Expose renderer variables to users

python/core/qgsexpressioncontext.sip

@@ -457,6 +457,10 @@ class QgsExpressionContext
     static const QString EXPR_GEOMETRY_PART_COUNT;
     //! Inbuilt variable name for geometry part number variable
     static const QString EXPR_GEOMETRY_PART_NUM;
+    //! Inbuilt variable name for cluster size variable
+    static const QString EXPR_CLUSTER_SIZE;
+    //! Inbuilt variable name for cluster color variable
+    static const QString EXPR_CLUSTER_COLOR;
 };
 
 /** \ingroup core

python/gui/symbology-ng/qgslayerpropertieswidget.sip

@@ -15,6 +15,13 @@ class QgsLayerPropertiesWidget : QgsPanelWidget
      */
     QgsExpressionContext* expressionContext() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the layer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -33,6 +40,7 @@ class QgsLayerPropertiesWidget : QgsPanelWidget
      * be kept alive for the lifetime of the properties widget.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context );
 

python/gui/symbology-ng/qgsrendererwidget.sip

@@ -29,6 +29,12 @@ class QgsRendererWidget : QgsPanelWidget
      */
     const QgsMapCanvas* mapCanvas() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the renderer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Returns the vector layer associated with the widget.
      * @note added in QGIS 2.12
      */
@@ -118,6 +124,12 @@ class QgsDataDefinedValueDialog : QDialog
      */
     const QgsVectorLayer* vectorLayer() const;
 
+    /** Sets a list of additional expression context scopes to show as available for the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
   public slots:
     void dataDefinedChanged();
 

python/gui/symbology-ng/qgssymbollayerwidget.sip

@@ -19,6 +19,13 @@ class QgsSymbolLayerWidget : QWidget
      */
     QgsExpressionContext* expressionContext() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the layer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -47,6 +54,7 @@ class QgsSymbolLayerWidget : QWidget
      * be kept alive for the lifetime of the layer widget.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context );
 

python/gui/symbology-ng/qgssymbolselectordialog.sip

@@ -16,6 +16,7 @@ class QgsSymbolSelectorWidget : QgsPanelWidget
      * @param context expression context pointer. Ownership is transferred to the dialog.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context /Transfer/ );
 
@@ -27,6 +28,13 @@ class QgsSymbolSelectorWidget : QgsPanelWidget
      */
     QgsExpressionContext* expressionContext() const;
 
+    /** Sets a list of additional expression context scopes to show as available for the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the dialog. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -106,6 +114,7 @@ class QgsSymbolSelectorDialog : QDialog
      * @param context expression context pointer. Ownership is transferred to the dialog.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context /Transfer/ );
 
@@ -117,6 +126,13 @@ class QgsSymbolSelectorDialog : QDialog
      */
     QgsExpressionContext* expressionContext() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the dialog. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas

python/gui/symbology-ng/qgssymbolslistwidget.sip

@@ -14,6 +14,12 @@ class QgsSymbolsListWidget : QWidget
      */
     QgsExpressionContext* expressionContext() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas

src/core/qgsexpression.cpp

@@ -5094,6 +5094,10 @@ void QgsExpression::initVariableHelp()
 
   gVariableHelpTexts.insert( "symbol_color", QCoreApplication::translate( "symbol_color", "Color of symbol used to render the feature." ) );
   gVariableHelpTexts.insert( "symbol_angle", QCoreApplication::translate( "symbol_angle", "Angle of symbol used to render the feature (valid for marker symbols only)." ) );
+
+  //cluster variables
+  gVariableHelpTexts.insert( "cluster_color", QCoreApplication::translate( "cluster_color", "Color of symbols within a cluster, or NULL if symbols have mixed colors." ) );
+  gVariableHelpTexts.insert( "cluster_size", QCoreApplication::translate( "cluster_size", "Number of symbols contained within a cluster." ) );
 }
 
