Add maximumExtentBuffer() function to QgsFeatureRenderer

python/PyQt6/core/auto_generated/symbology/qgsrenderer.sip.in

@@ -628,6 +628,18 @@ Currently clones
 :param destRenderer: destination renderer for copied effect
 
 .. versionadded:: 3.22
+%End
+
+    double maximumExtentBuffer( QgsRenderContext &context ) const;
+%Docstring
+Returns the maximum extent buffer found in this renderer's
+symbols' symbol layers.
+
+.. note::
+
+   Returns 0 if the renderer doesn't have any symbols.
+
+.. versionadded:: 3.42
 %End
 
   protected:

python/core/auto_generated/symbology/qgsrenderer.sip.in

@@ -628,6 +628,18 @@ Currently clones
 :param destRenderer: destination renderer for copied effect
 
 .. versionadded:: 3.22
+%End
+
+    double maximumExtentBuffer( QgsRenderContext &context ) const;
+%Docstring
+Returns the maximum extent buffer found in this renderer's
+symbols' symbol layers.
+
+.. note::
+
+   Returns 0 if the renderer doesn't have any symbols.
+
+.. versionadded:: 3.42
 %End
 
   protected:

src/core/symbology/qgsrenderer.cpp

@@ -38,6 +38,7 @@
 #include <QDomDocument>
 #include <QPolygonF>
 #include <QThread>
+#include <algorithm>
 
 QgsPropertiesDefinition QgsFeatureRenderer::sPropertyDefinitions;
 
@@ -417,6 +418,38 @@ QgsLegendSymbolList QgsFeatureRenderer::legendSymbolItems() const
   return QgsLegendSymbolList();
 }
 
+double QgsFeatureRenderer::maximumExtentBuffer( QgsRenderContext &context ) const
+{
+  const QgsSymbolList symbolList = symbols( context );
+
+  if ( symbolList.empty() )
+    return 0;
+
+  const QgsExpressionContext &expContext = context.expressionContext();
+
+  auto getValueFromSymbol = [ &expContext ]( const QgsSymbol * sym ) -> double
+  {
+    const QgsProperty property = sym->dataDefinedProperties().property( QgsSymbol::Property::ExtentBuffer );
+
+    if ( property.isActive() && ! property.expressionString().isEmpty() )
+    {
+      return sym->dataDefinedProperties().valueAsDouble( QgsSymbol::Property::ExtentBuffer, expContext );
+    }
+
+    return sym->extentBuffer();
+  };
+
+  if ( symbolList.size() == 1 )
+    return getValueFromSymbol( symbolList[0] );
+
+  auto it = std::max_element( symbolList.constBegin(), symbolList.constEnd(), [ &getValueFromSymbol ]( const QgsSymbol * a, const QgsSymbol * b ) -> bool
+  {
+    return getValueFromSymbol( a ) < getValueFromSymbol( b );
+  } );
+
+  return getValueFromSymbol( *it );
+}
+
 QList<QgsLayerTreeModelLegendNode *> QgsFeatureRenderer::createLegendNodes( QgsLayerTreeLayer *nodeLayer ) const
 {
   QList<QgsLayerTreeModelLegendNode *> nodes;

src/core/symbology/qgsrenderer.h

@@ -653,6 +653,15 @@ class CORE_EXPORT QgsFeatureRenderer
      */
     void copyRendererData( QgsFeatureRenderer *destRenderer ) const;
 
+    /**
+     * Returns the maximum extent buffer found in this renderer's
+     * symbols' symbol layers.
+     *
+     * \note Returns 0 if the renderer doesn't have any symbols.
+     * \since QGIS 3.42
+     */
+    double maximumExtentBuffer( QgsRenderContext &context ) const;
+
   protected:
     QgsFeatureRenderer( const QString &type );