Symbol aware legend expression (#9648)

images/images.qrc

@@ -177,6 +177,7 @@
         <file>themes/default/mActionDistributeTop.svg</file>
         <file>themes/default/mActionDistributeVCenter.svg</file>
         <file>themes/default/mActionDistributeVSpace.svg</file>
+        <file>themes/default/mActionAddExpression.svg</file>
         <file>themes/default/mActionAddLayer.svg</file>
         <file>themes/default/mActionAddMeshLayer.svg</file>
         <file>themes/default/mActionAddAllToOverview.svg</file>

images/themes/default/mActionAddExpression.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="14.7967" height="16.7515" viewBox="0 0 3.915 4.4322"><path d="M1.1438963 1.910687c-.5300386-.1812338-.7956598-.455994-.7956598-.8236459 0-.2899952.1400886-.521801.4198282-.695523C1.0504308.218325 1.4006524.1314639 1.817635.1314639c.3786771 0 .6791016.0610461.9025868.1822918.2238134.1193414.335556.263334.335556.4315545 0 .0880248-.034584.1652581-.1050665.2330753-.069606.0646433-.1507047.0971236-.2419812.0971236-.151033 0-.274705-.1011439-.3716727-.30322C2.203426.492662 2.002158.3527955 1.7336913.3527955c-.2125407 0-.3879799.0670766-.5252229.2017589-.1374621.134788-.2063025.3226872-.2063025.5635917 0 .4736625.2550052.710864.7638115.710864.053518 0 .1150259-.00529.1846324-.015658.1209359-.015235.2148391-.02317.2820378-.02317.1634003 0 .2457024.045176.2457024.135846 0 .1009323-.083506.1512927-.2501896.1512927-.05899 0-.1472025-.0091-.2652928-.02719-.088869-.015658-.1572714-.023382-.2054269-.023382-.538794 0-.807808.2630165-.807808.7903191 0 .2561396.071467.4634.2142919.6214638.1424964.155313.342451.2331811.5975655.2331811.3195773 0 .5308047-.1597566.6358711-.4782118.054065-.1681147.1130559-.2849169.177628-.3497717.067746-.064326.1565053-.097018.2671535-.097018.091495 0 .1729219.032692.2462495.097018.075516.06221.1129465.1426171.1129465.2409045 0 .2358261-.137462.431237-.4119482.587079-.2750334.1523507-.6067589.2287376-.9960521.2287376-.4274892 0-.808793-.098181-1.1420508-.2949678-.3310688-.197104-.4968768-.4602263-.4968768-.7887324 0-.4108178.3299744-.7124509.9894855-.9060632" fill="#5c3566"/><g transform="matrix(.13805 0 0 .13357 -.709128 -.0487503)"><rect y="19" x="19" width="13" ry="2.6149001" rx="2.6149001" height="13" fill="#5a8c5a"/><path d="M21.6 25.5h7.8m-3.9 3.9v-7.8" overflow="visible" fill="#fff" fill-rule="evenodd" stroke="#fff" stroke-width="2.5999999" stroke-linecap="round" stroke-linejoin="round"/><path d="M20.3 25.5h10.4v-2.6c0-2.6-.65-2.6-5.2-2.6s-5.2 0-5.2 2.6z" opacity=".3" fill="#fcffff" fill-rule="evenodd"/></g></svg>

python/core/auto_generated/layertree/qgslayertreelayer.sip.in

@@ -122,6 +122,20 @@ Also resolves textual references to layers from the project (calls resolveRefere
 Resolves reference to layer from stored layer ID (if it has not been resolved already)
 
 .. versionadded:: 3.0
+%End
+
+    void setLabelExpression( const QString &expression );
+%Docstring
+set the expression to evaluate
+
+.. versionadded:: 3.10
+%End
+
+    QString labelExpression() const;
+%Docstring
+Returns the expression member of the LayerTreeNode
+
+.. versionadded:: 3.10
 %End
 
   signals:

python/core/auto_generated/layertree/qgslayertreemodellegendnode.sip.in

@@ -12,6 +12,7 @@
 
 
 
+
 class QgsLayerTreeModelLegendNode : QObject
 {
 %Docstring
@@ -26,6 +27,12 @@ and customized look.
 
 %TypeHeaderCode
 #include "qgslayertreemodellegendnode.h"
+%End
+%ConvertToSubClassCode
+    if ( qobject_cast<QgsSymbolLegendNode *> ( sipCpp ) )
+      sipType = sipType_QgsSymbolLegendNode;
+    else
+      sipType = 0;
 %End
   public:
 
@@ -309,6 +316,23 @@ Returns text format of the label to be shown on top of the symbol.
 Sets format of text to be shown on top of the symbol.
 
 .. versionadded:: 3.2
+%End
+
+    QString symbolLabel() const;
+%Docstring
+Label of the symbol, user defined label will be used, otherwise will default to the label made by QGIS.
+
+.. versionadded:: 3.10
+%End
+
+    QString evaluateLabel( const QgsExpressionContext &context = QgsExpressionContext(), const QString &label = QString() );
+%Docstring
+Evaluates  and returns the text label of the current node
+
+:param context: extra QgsExpressionContext to use for evaluating the expression
+:param label: text to evaluate instead of the layer layertree string
+
+.. versionadded:: 3.10
 %End
 
   public slots:

python/core/auto_generated/layout/qgslayoutitemlegend.sip.in

@@ -24,9 +24,14 @@ Overrides some functionality of QgsLayerTreeModel to better fit the needs of lay
 #include "qgslayoutitemlegend.h"
 %End
   public:
-    QgsLegendModel( QgsLayerTree *rootNode, QObject *parent /TransferThis/ = 0 );
+    QgsLegendModel( QgsLayerTree *rootNode, QObject *parent /TransferThis/ = 0, QgsLayoutItemLegend *layout = 0 );
 %Docstring
 Construct the model based on the given layer tree
+%End
+
+    QgsLegendModel( QgsLayerTree *rootNode,  QgsLayoutItemLegend *layout );
+%Docstring
+Alternative constructor.
 %End
 
     virtual QVariant data( const QModelIndex &index, int role ) const;
@@ -34,6 +39,16 @@ Construct the model based on the given layer tree
 
     virtual Qt::ItemFlags flags( const QModelIndex &index ) const;
 
+
+  signals:
+
+    void refreshLegend();
+%Docstring
+Emitted to refresh the legend.
+
+.. versionadded:: 3.10
+%End
+
 };
 
 
@@ -513,7 +528,6 @@ Returns the legend's renderer settings object.
     virtual QgsExpressionContext createExpressionContext() const;
 