 QString QgsExpression::variableHelpText( const QString &variableName, bool showValue, const QVariant &value )

src/core/qgsexpressioncontext.cpp

@@ -40,6 +40,8 @@ const QString QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT( "geometry_part_cou
 const QString QgsExpressionContext::EXPR_GEOMETRY_PART_NUM( "geometry_part_num" );
 const QString QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT( "geometry_point_count" );
 const QString QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM( "geometry_point_num" );
+const QString QgsExpressionContext::EXPR_CLUSTER_SIZE( "cluster_size" );
+const QString QgsExpressionContext::EXPR_CLUSTER_COLOR( "cluster_color" );
 
 //
 // QgsExpressionContextScope

src/core/qgsexpressioncontext.h

@@ -503,6 +503,10 @@ class CORE_EXPORT QgsExpressionContext
     static const QString EXPR_GEOMETRY_POINT_COUNT;
     //! Inbuilt variable name for point number variable
     static const QString EXPR_GEOMETRY_POINT_NUM;
+    //! Inbuilt variable name for cluster size variable
+    static const QString EXPR_CLUSTER_SIZE;
+    //! Inbuilt variable name for cluster color variable
+    static const QString EXPR_CLUSTER_COLOR;
 
   private:
 

src/core/symbology-ng/qgspointdistancerenderer.cpp

@@ -438,15 +438,15 @@ QgsExpressionContextScope* QgsPointDistanceRenderer::createGroupScope( const Clu
 
     if ( groupColor.isValid() )
     {
-      clusterScope->setVariable( "cluster_color", QgsSymbolLayerUtils::encodeColor( groupColor ) );
+      clusterScope->setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, QgsSymbolLayerUtils::encodeColor( groupColor ) );
     }
     else
     {
       //mixed colors
-      clusterScope->setVariable( "cluster_color", "" );
+      clusterScope->setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, "" );
     }
 
-    clusterScope->setVariable( "cluster_size", group.size() );
+    clusterScope->setVariable( QgsExpressionContext::EXPR_CLUSTER_SIZE, group.size() );
   }
   return clusterScope;
 }

src/gui/symbology-ng/qgscategorizedsymbolrendererwidget.cpp

@@ -528,6 +528,7 @@ void QgsCategorizedSymbolRendererWidget::changeSelectedSymbols()
     QgsSymbol* newSymbol = mCategorizedSymbol->clone();
     QgsSymbolSelectorDialog dlg( newSymbol, mStyle, mLayer, this );
     dlg.setMapCanvas( mMapCanvas );
+    dlg.setAdditionalExpressionContextScopes( mAdditionalScopes );
     if ( !dlg.exec() )
     {
       delete newSymbol;
@@ -550,6 +551,7 @@ void QgsCategorizedSymbolRendererWidget::changeCategorizedSymbol()
   QgsSymbol* newSymbol = mCategorizedSymbol->clone();
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( newSymbol, mStyle, mLayer, nullptr );
   dlg->setMapCanvas( mMapCanvas );
+  dlg->setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateSymbolsFromWidget() ) );
   connect( dlg, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );
@@ -594,6 +596,7 @@ void QgsCategorizedSymbolRendererWidget::changeCategorySymbol()
 
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( symbol, mStyle, mLayer, nullptr );
   dlg->setMapCanvas( mMapCanvas );
+  dlg->setAdditionalExpressionContextScopes( mAdditionalScopes );
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateSymbolsFromWidget() ) );
   connect( dlg, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );
   openPanel( dlg );

src/gui/symbology-ng/qgsgraduatedsymbolrendererwidget.cpp

@@ -830,6 +830,7 @@ void QgsGraduatedSymbolRendererWidget::changeGraduatedSymbol()
   QgsSymbol* newSymbol = mGraduatedSymbol->clone();
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( newSymbol, mStyle, mLayer, nullptr );
   dlg->setMapCanvas( mMapCanvas );
+  dlg->setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateSymbolsFromWidget() ) );
   connect( dlg, SIGNAL( accepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );
@@ -908,6 +909,7 @@ void QgsGraduatedSymbolRendererWidget::changeRangeSymbol( int rangeIdx )
   QgsSymbol* newSymbol = mRenderer->ranges()[rangeIdx].symbol()->clone();
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( newSymbol, mStyle, mLayer, nullptr );
   dlg->setMapCanvas( mMapCanvas );
+  dlg->setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateSymbolsFromWidget() ) );
   connect( dlg, SIGNAL( accepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );

