[layouts] Don't needlessly calculate layer sets for maps linked

to a map theme

This is very expensive to do, so avoid calculating it as much
as possible

Refs #17027

python/core/layout/qgslayoutitemmap.sip.in

@@ -458,9 +458,13 @@ This is calculated using the width of the map item and the width of the
 current visible map extent.
 %End
 
-    QgsMapSettings mapSettings( const QgsRectangle &extent, QSizeF size, double dpi ) const;
+    QgsMapSettings mapSettings( const QgsRectangle &extent, QSizeF size, double dpi, bool includeLayerSettings ) const;
 %Docstring
 Return map settings that will be used for drawing of the map.
+
+If ``includeLayerSettings`` is true, than settings specifically relating to map layers and map layer styles
+will be calculated. This can be expensive to calculate, so if they are not required in the map settings
+(e.g. for map settings which are used for scale related calculations only) then ``includeLayerSettings`` should be false.
 %End
 
     virtual void finalizeRestoreFromXml();

src/core/layout/qgslayoutitemlegend.cpp

@@ -89,7 +89,7 @@ void QgsLayoutItemLegend::paint( QPainter *painter, const QStyleOptionGraphicsIt
     QSizeF mapSizePixels = QSizeF( mMap->rect().width() * dotsPerMM, mMap->rect().height() * dotsPerMM );
     QgsRectangle mapExtent = mMap->extent();
 
-    QgsMapSettings ms = mMap->mapSettings( mapExtent, mapSizePixels, dpi );
+    QgsMapSettings ms = mMap->mapSettings( mapExtent, mapSizePixels, dpi, false );
     mSettings.setMapScale( ms.scale() );
   }
   mInitialMapScaleCalculated = true;
@@ -761,7 +761,7 @@ void QgsLayoutItemLegend::doUpdateFilterByMap()
     QSizeF size( requestRectangle.width(), requestRectangle.height() );
     size *= mLayout->convertFromLayoutUnits( mMap->mapUnitsToLayoutUnits(), QgsUnitTypes::LayoutMillimeters ).length() * dpi / 25.4;
 
-    QgsMapSettings ms = mMap->mapSettings( requestRectangle, size, dpi );
+    QgsMapSettings ms = mMap->mapSettings( requestRectangle, size, dpi, true );
 
     QgsGeometry filterPolygon;
     if ( mInAtlas )

src/core/layout/qgslayoutitemmap.cpp

@@ -960,7 +960,7 @@ void QgsLayoutItemMap::drawMap( QPainter *painter, const QgsRectangle &extent, Q
   }
 
   // render
-  QgsMapRendererCustomPainterJob job( mapSettings( extent, size, dpi ), painter );
+  QgsMapRendererCustomPainterJob job( mapSettings( extent, size, dpi, true ), painter );
   // Render the map in this thread. This is done because of problems
   // with printing to printer on Windows (printing to PDF is fine though).
   // Raster images were not displayed - see #10599
@@ -1037,13 +1037,13 @@ void QgsLayoutItemMap::recreateCachedImageInBackground( double viewScaleFactor )
 
   mCacheInvalidated = false;
   mPainter.reset( new QPainter( mCacheRenderingImage.get() ) );
-  QgsMapSettings settings( mapSettings( ext, QSizeF( w, h ), mCacheRenderingImage->logicalDpiX() ) );
+  QgsMapSettings settings( mapSettings( ext, QSizeF( w, h ), mCacheRenderingImage->logicalDpiX(), true ) );
   mPainterJob.reset( new QgsMapRendererCustomPainterJob( settings, mPainter.get() ) );
   connect( mPainterJob.get(), &QgsMapRendererCustomPainterJob::finished, this, &QgsLayoutItemMap::painterJobFinished );
   mPainterJob->start();
 }
 