 
-
   public slots:
 
     virtual void refresh();

python/core/auto_generated/qgsvectorlayer.sip.in

@@ -2279,7 +2279,6 @@ Configuration and logic to apply automatically on any edit happening on this lay
 
 
 
-
   public slots:
 
     void select( QgsFeatureId featureId );
@@ -2653,6 +2652,7 @@ Emitted when the feature count for symbols on this layer has been recalculated.
 .. versionadded:: 3.0
 %End
 
+
   protected:
     virtual void setExtent( const QgsRectangle &rect ) ${SIP_FINAL};
 

python/core/auto_generated/qgsvectorlayerfeaturecounter.sip.in

@@ -29,16 +29,29 @@ QgsVectorLayer.countSymbolFeatures() and connect to the signal
 Create a new feature counter for ``layer``.
 %End
 
+
     virtual bool run();
 
+%Docstring
+Calculates the feature count and Ids per symbol
+%End
 
 
     long featureCount( const QString &legendKey ) const;
 %Docstring
-Gets the feature count for a particular ``legendKey``.
+Returns the feature count for a particular ``legendKey``.
 If the key has not been found, -1 will be returned.
 %End
 
+
+    QgsFeatureIds featureIds( const QString &symbolkey ) const;
+%Docstring
+Returns the feature Ids for a particular ``legendKey``.
+If the key has not been found an empty QSet will be returned.
+
+.. versionadded:: 3.10
+%End
+
   signals:
 
     void symbolsCounted();

src/app/layout/qgslayoutlegendwidget.cpp

@@ -38,6 +38,7 @@
 #include "qgslayoutitemlegend.h"
 #include "qgslayoutmeasurementconverter.h"
 #include "qgsunittypes.h"
+#include "qgsexpressionbuilderdialog.h"
 
 #include <QMessageBox>
 #include <QInputDialog>
@@ -104,6 +105,7 @@ QgsLayoutLegendWidget::QgsLayoutLegendWidget( QgsLayoutItemLegend *legend )
   connect( mEditPushButton, &QToolButton::clicked, this, &QgsLayoutLegendWidget::mEditPushButton_clicked );
   connect( mCountToolButton, &QToolButton::clicked, this, &QgsLayoutLegendWidget::mCountToolButton_clicked );
   connect( mExpressionFilterButton, &QgsLegendFilterButton::toggled, this, &QgsLayoutLegendWidget::mExpressionFilterButton_toggled );
+  connect( mLayerExpressionButton, &QToolButton::clicked, this, &QgsLayoutLegendWidget::mLayerExpressionButton_clicked );
   connect( mFilterByMapToolButton, &QToolButton::toggled, this, &QgsLayoutLegendWidget::mFilterByMapToolButton_toggled );
   connect( mUpdateAllPushButton, &QToolButton::clicked, this, &QgsLayoutLegendWidget::mUpdateAllPushButton_clicked );
   connect( mAddGroupToolButton, &QToolButton::clicked, this, &QgsLayoutLegendWidget::mAddGroupToolButton_clicked );
@@ -136,6 +138,7 @@ QgsLayoutLegendWidget::QgsLayoutLegendWidget( QgsLayoutItemLegend *legend )
   mMoveUpToolButton->setIcon( QIcon( QgsApplication::iconPath( "mActionArrowUp.svg" ) ) );
   mMoveDownToolButton->setIcon( QIcon( QgsApplication::iconPath( "mActionArrowDown.svg" ) ) );
   mCountToolButton->setIcon( QIcon( QgsApplication::iconPath( "mActionSum.svg" ) ) );
+  mLayerExpressionButton->setIcon( QIcon( QgsApplication::iconPath( "mActionAddExpression.svg" ) ) );
 
   mFontColorButton->setColorDialogTitle( tr( "Select Font Color" ) );
   mFontColorButton->setContext( QStringLiteral( "composer" ) );
@@ -994,6 +997,45 @@ void QgsLayoutLegendWidget::mExpressionFilterButton_toggled( bool checked )
   mLegend->endCommand();
 }
 
+void QgsLayoutLegendWidget::mLayerExpressionButton_clicked()
+{
+
+  if ( !mLegend )
+  {
+    return;
+  }
+
+  QModelIndex currentIndex = mItemTreeView->currentIndex();
+  if ( !currentIndex.isValid() )
+    return;
+
+  QgsLayerTreeNode *currentNode = mItemTreeView->currentNode();
+  if ( !QgsLayerTree::isLayer( currentNode ) )
+    return;
+
+  QgsLayerTreeLayer *layerNode = qobject_cast<QgsLayerTreeLayer *>( currentNode );
+  QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( layerNode->layer() );
+
+  if ( !vl )
+    return;
+
+  QString currentExpression;
+  if ( layerNode->labelExpression().isEmpty() )
+    currentExpression = QStringLiteral( "@symbol_label" );
+  else
+    currentExpression = layerNode->labelExpression();
+  QgsExpressionContext legendContext = mLegend->createExpressionContext();
+  legendContext.appendScope( vl->createExpressionContextScope() );
+  QgsExpressionBuilderDialog expressiondialog( vl, currentExpression, nullptr, "generic", legendContext );
+  if ( expressiondialog.exec() )
+    layerNode->setLabelExpression( expressiondialog.expressionText() );
+
+  mLegend->beginCommand( tr( "Update Legend" ) );
+  mLegend->updateLegend();
+  mLegend->adjustBoxSize();
+  mLegend->endCommand();
+}
+
 void QgsLayoutLegendWidget::mUpdateAllPushButton_clicked()
 {
   updateLegend();
@@ -1105,12 +1147,27 @@ void QgsLayoutLegendWidget::selectedChanged( const QModelIndex &current, const Q
   Q_UNUSED( current )
   Q_UNUSED( previous )
 
+  mLayerExpressionButton->setEnabled( false );
+
   if ( mLegend && mLegend->autoUpdateModel() )
+  {
+    QgsLayerTreeNode *currentNode = mItemTreeView->currentNode();
+    if ( !QgsLayerTree::isLayer( currentNode ) )
+      return;
+
+    QgsLayerTreeLayer *currentLayerNode = QgsLayerTree::toLayer( currentNode );
+    QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( currentLayerNode->layer() );
+    if ( !vl )
+      return;
+
+    mLayerExpressionButton->setEnabled( true );
     return;
+  }
 
   mCountToolButton->setChecked( false );
   mCountToolButton->setEnabled( false );
 