src/gui/symbology-ng/qgsinvertedpolygonrendererwidget.cpp

@@ -126,6 +126,7 @@ void QgsInvertedPolygonRendererWidget::on_mRendererComboBox_currentIndexChanged(
     mEmbeddedRendererWidget.reset( m->createRendererWidget( mLayer, mStyle, const_cast<QgsFeatureRenderer*>( mRenderer->embeddedRenderer() )->clone() ) );
     connect( mEmbeddedRendererWidget.data(), SIGNAL( widgetChanged() ), this, SIGNAL( widgetChanged() ) );
     mEmbeddedRendererWidget->setMapCanvas( mMapCanvas );
+    mEmbeddedRendererWidget->setAdditionalExpressionContextScopes( mAdditionalScopes );
 
     if ( layout()->count() > 2 )
     {

src/gui/symbology-ng/qgslayerpropertieswidget.cpp

@@ -126,6 +126,15 @@ QgsLayerPropertiesWidget::QgsLayerPropertiesWidget( QgsSymbolLayer* layer, const
   mEffectWidget->setPaintEffect( mLayer->paintEffect() );
 }
 
+void QgsLayerPropertiesWidget::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mAdditionalScopes = scopes;
+
+  QgsSymbolLayerWidget* w = dynamic_cast< QgsSymbolLayerWidget* >( stackedWidget->currentWidget() );
+  if ( w )
+    w->setAdditionalExpressionContextScopes( mAdditionalScopes );
+}
+
 void QgsLayerPropertiesWidget::setMapCanvas( QgsMapCanvas *canvas )
 {
   mMapCanvas = canvas;
@@ -191,6 +200,7 @@ void QgsLayerPropertiesWidget::updateSymbolLayerWidget( QgsSymbolLayer* layer )
     if ( w )
     {
       w->setExpressionContext( mPresetExpressionContext );
+      w->setAdditionalExpressionContextScopes( mAdditionalScopes );
       if ( mMapCanvas )
         w->setMapCanvas( mMapCanvas );
       w->setSymbolLayer( layer );

src/gui/symbology-ng/qgslayerpropertieswidget.h

@@ -17,6 +17,7 @@
 #define QGSLAYERPROPERTIESWIDGET_H
 
 #include "ui_widget_layerproperties.h"
+#include "qgsexpressioncontext.h"
 
 class QgsSymbol;
 class QgsSymbolLayer;
@@ -24,7 +25,6 @@ class QgsSymbolLayerWidget;
 class QgsVectorLayer;
 class QgsMapCanvas;
 class QgsPanelWidget;
-class QgsExpressionContext;
 
 class SymbolLayerItem;
 
@@ -49,6 +49,13 @@ class GUI_EXPORT QgsLayerPropertiesWidget : public QgsPanelWidget, private Ui::L
      */
     QgsExpressionContext* expressionContext() const { return mPresetExpressionContext; }
 
+    /** Sets a list of additional expression context scopes to show as available within the layer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -74,6 +81,7 @@ class GUI_EXPORT QgsLayerPropertiesWidget : public QgsPanelWidget, private Ui::L
      * be kept alive for the lifetime of the properties widget.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context );
 
@@ -96,6 +104,7 @@ class GUI_EXPORT QgsLayerPropertiesWidget : public QgsPanelWidget, private Ui::L
 
   private:
     QgsExpressionContext* mPresetExpressionContext;
+    QList< QgsExpressionContextScope > mAdditionalScopes;
     QgsMapCanvas* mMapCanvas;
 
