Reduce number of expression evaluations in legend rendering

...by caching previous results and using them, until the layout is refreshed
qgis · Jun 2, 2020 · 4a125f6 · 4a125f6
1 parent 08542ef
commit 4a125f6
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Showing 3 changed files with 56 additions and 4 deletions.
diff --git a/python/core/auto_generated/layout/qgslayoutitemlegend.sip.in b/python/core/auto_generated/layout/qgslayoutitemlegend.sip.in
@@ -41,6 +41,13 @@ Alternative constructor.
 
 
 
+    void clearCachedData( QgsLayerTreeNode *node ) const;
+%Docstring
+Clears any previously cached data for the specified ``node``.
+
+.. versionadded:: 3.14
+%End
+
   signals:
 
     void refreshLegend();

diff --git a/src/core/layout/qgslayoutitemlegend.cpp b/src/core/layout/qgslayoutitemlegend.cpp
@@ -168,6 +168,7 @@ void QgsLayoutItemLegend::finalizeRestoreFromXml()
 void QgsLayoutItemLegend::refresh()
 {
   QgsLayoutItem::refresh();
+  clearLegendCachedData();
   onAtlasFeature();
 }
 
@@ -780,6 +781,26 @@ void QgsLayoutItemLegend::setModelStyleOverrides( const QMap<QString, QString> &
 
 }
 
+void QgsLayoutItemLegend::clearLegendCachedData()
+{
+  std::function< void( QgsLayerTreeNode * ) > clearNodeCache;
+  clearNodeCache = [&]( QgsLayerTreeNode * node )
+  {
+    mLegendModel->clearCachedData( node );
+    if ( QgsLayerTree::isGroup( node ) )
+    {
+      QgsLayerTreeGroup *group = QgsLayerTree::toGroup( node );
+      const QList< QgsLayerTreeNode * > children = group->children();
+      for ( QgsLayerTreeNode *child : children )
+      {
+        clearNodeCache( child );
+      }
+    }
+  };
+
+  clearNodeCache( mLegendModel->rootGroup() );
+}
+
 void QgsLayoutItemLegend::mapLayerStyleOverridesChanged()
 {
   if ( !mMap )
@@ -1022,7 +1043,10 @@ QVariant QgsLegendModel::data( const QModelIndex &index, int role ) const
   QgsLayerTreeLayer *nodeLayer = QgsLayerTree::isLayer( node ) ? QgsLayerTree::toLayer( node ) : nullptr;
   if ( nodeLayer && ( role == Qt::DisplayRole || role == Qt::EditRole ) )
   {
-    QString name;
+    QString name = node->customProperty( QStringLiteral( "cached_name" ) ).toString();
+    if ( !name.isEmpty() )
+      return name;
+
     QgsVectorLayer *vlayer = qobject_cast<QgsVectorLayer *>( nodeLayer->layer() );
 
     //finding the first label that is stored
@@ -1038,13 +1062,13 @@ QVariant QgsLegendModel::data( const QModelIndex &index, int role ) const
       if ( vlayer && vlayer->featureCount() >= 0 )
       {
         name += QStringLiteral( " [%1]" ).arg( vlayer->featureCount() );
+        node->setCustomProperty( QStringLiteral( "cached_name" ), name );
         return name;
       }
     }
 
-    bool evaluate = vlayer ? !nodeLayer->labelExpression().isEmpty() : false;
-
-    if ( evaluate || name.contains( "[%" ) )
+    const bool evaluate = ( vlayer && !nodeLayer->labelExpression().isEmpty() ) || name.contains( "[%" );
+    if ( evaluate )
     {
       QgsExpressionContext expressionContext;
       if ( vlayer )
@@ -1069,6 +1093,7 @@ QVariant QgsLegendModel::data( const QModelIndex &index, int role ) const
       else if ( QgsSymbolLegendNode *symnode = qobject_cast<QgsSymbolLegendNode *>( legendnodes.first() ) )
         name = symnode->evaluateLabel( expressionContext );
     }
+    node->setCustomProperty( QStringLiteral( "cached_name" ), name );
     return name;
   }
   return QgsLayerTreeModel::data( index, role );
@@ -1095,6 +1120,11 @@ QList<QgsLayerTreeModelLegendNode *> QgsLegendModel::layerLegendNodes( QgsLayerT
   return lst;
 }
 
+void QgsLegendModel::clearCachedData( QgsLayerTreeNode *node ) const
+{
+  node->removeCustomProperty( QStringLiteral( "cached_name" ) );
+}
+
 void QgsLegendModel::forceRefresh()
 {
   emit refreshLegend();

diff --git a/src/core/layout/qgslayoutitemlegend.h b/src/core/layout/qgslayoutitemlegend.h
@@ -65,6 +65,12 @@ class CORE_EXPORT QgsLegendModel : public QgsLayerTreeModel
      */
     QList<QgsLayerTreeModelLegendNode *> layerLegendNodes( QgsLayerTreeLayer *nodeLayer, bool skipNodeEmbeddedInParent = false ) const SIP_SKIP;
 
+    /**
+     * Clears any previously cached data for the specified \a node.
+     * \since QGIS 3.14
+     */
+    void clearCachedData( QgsLayerTreeNode *node ) const;
+
   signals:
 
     /**
@@ -89,6 +95,12 @@ class CORE_EXPORT QgsLegendModel : public QgsLayerTreeModel
      */
     QgsLayoutItemLegend *mLayoutLegend = nullptr;
 
+    /**
+     * Evaluate the expression or symbol expressions of a given layer node.
+     * \since QGIS 3.14
+     */
+    QString evaluateLayerExpressions( QgsLayerTreeLayer *nodeLayer ) const;
+
 };
 
 
@@ -549,6 +561,9 @@ class CORE_EXPORT QgsLayoutItemLegend : public QgsLayoutItem
 
     void setModelStyleOverrides( const QMap<QString, QString> &overrides );
 
+    //! Clears any data cached for the legend model
+    void clearLegendCachedData();
+
     std::unique_ptr< QgsLegendModel > mLegendModel;
     std::unique_ptr< QgsLayerTreeGroup > mCustomLayerTree;