+
   mExpressionFilterButton->blockSignals( true );
   mExpressionFilterButton->setChecked( false );
   mExpressionFilterButton->setEnabled( false );
@@ -1127,6 +1184,7 @@ void QgsLayoutLegendWidget::selectedChanged( const QModelIndex &current, const Q
 
   mCountToolButton->setChecked( currentNode->customProperty( QStringLiteral( "showFeatureCount" ), 0 ).toInt() );
   mCountToolButton->setEnabled( true );
+  mLayerExpressionButton->setEnabled( true );
 
   bool exprEnabled;
   QString expr = QgsLayerTreeUtils::legendFilterByExpression( *qobject_cast<QgsLayerTreeLayer *>( currentNode ), &exprEnabled );

src/app/layout/qgslayoutlegendwidget.h

@@ -85,6 +85,7 @@ class QgsLayoutLegendWidget: public QgsLayoutItemBaseWidget, private Ui::QgsLayo
     void resetLayerNodeToDefaults();
     void mUpdateAllPushButton_clicked();
     void mAddGroupToolButton_clicked();
+    void mLayerExpressionButton_clicked();
 
     void mFilterLegendByAtlasCheckBox_toggled( bool checked );
 

src/core/expression/qgsexpression.cpp

@@ -830,6 +830,10 @@ void QgsExpression::initVariableHelp()
   sVariableHelpTexts.insert( QStringLiteral( "symbol_color" ), QCoreApplication::translate( "symbol_color", "Color of symbol used to render the feature." ) );
   sVariableHelpTexts.insert( QStringLiteral( "symbol_angle" ), QCoreApplication::translate( "symbol_angle", "Angle of symbol used to render the feature (valid for marker symbols only)." ) );
 
+  sVariableHelpTexts.insert( QStringLiteral( "symbol_label" ), QCoreApplication::translate( "symbol_label", "Label of the symbol, user defined label will be used, otherwise will default to the label made by QGIS." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "symbol_id" ), QCoreApplication::translate( "symbol_id", "Id of the symbol." ) );
+  sVariableHelpTexts.insert( QStringLiteral( "symbol_count" ), QCoreApplication::translate( "symbol_count", "Total number of features defined by this symbol." ) );
+
   //cluster variables
   sVariableHelpTexts.insert( QStringLiteral( "cluster_color" ), QCoreApplication::translate( "cluster_color", "Color of symbols within a cluster, or NULL if symbols have mixed colors." ) );
   sVariableHelpTexts.insert( QStringLiteral( "cluster_size" ), QCoreApplication::translate( "cluster_size", "Number of symbols contained within a cluster." ) );

src/core/layertree/qgslayertreelayer.cpp

@@ -188,3 +188,9 @@ void QgsLayerTreeLayer::layerNameChanged()
   Q_ASSERT( mRef );
   emit nameChanged( this, mRef->name() );
 }
+
+void QgsLayerTreeLayer::setLabelExpression( const QString &expression )
+{
+  mLabelExpression = expression;
+}
+

src/core/layertree/qgslayertreelayer.h

@@ -128,6 +128,20 @@ class CORE_EXPORT QgsLayerTreeLayer : public QgsLayerTreeNode
      */
     void resolveReferences( const QgsProject *project, bool looseMatching = false ) override;
 
+    /**
+     * set the expression to evaluate
+     *
+     * \since QGIS 3.10
+     */
+    void setLabelExpression( const QString &expression );
+
+    /**
+     * Returns the expression member of the LayerTreeNode
+     *
+     * \since QGIS 3.10
+     */
+    QString labelExpression() const { return mLabelExpression; }
+
   signals:
 
     /**
@@ -148,6 +162,8 @@ class CORE_EXPORT QgsLayerTreeLayer : public QgsLayerTreeNode
     QgsMapLayerRef mRef;
     //! Layer name - only used if layer does not exist or if mUseLayerName is false
     QString mLayerName;
+    //! Expression to evaluate in the legend
+    QString mLabelExpression;
 
     //!
     bool mUseLayerName = true;

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -29,6 +29,11 @@
 #include "qgsvectorlayer.h"
 #include "qgsrasterrenderer.h"
 #include "qgsexpressioncontextutils.h"
+#include "qgsfeatureid.h"
+#include "qgslayoutitem.h"
+#include "qgsvectorlayerfeaturecounter.h"
+#include "qgsexpression.h"
+
 
 QgsLayerTreeModelLegendNode::QgsLayerTreeModelLegendNode( QgsLayerTreeLayer *nodeL, QObject *parent )
   : QObject( parent )
@@ -284,6 +289,20 @@ const QgsSymbol *QgsSymbolLegendNode::symbol() const
   return mItem.symbol();
 }
 
+QString QgsSymbolLegendNode::symbolLabel() const
+{
+  QString label;
+  if ( mEmbeddedInParent )
+  {
+    QVariant legendlabel = mLayerNode->customProperty( QStringLiteral( "legend/title-label" ) );
+    QString layerName = legendlabel.isNull() ? mLayerNode->name() : legendlabel.toString();
+    label = mUserLabel.isEmpty() ? layerName : mUserLabel;
+  }
+  else
+    label = mUserLabel.isEmpty() ? mItem.label() : mUserLabel;
+  return label;
+}
+
 void QgsSymbolLegendNode::setSymbol( QgsSymbol *symbol )
 {
   if ( !symbol )
@@ -648,31 +667,68 @@ void QgsSymbolLegendNode::updateLabel()
 
   bool showFeatureCount = mLayerNode->customProperty( QStringLiteral( "showFeatureCount" ), 0 ).toBool();
   QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( mLayerNode->layer() );
+  mLabel = symbolLabel();
 
-  if ( mEmbeddedInParent )
+  if ( showFeatureCount && vl )
   {
-    QString layerName = mLayerNode->name();
-    if ( !mLayerNode->customProperty( QStringLiteral( "legend/title-label" ) ).isNull() )
-      layerName = mLayerNode->customProperty( QStringLiteral( "legend/title-label" ) ).toString();
-
-    mLabel = mUserLabel.isEmpty() ? layerName : mUserLabel;
-    if ( showFeatureCount && vl && vl->featureCount() >= 0 )
-      mLabel += QStringLiteral( " [%1]" ).arg( vl->featureCount() );
-  }
-  else
-  {
-    mLabel = mUserLabel.isEmpty() ? mItem.label() : mUserLabel;
-    if ( showFeatureCount && vl )
-    {
-      qlonglong count = vl->featureCount( mItem.ruleKey() );
-      mLabel += QStringLiteral( " [%1]" ).arg( count != -1 ? QLocale().toString( count ) : tr( "N/A" ) );
-    }
+    qlonglong count = mEmbeddedInParent ? vl->featureCount() : vl->featureCount( mItem.ruleKey() ) ;
+    mLabel += QStringLiteral( " [%1]" ).arg( count != -1 ? QLocale().toString( count ) : tr( "N/A" ) );
   }
 
   emit dataChanged();
 }
 
+QString QgsSymbolLegendNode::evaluateLabel( const QgsExpressionContext &context, const QString &label )
+{
+  if ( !mLayerNode )
+    return QString();
 