 };

src/gui/symbology-ng/qgspointclusterrendererwidget.cpp

@@ -135,6 +135,12 @@ void QgsPointClusterRendererWidget::on_mRendererSettingsButton_clicked()
   {
     QgsRendererWidget* w = m->createRendererWidget( mLayer, mStyle, mRenderer->embeddedRenderer()->clone() );
     w->setMapCanvas( mMapCanvas );
+    QgsExpressionContextScope scope;
+    scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, "" );
+    scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_SIZE, 0 );
+    QList< QgsExpressionContextScope > scopes = mAdditionalScopes;
+    scopes << scope;
+    w->setAdditionalExpressionContextScopes( scopes );
     connect( w, SIGNAL( widgetChanged() ), this, SLOT( updateRendererFromWidget() ) );
     w->setDockMode( this->dockMode() );
     openPanel( w );
@@ -178,6 +184,12 @@ void QgsPointClusterRendererWidget::on_mCenterSymbolPushButton_clicked()
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( markerSymbol, QgsStyle::defaultStyle(), mLayer, this );
   dlg->setDockMode( this->dockMode() );
   dlg->setMapCanvas( mMapCanvas );
+  QgsExpressionContextScope scope;
+  scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, "" );
+  scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_SIZE, 0 );
+  QList< QgsExpressionContextScope > scopes = mAdditionalScopes;
+  scopes << scope;
+  dlg->setAdditionalExpressionContextScopes( scopes );
 
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateCenterSymbolFromWidget() ) );
   connect( dlg, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );

src/gui/symbology-ng/qgspointdisplacementrendererwidget.cpp

@@ -214,6 +214,12 @@ void QgsPointDisplacementRendererWidget::on_mRendererSettingsButton_clicked()
   {
     QgsRendererWidget* w = m->createRendererWidget( mLayer, mStyle, mRenderer->embeddedRenderer()->clone() );
     w->setMapCanvas( mMapCanvas );
+    QgsExpressionContextScope scope;
+    scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, "" );
+    scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_SIZE, 0 );
+    QList< QgsExpressionContextScope > scopes = mAdditionalScopes;
+    scopes << scope;
+    w->setAdditionalExpressionContextScopes( scopes );
     connect( w, SIGNAL( widgetChanged() ), this, SLOT( updateRendererFromWidget() ) );
     openPanel( w );
   }
@@ -351,6 +357,13 @@ void QgsPointDisplacementRendererWidget::on_mCenterSymbolPushButton_clicked()
   QgsMarkerSymbol* markerSymbol = mRenderer->centerSymbol()->clone();
   QgsSymbolSelectorWidget* dlg = new QgsSymbolSelectorWidget( markerSymbol, QgsStyle::defaultStyle(), mLayer, this );
   dlg->setMapCanvas( mMapCanvas );
+
+  QgsExpressionContextScope scope;
+  scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_COLOR, "" );
+  scope.setVariable( QgsExpressionContext::EXPR_CLUSTER_SIZE, 0 );
+  QList< QgsExpressionContextScope > scopes = mAdditionalScopes;
+  scopes << scope;
+  dlg->setAdditionalExpressionContextScopes( scopes );
   connect( dlg, SIGNAL( widgetChanged() ), this, SLOT( updateCenterSymbolFromWidget() ) );
   connect( dlg, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( cleanUpSymbolSelector( QgsPanelWidget* ) ) );
   openPanel( dlg );

src/gui/symbology-ng/qgsrendererwidget.cpp

@@ -172,6 +172,7 @@ void QgsRendererWidget::changeSymbolWidth()
 
   QgsDataDefinedWidthDialog dlg( symbolList, mLayer );
   dlg.setMapCanvas( mMapCanvas );
+  dlg.setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   if ( QDialog::Accepted == dlg.exec() )
   {
@@ -200,6 +201,7 @@ void QgsRendererWidget::changeSymbolSize()
 
   QgsDataDefinedSizeDialog dlg( symbolList, mLayer );
   dlg.setMapCanvas( mMapCanvas );
+  dlg.setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   if ( QDialog::Accepted == dlg.exec() )
   {
@@ -228,6 +230,7 @@ void QgsRendererWidget::changeSymbolAngle()
 
   QgsDataDefinedRotationDialog dlg( symbolList, mLayer );
   dlg.setMapCanvas( mMapCanvas );
+  dlg.setAdditionalExpressionContextScopes( mAdditionalScopes );
 
   if ( QDialog::Accepted == dlg.exec() )
   {
@@ -269,6 +272,11 @@ const QgsMapCanvas* QgsRendererWidget::mapCanvas() const
   return mMapCanvas;
 }
 