-QgsMapSettings QgsLayoutItemMap::mapSettings( const QgsRectangle &extent, QSizeF size, double dpi ) const
+QgsMapSettings QgsLayoutItemMap::mapSettings( const QgsRectangle &extent, QSizeF size, double dpi, bool includeLayerSettings ) const
 {
   QgsExpressionContext expressionContext = createExpressionContext();
   QgsCoordinateReferenceSystem renderCrs = crs();
@@ -1058,26 +1058,29 @@ QgsMapSettings QgsLayoutItemMap::mapSettings( const QgsRectangle &extent, QSizeF
   if ( mLayout )
     jobMapSettings.setEllipsoid( mLayout->project()->ellipsoid() );
 
-  //set layers to render
+  if ( includeLayerSettings )
-  QList<QgsMapLayer *> layers = layersToRender( &expressionContext );
-  if ( mLayout && -1 != mLayout->renderContext().currentExportLayer() )
   {
-    const int layerIdx = mLayout->renderContext().currentExportLayer() - ( hasBackground() ? 1 : 0 );
+    //set layers to render
-    if ( layerIdx >= 0 && layerIdx < layers.length() )
+    QList<QgsMapLayer *> layers = layersToRender( &expressionContext );
+    if ( mLayout && -1 != mLayout->renderContext().currentExportLayer() )
     {
-      // exporting with separate layers (e.g., to svg layers), so we only want to render a single map layer
+      const int layerIdx = mLayout->renderContext().currentExportLayer() - ( hasBackground() ? 1 : 0 );
-      QgsMapLayer *ml = layers[ layers.length() - layerIdx - 1 ];
+      if ( layerIdx >= 0 && layerIdx < layers.length() )
-      layers.clear();
+      {
-      layers << ml;
+        // exporting with separate layers (e.g., to svg layers), so we only want to render a single map layer
-    }
+        QgsMapLayer *ml = layers[ layers.length() - layerIdx - 1 ];
-    else
+        layers.clear();
-    {
+        layers << ml;
-      // exporting decorations such as map frame/grid/overview, so no map layers required
+      }
-      layers.clear();
+      else
+      {
+        // exporting decorations such as map frame/grid/overview, so no map layers required
+        layers.clear();
+      }
     }
+    jobMapSettings.setLayers( layers );
+    jobMapSettings.setLayerStyleOverrides( layerStyleOverridesToRender( expressionContext ) );
   }
-  jobMapSettings.setLayers( layers );
-  jobMapSettings.setLayerStyleOverrides( layerStyleOverridesToRender( expressionContext ) );
 
   if ( !mLayout->renderContext().isPreviewRender() )
   {

src/core/layout/qgslayoutitemmap.h

@@ -408,8 +408,12 @@ class CORE_EXPORT QgsLayoutItemMap : public QgsLayoutItem
 
     /**
      * Return map settings that will be used for drawing of the map.
+     *
+     * If \a includeLayerSettings is true, than settings specifically relating to map layers and map layer styles
+     * will be calculated. This can be expensive to calculate, so if they are not required in the map settings
+     * (e.g. for map settings which are used for scale related calculations only) then \a includeLayerSettings should be false.
      */
-    QgsMapSettings mapSettings( const QgsRectangle &extent, QSizeF size, double dpi ) const;
+    QgsMapSettings mapSettings( const QgsRectangle &extent, QSizeF size, double dpi, bool includeLayerSettings ) const;
 
     void finalizeRestoreFromXml() override;
 

src/core/layout/qgslayoututils.cpp

@@ -123,7 +123,7 @@ QgsRenderContext QgsLayoutUtils::createRenderContextForMap( QgsLayoutItemMap *ma
     QgsRectangle extent = map->extent();
     QSizeF mapSizeLayoutUnits = map->rect().size();
     QSizeF mapSizeMM = map->layout()->convertFromLayoutUnits( mapSizeLayoutUnits, QgsUnitTypes::LayoutMillimeters ).toQSizeF();
-    QgsMapSettings ms = map->mapSettings( extent, mapSizeMM * dotsPerMM, dpi );
+    QgsMapSettings ms = map->mapSettings( extent, mapSizeMM * dotsPerMM, dpi, false );
     QgsRenderContext context = QgsRenderContext::fromMapSettings( ms );
     if ( painter )
       context.setPainter( painter );