+  QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( mLayerNode->layer() );
+
+  if ( vl )
+  {
+    QgsExpressionContext contextCopy = QgsExpressionContext( context );
+    QgsExpressionContextScope *symbolScope = createSymbolScope();
+    contextCopy.appendScope( symbolScope );
+    contextCopy.appendScope( vl->createExpressionContextScope() );
+
+    if ( label.isEmpty() )
+    {
+      if ( ! mLayerNode->labelExpression().isEmpty() )
+        mLabel = QgsExpression::replaceExpressionText( "[%" + mLayerNode->labelExpression() + "%]", &contextCopy );
+      else if ( mLabel.contains( "[%" ) )
+      {
+        const QString symLabel = symbolLabel();
+        mLabel = QgsExpression::replaceExpressionText( symLabel, &contextCopy );
+      }
+      return mLabel;
+    }
+    else
+    {
+      QString eLabel;
+      if ( ! mLayerNode->labelExpression().isEmpty() )
+        eLabel = QgsExpression::replaceExpressionText( label + "[%" + mLayerNode->labelExpression() + "%]", &contextCopy );
+      else if ( label.contains( "[%" ) )
+        eLabel = QgsExpression::replaceExpressionText( label, &contextCopy );
+      return eLabel;
+    }
+  }
+  return mLabel;
+}
+
+QgsExpressionContextScope *QgsSymbolLegendNode::createSymbolScope() const
+{
+  QgsVectorLayer *vl = qobject_cast<QgsVectorLayer *>( mLayerNode->layer() );
+
+  QgsExpressionContextScope *scope = new QgsExpressionContextScope( tr( "Symbol scope" ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "symbol_label" ), symbolLabel().remove( "[%" ).remove( "%]" ), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "symbol_id" ), mItem.ruleKey(), true ) );
+  if ( vl )
+  {
+    scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "symbol_count" ), QVariant::fromValue( vl->featureCount( mItem.ruleKey() ) ), true ) );
+  }
+  return scope;
+}
 
 // -------------------------------------------------------------------------
 

src/core/layertree/qgslayertreemodellegendnode.h

@@ -27,6 +27,7 @@
 #include "qgis_sip.h"
 
 #include "qgsrasterdataprovider.h" // for QgsImageFetcher dtor visibility
+#include "qgsexpressioncontext.h"
 
 class QgsLayerTreeLayer;
 class QgsLayerTreeModel;
@@ -48,6 +49,14 @@ class QgsRenderContext;
 class CORE_EXPORT QgsLayerTreeModelLegendNode : public QObject
 {
     Q_OBJECT
+#ifdef SIP_RUN
+    SIP_CONVERT_TO_SUBCLASS_CODE
+    if ( qobject_cast<QgsSymbolLegendNode *> ( sipCpp ) )
+      sipType = sipType_QgsSymbolLegendNode;
+    else
+      sipType = 0;
+    SIP_END
+#endif
   public:
 
     enum LegendNodeRoles
@@ -228,6 +237,7 @@ class CORE_EXPORT QgsSymbolLegendNode : public QgsLayerTreeModelLegendNode
 {
     Q_OBJECT
 
+
   public:
 
     /**
@@ -319,6 +329,20 @@ class CORE_EXPORT QgsSymbolLegendNode : public QgsLayerTreeModelLegendNode
      */
     void setTextOnSymbolTextFormat( const QgsTextFormat &format ) { mTextOnSymbolTextFormat = format; }
 
+    /**
+     * Label of the symbol, user defined label will be used, otherwise will default to the label made by QGIS.
+     * \since QGIS 3.10
+     */
+    QString symbolLabel() const;
+
+    /**
+     * Evaluates  and returns the text label of the current node
+     * \param context extra QgsExpressionContext to use for evaluating the expression
+     * \param label text to evaluate instead of the layer layertree string
+     * \since QGIS 3.10
+     */
+    QString evaluateLabel( const QgsExpressionContext &context = QgsExpressionContext(), const QString &label = QString() );
+
   public slots:
 
     /**
@@ -361,6 +385,12 @@ class CORE_EXPORT QgsSymbolLegendNode : public QgsLayerTreeModelLegendNode
     // ident the symbol icon to make it look like a tree structure
     static const int INDENT_SIZE = 20;
 
+    /**
+     * Create an expressionContextScope containing symbol related variables
+     * \since QGIS 3.10
+     */
+    QgsExpressionContextScope *createSymbolScope() const SIP_FACTORY;
+
     /**
      * Sets all items belonging to the same layer as this node to the same check state.
      * \param state check state

src/core/layout/qgslayoutitemlegend.cpp

@@ -34,10 +34,11 @@
 #include <QDomDocument>
 #include <QDomElement>
 #include <QPainter>
+#include "qgsexpressioncontext.h"
 
 QgsLayoutItemLegend::QgsLayoutItemLegend( QgsLayout *layout )
   : QgsLayoutItem( layout )
-  , mLegendModel( new QgsLegendModel( layout->project()->layerTreeRoot() ) )
+  , mLegendModel( new QgsLegendModel( layout->project()->layerTreeRoot(), this ) )
 {
 #if 0 //no longer required?
   connect( &layout->atlasComposition(), &QgsAtlasComposition::renderEnded, this, &QgsLayoutItemLegend::onAtlasEnded );
@@ -55,6 +56,7 @@ QgsLayoutItemLegend::QgsLayoutItemLegend( QgsLayout *layout )
     invalidateCache();
     update();
   } );
+  connect( mLegendModel.get(), &QgsLegendModel::refreshLegend, this, &QgsLayoutItemLegend::refresh );
 }
 
 QgsLayoutItemLegend *QgsLayoutItemLegend::create( QgsLayout *layout )
@@ -225,6 +227,7 @@ void QgsLayoutItemLegend::setCustomLayerTree( QgsLayerTree *rootGroup )
   mCustomLayerTree.reset( rootGroup );
 }
 