+void QgsRendererWidget::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mAdditionalScopes = scopes;
+}
+
 void QgsRendererWidget::applyChanges()
 {
   apply();
@@ -325,6 +333,12 @@ QgsExpressionContext QgsDataDefinedValueDialog::createExpressionContext() const
   if ( vectorLayer() )
     expContext << QgsExpressionContextUtils::layerScope( vectorLayer() );
 
+  // additional scopes
+  Q_FOREACH ( const QgsExpressionContextScope& scope, mAdditionalScopes )
+  {
+    expContext.appendScope( new QgsExpressionContextScope( scope ) );
+  }
+
   return expContext;
 }
 

src/gui/symbology-ng/qgsrendererwidget.h

@@ -65,6 +65,12 @@ class GUI_EXPORT QgsRendererWidget : public QgsPanelWidget
      */
     const QgsMapCanvas* mapCanvas() const;
 
+    /** Sets a list of additional expression context scopes to show as available within the renderer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Returns the vector layer associated with the widget.
      * @note added in QGIS 2.12
      */
@@ -91,6 +97,9 @@ class GUI_EXPORT QgsRendererWidget : public QgsPanelWidget
     QAction* mPasteAction;
     QgsMapCanvas* mMapCanvas;
 
+    //! List of additional expression context scopes to add to default expression context
+    QList< QgsExpressionContextScope > mAdditionalScopes;
+
     /** Subclasses may provide the capability of changing multiple symbols at once by implementing the following two methods
       and by connecting the slot contextMenuViewCategories(const QPoint&)*/
     virtual QList<QgsSymbol*> selectedSymbols() { return QList<QgsSymbol*>(); }
@@ -121,6 +130,7 @@ class GUI_EXPORT QgsRendererWidget : public QgsPanelWidget
      */
     virtual void apply();
 
+
 };
 
 
@@ -170,6 +180,12 @@ class GUI_EXPORT QgsDataDefinedValueDialog : public QDialog, public Ui::QgsDataD
      */
     const QgsVectorLayer* vectorLayer() const { return mLayer; }
 
+    /** Sets a list of additional expression context scopes to show as available for the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes ) { mAdditionalScopes = scopes; }
+
   public slots:
     void dataDefinedChanged();
 
@@ -181,6 +197,9 @@ class GUI_EXPORT QgsDataDefinedValueDialog : public QDialog, public Ui::QgsDataD
      */
     void init( const QString& description ); // needed in children ctor to call virtual
 
+    //! List of additional scopes to add to default expression context
+    QList< QgsExpressionContextScope > mAdditionalScopes;
+
   private:
     QgsDataDefined symbolDataDefined() const;
 

src/gui/symbology-ng/qgsrulebasedrendererwidget.cpp

@@ -261,6 +261,7 @@ void QgsRuleBasedRendererWidget::refineRuleCategoriesGui()
   w->setPanelTitle( tr( "Add categories to rules" ) );
   connect( w, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( refineRuleCategoriesAccepted( QgsPanelWidget* ) ) );
   w->setMapCanvas( mMapCanvas );
+  w->setAdditionalExpressionContextScopes( mAdditionalScopes );
   openPanel( w );
 }
 
@@ -270,6 +271,7 @@ void QgsRuleBasedRendererWidget::refineRuleRangesGui()
   w->setPanelTitle( tr( "Add ranges to rules" ) );
   connect( w, SIGNAL( panelAccepted( QgsPanelWidget* ) ), this, SLOT( refineRuleRangesAccepted( QgsPanelWidget* ) ) );
   w->setMapCanvas( mMapCanvas );
+  w->setAdditionalExpressionContextScopes( mAdditionalScopes );
   openPanel( w );
 }
 

src/gui/symbology-ng/qgssymbollayerwidget.cpp

@@ -84,15 +84,28 @@ QgsExpressionContext QgsSymbolLayerWidget::createExpressionContext() const
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT, 1, true ) );
   expContext.lastScope()->addVariable( QgsExpressionContextScope::StaticVariable( QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM, 1, true ) );
 
