ) ).value<QgsCoordinateReferenceSystem>();
-    QgsCoordinateTransformContext tContext = context->variable( QStringLiteral( "_project_transform_context" ) ).value<QgsCoordinateTransformContext>();
-    if ( crs.isValid() )
-    {
-      d->mCalc = std::shared_ptr<QgsDistanceArea>( new QgsDistanceArea() );
-      d->mCalc->setEllipsoid( ellipsoid.isEmpty() ? geoNone() : ellipsoid );
-      d->mCalc->setSourceCrs( crs, tContext );
-    }
+    // actually don't do it right away, cos it's expensive to create and only a very small number of expression
+    // functions actually require it. Let's lazily construct it when needed
+    d->mDaEllipsoid = context->variable( QStringLiteral( "project_ellipsoid" ) ).toString();
+    d->mDaCrs = context->variable( QStringLiteral( "_layer_crs" ) ).value<QgsCoordinateReferenceSystem>();
+    d->mDaTransformContext = context->variable( QStringLiteral( "_project_transform_context" ) ).value<QgsCoordinateTransformContext>();
   }
 
   // Set the distance units from the context if it has not been set by setDistanceUnits()
@@ -397,6 +393,14 @@ QString QgsExpression::dump() const
 
 QgsDistanceArea *QgsExpression::geomCalculator()
 {
+  if ( !d->mCalc && d->mDaCrs.isValid() )
+  {
+    // calculator IS required, so initialize it now...
+    d->mCalc = std::shared_ptr<QgsDistanceArea>( new QgsDistanceArea() );
+    d->mCalc->setEllipsoid( d->mDaEllipsoid.isEmpty() ? geoNone() : d->mDaEllipsoid );
+    d->mCalc->setSourceCrs( d->mDaCrs, d->mDaTransformContext );
+  }
+
   return d->mCalc.get();
 }
 
diff --git a/src/core/expression/qgsexpression_p.h b/src/core/expression/qgsexpression_p.h
@@ -67,6 +67,10 @@ class QgsExpressionPrivate
 
     QString mExp;
 
+    QString mDaEllipsoid;
+    QgsCoordinateReferenceSystem mDaCrs;
+    QgsCoordinateTransformContext mDaTransformContext;
+
     std::shared_ptr<QgsDistanceArea> mCalc;
     QgsUnitTypes::DistanceUnit mDistanceUnit = QgsUnitTypes::DistanceUnknownUnit;
     QgsUnitTypes::AreaUnit mAreaUnit = QgsUnitTypes::AreaUnknownUnit;