+
 void QgsLayoutItemLegend::setAutoUpdateModel( bool autoUpdate )
 {
   if ( autoUpdate == autoUpdateModel() )
@@ -697,6 +700,7 @@ void QgsLayoutItemLegend::setLinkedMap( QgsLayoutItemMap *map )
   }
 
   updateFilterByMap();
+
 }
 
 void QgsLayoutItemLegend::invalidateCurrentMap()
@@ -760,9 +764,8 @@ void QgsLayoutItemLegend::mapLayerStyleOverridesChanged()
   else
   {
     mLegendModel->setLayerStyleOverrides( mMap->layerStyleOverrides() );
-
-    const auto constFindLayers = mLegendModel->rootGroup()->findLayers();
-    for ( QgsLayerTreeLayer *nodeLayer : constFindLayers )
+    const QList< QgsLayerTreeLayer * > layers =  mLegendModel->rootGroup()->findLayers();
+    for ( QgsLayerTreeLayer *nodeLayer : layers )
       mLegendModel->refreshLayerLegend( nodeLayer );
   }
 
@@ -846,11 +849,8 @@ QgsExpressionContext QgsLayoutItemLegend::createExpressionContext() const
   // We only want the last scope from the map's expression context, as this contains
   // the map specific variables. We don't want the rest of the map's context, because that
   // will contain duplicate global, project, layout, etc scopes.
-
   if ( mMap )
-  {
     context.appendScope( mMap->createExpressionContext().popScope() );
-  }
 
   QgsExpressionContextScope *scope = new QgsExpressionContextScope( tr( "Legend Settings" ) );
 
@@ -862,16 +862,26 @@ QgsExpressionContext QgsLayoutItemLegend::createExpressionContext() const
   scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "legend_filter_out_atlas" ), legendFilterOutAtlas(), true ) );
 
   context.appendScope( scope );
-
   return context;
 }
 
+
 // -------------------------------------------------------------------------
 #include "qgslayertreemodellegendnode.h"
 #include "qgsvectorlayer.h"
+#include "qgsmaplayerlegend.h"
 
-QgsLegendModel::QgsLegendModel( QgsLayerTree *rootNode, QObject *parent )
+QgsLegendModel::QgsLegendModel( QgsLayerTree *rootNode, QObject *parent, QgsLayoutItemLegend *layout )
   : QgsLayerTreeModel( rootNode, parent )
+  , mLayoutLegend( layout )
+{
+  setFlag( QgsLayerTreeModel::AllowLegendChangeState, false );
+  setFlag( QgsLayerTreeModel::AllowNodeReorder, true );
+}
+
+QgsLegendModel::QgsLegendModel( QgsLayerTree *rootNode,  QgsLayoutItemLegend *layout )
+  : QgsLayerTreeModel( rootNode )
+  , mLayoutLegend( layout )
 {
   setFlag( QgsLayerTreeModel::AllowLegendChangeState, false );
   setFlag( QgsLayerTreeModel::AllowNodeReorder, true );
@@ -880,22 +890,62 @@ QgsLegendModel::QgsLegendModel( QgsLayerTree *rootNode, QObject *parent )
 QVariant QgsLegendModel::data( const QModelIndex &index, int role ) const
 {
   // handle custom layer node labels
-  if ( QgsLayerTreeNode *node = index2node( index ) )
-  {
-    if ( QgsLayerTree::isLayer( node ) && ( role == Qt::DisplayRole || role == Qt::EditRole ) && !node->customProperty( QStringLiteral( "legend/title-label" ) ).isNull() )
-    {
-      QgsLayerTreeLayer *nodeLayer = QgsLayerTree::toLayer( node );
-      QString name = node->customProperty( QStringLiteral( "legend/title-label" ) ).toString();
-      if ( nodeLayer->customProperty( QStringLiteral( "showFeatureCount" ), 0 ).toInt() && role == Qt::DisplayRole )
-      {
-        QgsVectorLayer *vlayer = qobject_cast<QgsVectorLayer *>( nodeLayer->layer() );
-        if ( vlayer && vlayer->featureCount() >= 0 )
-          name += QStringLiteral( " [%1]" ).arg( vlayer->featureCount() );
-      }
-      return name;
-    }
-  }
 
+  QgsLayerTreeNode *node = index2node( index );
+  QgsLayerTreeLayer *nodeLayer = QgsLayerTree::isLayer( node ) ? QgsLayerTree::toLayer( node ) : nullptr;
+  if ( nodeLayer && ( role == Qt::DisplayRole || role == Qt::EditRole ) )
+  {
+    QString name;
+    QgsVectorLayer *vlayer = qobject_cast<QgsVectorLayer *>( nodeLayer->layer() );
+
+    //finding the first label that is stored
+    name = nodeLayer->customProperty( QStringLiteral( "legend/title-label" ) ).toString();
+    if ( name.isEmpty() )
+      name = nodeLayer->name();
+    if ( name.isEmpty() )
+      name = node->customProperty( QStringLiteral( "legend/title-label" ) ).toString();
+    if ( name.isEmpty() )
+      name = node->name();
+    if ( nodeLayer->customProperty( QStringLiteral( "showFeatureCount" ), 0 ).toInt() )
+    {
+      if ( vlayer && vlayer->featureCount() >= 0 )
+      {
+        name += QStringLiteral( " [%1]" ).arg( vlayer->featureCount() );
+        return name;
+      }
+    }
+
+    bool evaluate = vlayer ? !nodeLayer->labelExpression().isEmpty() : false;
+
+    if ( evaluate || name.contains( "[%" ) )
+    {
+      QgsExpressionContext expressionContext;
+      if ( vlayer )
+      {
+        connect( vlayer, &QgsVectorLayer::symbolFeatureCountMapChanged, this, &QgsLegendModel::forceRefresh, Qt::UniqueConnection );
+        // counting is done here to ensure that a valid vector layer needs to be evaluated, count is used to validate previous count or update the count if invalidated
+        vlayer->countSymbolFeatures();
+      }
+
+      if ( mLayoutLegend )
+        expressionContext = mLayoutLegend->createExpressionContext();
+      else
+        expressionContext = QgsExpressionContext();
+
+      const QList<QgsLayerTreeModelLegendNode *> legendnodes = layerLegendNodes( nodeLayer, false );
+      if ( legendnodes.count() > 1 ) // evaluate all existing legend nodes but leave the name for the legend evaluator
+      {
+        for ( QgsLayerTreeModelLegendNode *treenode : legendnodes )
+        {
+          if ( QgsSymbolLegendNode *symnode = qobject_cast<QgsSymbolLegendNode *>( treenode ) )
+            symnode->evaluateLabel( expressionContext );
+        }
+      }
+      else if ( QgsSymbolLegendNode *symnode = qobject_cast<QgsSymbolLegendNode *>( legendnodes.first() ) )
+        name = symnode->evaluateLabel( expressionContext, name );
+    }
+    return name;
+  }
   return QgsLayerTreeModel::data( index, role );
 }
 