+  // additional scopes
+  Q_FOREACH ( const QgsExpressionContextScope& scope, mAdditionalScopes )
+  {
+    expContext.appendScope( new QgsExpressionContextScope( scope ) );
+  }
+
   //TODO - show actual value
   expContext.setOriginalValueVariable( QVariant() );
+
   expContext.setHighlightedVariables( QStringList() << QgsExpressionContext::EXPR_ORIGINAL_VALUE << QgsExpressionContext::EXPR_SYMBOL_COLOR
                                       << QgsExpressionContext::EXPR_GEOMETRY_PART_COUNT << QgsExpressionContext::EXPR_GEOMETRY_PART_NUM
-                                      << QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT << QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM );
+                                      << QgsExpressionContext::EXPR_GEOMETRY_POINT_COUNT << QgsExpressionContext::EXPR_GEOMETRY_POINT_NUM
+                                      << QgsExpressionContext::EXPR_CLUSTER_COLOR << QgsExpressionContext::EXPR_CLUSTER_SIZE );
 
   return expContext;
 }
 
+void QgsSymbolLayerWidget::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mAdditionalScopes = scopes;
+}
+
 void QgsSymbolLayerWidget::setMapCanvas( QgsMapCanvas *canvas )
 {
   mMapCanvas = canvas;

src/gui/symbology-ng/qgssymbollayerwidget.h

@@ -53,6 +53,13 @@ class GUI_EXPORT QgsSymbolLayerWidget : public QWidget, protected QgsExpressionC
      */
     QgsExpressionContext* expressionContext() const { return mPresetExpressionContext; }
 
+    /** Sets a list of additional expression context scopes to show as available within the layer.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -81,6 +88,7 @@ class GUI_EXPORT QgsSymbolLayerWidget : public QWidget, protected QgsExpressionC
      * be kept alive for the lifetime of the layer widget.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context ) { mPresetExpressionContext = context; }
 
@@ -96,6 +104,7 @@ class GUI_EXPORT QgsSymbolLayerWidget : public QWidget, protected QgsExpressionC
     const QgsVectorLayer* mVectorLayer;
 
     QgsMapCanvas* mMapCanvas;
+    QList< QgsExpressionContextScope > mAdditionalScopes;
 
   signals:
     /**

src/gui/symbology-ng/qgssymbolselectordialog.cpp

@@ -286,6 +286,11 @@ void QgsSymbolSelectorWidget::setExpressionContext( QgsExpressionContext *contex
   updatePreview();
 }
 
+void QgsSymbolSelectorWidget::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mAdditionalScopes = scopes;
+}
+
 void QgsSymbolSelectorWidget::setMapCanvas( QgsMapCanvas *canvas )
 {
   mMapCanvas = canvas;
@@ -416,6 +421,7 @@ void QgsSymbolSelectorWidget::layerChanged()
     QgsLayerPropertiesWidget *layerProp = new QgsLayerPropertiesWidget( currentItem->layer(), parent->symbol(), mVectorLayer );
     layerProp->setDockMode( this->dockMode() );
     layerProp->setExpressionContext( mPresetExpressionContext.data() );
+    layerProp->setAdditionalExpressionContextScopes( mAdditionalScopes );
     layerProp->setMapCanvas( mMapCanvas );
     setWidget( layerProp );
     connect( layerProp, SIGNAL( changed() ), mDataDefineRestorer.data(), SLOT( restore() ) );
@@ -433,6 +439,7 @@ void QgsSymbolSelectorWidget::layerChanged()
     // Now populate symbols of that type using the symbols list widget:
     QgsSymbolsListWidget *symbolsList = new QgsSymbolsListWidget( currentItem->symbol(), mStyle, mAdvancedMenu, this, mVectorLayer );
     symbolsList->setExpressionContext( mPresetExpressionContext.data() );
+    symbolsList->setAdditionalExpressionContextScopes( mAdditionalScopes );
     symbolsList->setMapCanvas( mMapCanvas );
 
     setWidget( symbolsList );
@@ -711,6 +718,11 @@ void QgsSymbolSelectorDialog::setExpressionContext( QgsExpressionContext *contex
   mSelectorWidget->setExpressionContext( context );
 }
 
+void QgsSymbolSelectorDialog::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mSelectorWidget->setAdditionalExpressionContextScopes( scopes );
+}
+
 QgsExpressionContext *QgsSymbolSelectorDialog::expressionContext() const
 {
   return mSelectorWidget->expressionContext();

src/gui/symbology-ng/qgssymbolselectordialog.h

@@ -104,6 +104,7 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
      * @param context expression context pointer. Ownership is transferred to the dialog.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context );
 
@@ -115,6 +116,13 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
      */
     QgsExpressionContext* expressionContext() const { return mPresetExpressionContext.data(); }
 
+    /** Sets a list of additional expression context scopes to show as available for the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the dialog. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -245,6 +253,7 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
   private:
     QScopedPointer<DataDefinedRestorer> mDataDefineRestorer;
     QScopedPointer< QgsExpressionContext > mPresetExpressionContext;
+    QList< QgsExpressionContextScope > mAdditionalScopes;
 
     QgsMapCanvas* mMapCanvas;
 };
@@ -269,9 +278,17 @@ class GUI_EXPORT QgsSymbolSelectorDialog : public QDialog
      * @param context expression context pointer. Ownership is transferred to the dialog.
      * @note added in QGIS 2.12
      * @see expressionContext()
+     * @see setAdditionalExpressionContextScopes()
      */
     void setExpressionContext( QgsExpressionContext* context );
 
+    /** Sets a list of additional expression context scopes to show as available within the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     * @see setExpressionContext()
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Returns the expression context used for the dialog, if set. This expression context is used for
      * evaluating data defined symbol properties and for populating based expression widgets in
      * the dialog.

src/gui/symbology-ng/qgssymbolslistwidget.cpp

@@ -130,6 +130,11 @@ QgsSymbolsListWidget::~QgsSymbolsListWidget()
   btnAdvanced->menu()->removeAction( mClipFeaturesAction );
 }
 
+void QgsSymbolsListWidget::setAdditionalExpressionContextScopes( const QList<QgsExpressionContextScope>& scopes )
+{
+  mAdditionalScopes = scopes;
+}
+
 void QgsSymbolsListWidget::setMapCanvas( QgsMapCanvas* canvas )
 {
   mMapCanvas = canvas;
@@ -448,6 +453,12 @@ QgsExpressionContext QgsSymbolsListWidget::createExpressionContext() const
 
   expContext << QgsExpressionContextUtils::layerScope( layer() );
 
+  // additional scopes
+  Q_FOREACH ( const QgsExpressionContextScope& scope, mAdditionalScopes )
+  {
+    expContext.appendScope( new QgsExpressionContextScope( scope ) );
+  }
+
   return expContext;
 }
 

src/gui/symbology-ng/qgssymbolslistwidget.h

@@ -46,6 +46,12 @@ class GUI_EXPORT QgsSymbolsListWidget : public QWidget, private Ui::SymbolsListW
      */
     QgsExpressionContext* expressionContext() const { return mPresetExpressionContext; }
 
+    /** Sets a list of additional expression context scopes to show as available within the symbol.
+     * @param scopes list of additional scopes which will be added in order to the end of the default expression context
+     * @note added in QGIS 3.0
+     */
+    void setAdditionalExpressionContextScopes( const QList< QgsExpressionContextScope >& scopes );
+
     /** Sets the map canvas associated with the widget. This allows the widget to retrieve the current
      * map scale and other properties from the canvas.
      * @param canvas map canvas
@@ -120,6 +126,7 @@ class GUI_EXPORT QgsSymbolsListWidget : public QWidget, private Ui::SymbolsListW
     void populateGroups( const QString& parent = "", const QString& prepend = "" );
 
     QgsExpressionContext* mPresetExpressionContext;
+    QList< QgsExpressionContextScope > mAdditionalScopes;
 
     QgsExpressionContext createExpressionContext() const override;
 };