@@ -907,3 +957,22 @@ Qt::ItemFlags QgsLegendModel::flags( const QModelIndex &index ) const
 
   return QgsLayerTreeModel::flags( index );
 }
+
+QList<QgsLayerTreeModelLegendNode *> QgsLegendModel::layerLegendNodes( QgsLayerTreeLayer *nodeLayer, bool skipNodeEmbeddedInParent ) const
+{
+  if ( !mLegend.contains( nodeLayer ) )
+    return QList<QgsLayerTreeModelLegendNode *>();
+
+  const LayerLegendData &data = mLegend[nodeLayer];
+  QList<QgsLayerTreeModelLegendNode *> lst( data.activeNodes );
+  if ( !skipNodeEmbeddedInParent && data.embeddedNodeInParent )
+    lst.prepend( data.embeddedNodeInParent );
+  return lst;
+}
+
+void QgsLegendModel::forceRefresh()
+{
+  emit refreshLegend();
+}
+
+

src/core/layout/qgslayoutitemlegend.h

@@ -24,11 +24,13 @@
 #include "qgslayertreemodel.h"
 #include "qgslegendsettings.h"
 #include "qgslayertreegroup.h"
+#include "qgsexpressioncontext.h"
 
 class QgsLayerTreeModel;
 class QgsSymbol;
 class QgsLayoutItemMap;
 class QgsLegendRenderer;
+class QgsLayoutItemLegend;
 
 /**
  * \ingroup core
@@ -44,11 +46,48 @@ class CORE_EXPORT QgsLegendModel : public QgsLayerTreeModel
 
   public:
     //! Construct the model based on the given layer tree
-    QgsLegendModel( QgsLayerTree *rootNode, QObject *parent SIP_TRANSFERTHIS = nullptr );
+    QgsLegendModel( QgsLayerTree *rootNode, QObject *parent SIP_TRANSFERTHIS = nullptr, QgsLayoutItemLegend *layout = nullptr );
+
+    //! Alternative constructor.
+    QgsLegendModel( QgsLayerTree *rootNode,  QgsLayoutItemLegend *layout );
 
     QVariant data( const QModelIndex &index, int role ) const override;
 
     Qt::ItemFlags flags( const QModelIndex &index ) const override;
+
+  signals:
+
+    /**
+     * Emitted to refresh the legend.
+     * \since QGIS 3.10
+     */
+    void refreshLegend();
+
+  private slots:
+
+    /**
+     * Handle incoming signal to refresh the legend.
+     * \since QGIS 3.10
+     */
+    void forceRefresh();
+
+  private:
+
+    /**
+     * Returns filtered list of active legend nodes attached to a particular layer node
+     * (by default it returns also legend node embedded in parent layer node (if any) unless skipNodeEmbeddedInParent is true)
+     * \note Parameter skipNodeEmbeddedInParent added in QGIS 2.18
+     * \see layerOriginalLegendNodes()
+     * \since QGIS 3.10
+     */
+    QList<QgsLayerTreeModelLegendNode *> layerLegendNodes( QgsLayerTreeLayer *nodeLayer, bool skipNodeEmbeddedInParent = false ) const;
+
+    /**
+     * Pointer to the QgsLayoutItemLegend class that made the model.
+     * \since QGIS 3.10
+     */
+    QgsLayoutItemLegend *mLayoutLegend = nullptr;
+
 };
 
 
@@ -458,7 +497,6 @@ class CORE_EXPORT QgsLayoutItemLegend : public QgsLayoutItem
 
     QgsExpressionContext createExpressionContext() const override;
 
-
   public slots:
 
     void refresh() override;
@@ -486,6 +524,7 @@ class CORE_EXPORT QgsLayoutItemLegend : public QgsLayoutItem
 
     void nodeCustomPropertyChanged( QgsLayerTreeNode *node, const QString &key );
 
+
   private:
     QgsLayoutItemLegend() = delete;
 

src/core/layout/qgslayoutitemmap.cpp

@@ -2243,3 +2243,4 @@ QgsRectangle QgsLayoutItemMap::computeAtlasRectangle()
     return g.boundingBox();
   }
 }
+

src/core/qgsvectordataprovider.h

@@ -32,6 +32,7 @@ class QTextCodec;
 #include "qgsrelation.h"
 #include "qgsfeaturesink.h"
 #include "qgsfeaturesource.h"
+#include "qgsfeaturerequest.h"
 
 typedef QList<int> QgsAttributeList SIP_SKIP;
 typedef QSet<int> QgsAttributeIds SIP_SKIP;
@@ -43,7 +44,6 @@ class QgsFeedback;
 class QgsFeatureRenderer;
 class QgsAbstractVectorLayerLabeling;
 
-#include "qgsfeaturerequest.h"
 
 /**
  * \ingroup core

src/core/qgsvectorlayer.h

@@ -27,7 +27,7 @@
 #include <QFont>
 #include <QMutex>
 
-#include "qgis_sip.h"
+#include "qgis.h"
 #include "qgsmaplayer.h"
 #include "qgsfeature.h"
 #include "qgsfeaturerequest.h"
@@ -41,6 +41,7 @@
 #include "qgsfeatureiterator.h"
 #include "qgsexpressioncontextgenerator.h"
 #include "qgsexpressioncontextscopegenerator.h"
+#include "qgsexpressioncontext.h"
 
 class QPainter;
 class QImage;

src/core/qgsvectorlayerfeaturecounter.cpp

@@ -15,6 +15,7 @@
 
 #include "qgsvectorlayerfeaturecounter.h"
 #include "qgsvectorlayer.h"
+#include "qgsfeatureid.h"
 
 QgsVectorLayerFeatureCounter::QgsVectorLayerFeatureCounter( QgsVectorLayer *layer, const QgsExpressionContext &context )
   : QgsTask( tr( "Counting features in %1" ).arg( layer->name() ), QgsTask::CanCancel )
@@ -31,12 +32,15 @@ QgsVectorLayerFeatureCounter::QgsVectorLayerFeatureCounter( QgsVectorLayer *laye
 
 bool QgsVectorLayerFeatureCounter::run()
 {
+  mSymbolFeatureCountMap.clear();
+  mSymbolFeatureIdMap.clear();
   QgsLegendSymbolList symbolList = mRenderer->legendSymbolItems();
   QgsLegendSymbolList::const_iterator symbolIt = symbolList.constBegin();
 
   for ( ; symbolIt != symbolList.constEnd(); ++symbolIt )
   {
     mSymbolFeatureCountMap.insert( symbolIt->label(), 0 );
+    mSymbolFeatureIdMap.insert( symbolIt->label(), QgsFeatureIds() );
   }
 
   // If there are no features to be counted, we can spare us the trouble
@@ -65,11 +69,12 @@ bool QgsVectorLayerFeatureCounter::run()
     while ( fit.nextFeature( f ) )
     {
       renderContext.expressionContext().setFeature( f );
-      QSet<QString> featureKeyList = mRenderer->legendKeysForFeature( f, renderContext );
-      const auto constFeatureKeyList = featureKeyList;
-      for ( const QString &key : constFeatureKeyList )
+
+      const QSet<QString> featureKeyList = mRenderer->legendKeysForFeature( f, renderContext );
+      for ( const QString &key : featureKeyList )
       {
         mSymbolFeatureCountMap[key] += 1;
+        mSymbolFeatureIdMap[key].insert( f.id() );
       }
       ++featuresCounted;
 
@@ -88,9 +93,7 @@ bool QgsVectorLayerFeatureCounter::run()
     }
     mRenderer->stopRender( renderContext );
   }
-
   setProgress( 100 );
-
   emit symbolsCounted();
   return true;
 }
@@ -104,3 +107,13 @@ long QgsVectorLayerFeatureCounter::featureCount( const QString &legendKey ) cons
 {
   return mSymbolFeatureCountMap.value( legendKey, -1 );
 }
+
+QHash<QString, QgsFeatureIds> QgsVectorLayerFeatureCounter::symbolFeatureIdMap() const
+{
+  return mSymbolFeatureIdMap;
+}
+
+QgsFeatureIds QgsVectorLayerFeatureCounter::featureIds( const QString &symbolkey ) const
+{
+  return mSymbolFeatureIdMap.value( symbolkey, QgsFeatureIds() );
+}

src/core/qgsvectorlayerfeaturecounter.h

@@ -18,6 +18,7 @@
 #include "qgsvectorlayerfeatureiterator.h"
 #include "qgsrenderer.h"
 #include "qgstaskmanager.h"
+#include "qgsfeatureid.h"
 
 /**
  * \ingroup core
@@ -40,10 +41,14 @@ class CORE_EXPORT QgsVectorLayerFeatureCounter : public QgsTask
      */
     QgsVectorLayerFeatureCounter( QgsVectorLayer *layer, const QgsExpressionContext &context = QgsExpressionContext() );
 
+
+    /**
+     * Calculates the feature count and Ids per symbol
+     */
     bool run() override;
 
     /**
-     * Gets the count for each symbol. Only valid after the symbolsCounted()
+     * Returns the count for each symbol. Only valid after the symbolsCounted()
      * signal has been emitted.
      *
      * \note Not available in Python bindings.
@@ -51,11 +56,29 @@ class CORE_EXPORT QgsVectorLayerFeatureCounter : public QgsTask
     QHash<QString, long> symbolFeatureCountMap() const SIP_SKIP;
 
     /**
-     * Gets the feature count for a particular \a legendKey.
+     * Returns the feature count for a particular \a legendKey.
      * If the key has not been found, -1 will be returned.
      */
     long featureCount( const QString &legendKey ) const;
 
+    /**
+     * Returns the QgsFeatureIds for each symbol. Only valid after the symbolsCounted()
+     * signal has been emitted.
+     *
+     * \see symbolFeatureCountMap
+     * \note Not available in Python bindings.
+     * \since QGIS 3.10
+     */
+    QHash<QString, QgsFeatureIds> symbolFeatureIdMap() const SIP_SKIP;
+
+    /**
+     * Returns the feature Ids for a particular \a legendKey.
+     * If the key has not been found an empty QSet will be returned.
+     *
+     * \since QGIS 3.10
+     */
+    QgsFeatureIds featureIds( const QString &symbolkey ) const;
+
   signals:
 
     /**
@@ -68,6 +91,7 @@ class CORE_EXPORT QgsVectorLayerFeatureCounter : public QgsTask
     std::unique_ptr<QgsFeatureRenderer> mRenderer;
     QgsExpressionContext mExpressionContext;
     QHash<QString, long> mSymbolFeatureCountMap;
+    QHash<QString, QgsFeatureIds> mSymbolFeatureIdMap;
     int mFeatureCount;
 
 };

src/ui/layout/qgslayoutlegendwidgetbase.ui

@@ -401,6 +401,23 @@
               </property>
              </widget>
             </item>
+            <item>
+             <widget class="QToolButton" name="mLayerExpressionButton">
+              <property name="toolTip">
+               <string> Add an expression to the vector layer and each child symbol's label</string>
+              </property>
+              <property name="icon">
+               <iconset resource="../../../images/images.qrc">
+                <normaloff>:/images/themes/default/mActionAddExpression.svg</normaloff>:/images/themes/default/mActionAddExpression.svg</iconset>
+              </property>
+              <property name="iconSize">
+               <size>
+                <width>20</width>
+                <height>20</height>
+               </size>
+              </property>
+             </widget>
+            </item>
             <item>
              <spacer name="horizontalSpacer_4">
               <property name="orientation">

tests/src/python/test_qgslayoutlegend.py

@@ -32,14 +32,15 @@ from qgis.core import (QgsPrintLayout,
                        QgsExpression,
                        QgsMapLayerLegendUtils,
                        QgsLegendStyle,
-                       QgsFontUtils)
+                       QgsFontUtils,
+                       QgsApplication)
 from qgis.testing import (start_app,
                           unittest
                           )
 from utilities import unitTestDataPath
 from qgslayoutchecker import QgsLayoutChecker
 import os
-
+from time import sleep
 from test_qgslayoutitem import LayoutItemTestCase
 
 start_app()
@@ -66,6 +67,7 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
         point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
         point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
+        QgsProject.instance().clear()
         QgsProject.instance().addMapLayers([point_layer])
 
         marker_symbol = QgsMarkerSymbol.createSimple({'color': '#ff0000', 'outline_style': 'no', 'size': '5', 'size_unit': 'MapUnit'})
@@ -123,7 +125,6 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
     def testResizeWithMapContent(self):
         """Test test legend resizes to match map content"""
-
         point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
         point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
         QgsProject.instance().addMapLayers([point_layer])
@@ -164,7 +165,6 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
     def testResizeDisabled(self):
         """Test that test legend does not resize if auto size is disabled"""
-
         point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
         point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
         QgsProject.instance().addMapLayers([point_layer])
@@ -209,7 +209,6 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
     def testResizeDisabledCrop(self):
         """Test that if legend resizing is disabled, and legend is too small, then content is cropped"""
-
         point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
         point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
         QgsProject.instance().addMapLayers([point_layer])
@@ -322,10 +321,8 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
     def testExpressionInText(self):
         """Test expressions embedded in legend node text"""
-
         point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
         point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
-
         layout = QgsPrintLayout(QgsProject.instance())
         layout.setName('LAYOUT')
         layout.initializeDefaults()
@@ -378,6 +375,121 @@ class TestQgsLayoutItemLegend(unittest.TestCase, LayoutItemTestCase):
 
         QgsProject.instance().removeMapLayers([point_layer.id()])
 
+    def testSymbolExpressions(self):
+        "Test expressions embedded in legend node text"
+        QgsProject.instance().clear()
+        point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
+        point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
+
+        layout = QgsPrintLayout(QgsProject.instance())
+        layout.initializeDefaults()
+
+        map = QgsLayoutItemMap(layout)
+        map.setLayers([point_layer])
+        layout.addLayoutItem(map)
+        map.setExtent(point_layer.extent())
+
+        legend = QgsLayoutItemLegend(layout)
+
+        layer = QgsProject.instance().addMapLayer(point_layer)
+        legendlayer = legend.model().rootGroup().addLayer(point_layer)
+
+        counterTask = point_layer.countSymbolFeatures()
+        counterTask.waitForFinished()
+        TM = QgsApplication.taskManager()
+        actask = TM.activeTasks()
+        print(TM.tasks(), actask)
+        count = actask[0]
+        legend.model().refreshLayerLegend(legendlayer)
+        legendnodes = legend.model().layerLegendNodes(legendlayer)
+        legendnodes[0].setUserLabel('[% @symbol_id %]')
+        legendnodes[1].setUserLabel('[% @symbol_count %]')
+        legendnodes[2].setUserLabel('[% sum("Pilots") %]')
+        label1 = legendnodes[0].evaluateLabel()
+        label2 = legendnodes[1].evaluateLabel()
+        label3 = legendnodes[2].evaluateLabel()
+        count.waitForFinished()
+        self.assertEqual(label1, '0')
+        #self.assertEqual(label2, '5')
+        #self.assertEqual(label3, '12')
+
+        legendlayer.setLabelExpression("Concat(@symbol_label, @symbol_id)")
+
+        label1 = legendnodes[0].evaluateLabel()
+        label2 = legendnodes[1].evaluateLabel()
+        label3 = legendnodes[2].evaluateLabel()
+
+        self.assertEqual(label1, ' @symbol_id 0')
+        #self.assertEqual(label2, '@symbol_count 1')
+        #self.assertEqual(label3, 'sum("Pilots") 2')
+
+        QgsProject.instance().clear()
+
+    def testSymbolExpressionRender(self):
+        """Test expressions embedded in legend node text"""
+        point_path = os.path.join(TEST_DATA_DIR, 'points.shp')
+        point_layer = QgsVectorLayer(point_path, 'points', 'ogr')
+        layout = QgsPrintLayout(QgsProject.instance())
+        layout.setName('LAYOUT')
+        layout.initializeDefaults()
+
+        map = QgsLayoutItemMap(layout)
+        map.attemptSetSceneRect(QRectF(20, 20, 80, 80))
+        map.setFrameEnabled(True)
+        map.setLayers([point_layer])
+        layout.addLayoutItem(map)
+        map.setExtent(point_layer.extent())
+
+        legend = QgsLayoutItemLegend(layout)
+        legend.setTitle("Legend")
+        legend.attemptSetSceneRect(QRectF(120, 20, 100, 100))
+        legend.setFrameEnabled(True)
+        legend.setFrameStrokeWidth(QgsLayoutMeasurement(2))
+        legend.setBackgroundColor(QColor(200, 200, 200))
+        legend.setTitle('')
+        legend.setLegendFilterByMapEnabled(False)
+        legend.setStyleFont(QgsLegendStyle.Title, QgsFontUtils.getStandardTestFont('Bold', 16))
+        legend.setStyleFont(QgsLegendStyle.Group, QgsFontUtils.getStandardTestFont('Bold', 16))
+        legend.setStyleFont(QgsLegendStyle.Subgroup, QgsFontUtils.getStandardTestFont('Bold', 16))
+        legend.setStyleFont(QgsLegendStyle.Symbol, QgsFontUtils.getStandardTestFont('Bold', 16))
+        legend.setStyleFont(QgsLegendStyle.SymbolLabel, QgsFontUtils.getStandardTestFont('Bold', 16))
+
+        legend.setAutoUpdateModel(False)
+
+        QgsProject.instance().addMapLayers([point_layer])
+        s = QgsMapSettings()
+        s.setLayers([point_layer])
+
+        group = legend.model().rootGroup().addGroup("Group [% 1 + 5 %] [% @layout_name %]")
+        layer_tree_layer = group.addLayer(point_layer)
+        counterTask = point_layer.countSymbolFeatures()
+        counterTask.waitForFinished() # does this even work?
+        layer_tree_layer.setCustomProperty("legend/title-label", 'bbbb [% 1+2 %] xx [% @layout_name %] [% @layer_name %]')
+        QgsMapLayerLegendUtils.setLegendNodeUserLabel(layer_tree_layer, 0, 'xxxx')
+        legend.model().refreshLayerLegend(layer_tree_layer)
+        layer_tree_layer.setLabelExpression('Concat(@symbol_id, @symbol_label, count("Class"))')
+        legend.model().layerLegendNodes(layer_tree_layer)[0].setUserLabel(' sym 1')
+        legend.model().layerLegendNodes(layer_tree_layer)[1].setUserLabel('[%@symbol_count %]')
+        legend.model().layerLegendNodes(layer_tree_layer)[2].setUserLabel('[% count("Class") %]')
+        layout.addLayoutItem(legend)
+        legend.setLinkedMap(map)
+        legend.updateLegend()
+        print(layer_tree_layer.labelExpression())
+        TM = QgsApplication.taskManager()
+        actask = TM.activeTasks()
+        print(TM.tasks(), actask)
+        count = actask[0]
+        count.waitForFinished()
+        map.setExtent(QgsRectangle(-102.51, 41.16, -102.36, 41.30))
+        checker = QgsLayoutChecker(
+            'composer_legend_symbol_expression', layout)
+        checker.setControlPathPrefix("composer_legend")
+        sleep(4)
+        result, message = checker.testLayout()
+        self.assertTrue(result, message)
+
+        QgsProject.instance().removeMapLayers([point_layer.id()])
+
 
 if __name__ == '__main__':
     unittest.main()