Avoid creating multiple render contexts when calculating legend

node size Partial fix for qgis slowdown when using layer with large number of legend nodes
qgis · Oct 19, 2016 · c9251c5 · c9251c5
1 parent cf60049
commit c9251c5
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Showing 6 changed files with 66 additions and 12 deletions.
diff --git a/python/core/layertree/qgslayertreemodel.sip b/python/core/layertree/qgslayertreemodel.sip
@@ -166,7 +166,7 @@ class QgsLayerTreeModel : QAbstractItemModel
     //! Get hints about map view - to be used in legend nodes. Arguments that are not null will receive values.
     //! If there are no valid map view data (from previous call to setLegendMapViewData()), returned values are zeros.
     //! @note added in 2.6
-    void legendMapViewData( double *mapUnitsPerPixel /Out/, int *dpi /Out/, double *scale /Out/ );
+    void legendMapViewData( double *mapUnitsPerPixel /Out/, int *dpi /Out/, double *scale /Out/ ) const;
 
     //! Get map of map layer style overrides (key: layer ID, value: style name) where a different style should be used instead of the current one
     //! @note added in 2.10

diff --git a/python/core/layertree/qgslayertreemodellegendnode.sip b/python/core/layertree/qgslayertreemodellegendnode.sip
@@ -141,10 +141,23 @@ class QgsSymbolLegendNode : QgsLayerTreeModelLegendNode
     //! @note added in 2.10
     QSize iconSize() const;
 
-    //! Get the minimum icon size to prevent cropping
-    //! @note added in 2.10
+    /**
+     * Calculates the minimum icon size to prevent cropping. When evaluating
+     * the size for multiple icons it is more efficient to create a single
+     * render context in advance and use the variant which accepts a QgsRenderContext
+     * argument.
+     * @note added in 2.10
+     */
     QSize minimumIconSize() const;
 
+    /**
+     * Calculates the minimum icon size to prevent cropping. When evaluating
+     * the size for multiple icons it is more efficient to create a single
+     * render context in advance and call this method instead of minimumIconSize().
+     * @note added in QGIS 2.18
+     */
+    QSize minimumIconSize( QgsRenderContext &context ) const;
+
     /** Returns the symbol used by the legend node.
      * @see setSymbol()
      * @note added in QGIS 2.14

diff --git a/src/core/layertree/qgslayertreemodel.cpp b/src/core/layertree/qgslayertreemodel.cpp
@@ -663,7 +663,7 @@ void QgsLayerTreeModel::setLegendMapViewData( double mapUnitsPerPixel, int dpi,
   refreshScaleBasedLayers();
 }
 
-void QgsLayerTreeModel::legendMapViewData( double* mapUnitsPerPixel, int* dpi, double* scale )
+void QgsLayerTreeModel::legendMapViewData( double* mapUnitsPerPixel, int* dpi, double* scale ) const
 {
   if ( mapUnitsPerPixel ) *mapUnitsPerPixel = mLegendMapViewMupp;
   if ( dpi ) *dpi = mLegendMapViewDpi;
@@ -1269,6 +1269,22 @@ void QgsLayerTreeModel::tryBuildLegendTree( LayerLegendData& data )
   }
 }
 
+QgsRenderContext* QgsLayerTreeModel::createTemporaryRenderContext() const
+{
+  double scale = 0.0;
+  double mupp = 0.0;
+  int dpi = 0;
+  legendMapViewData( &mupp, &dpi, &scale );
+  bool validData = !qgsDoubleNear( mupp, 0.0 ) && dpi != 0 && !qgsDoubleNear( scale, 0.0 );
+
+  // setup temporary render context
+  QScopedPointer<QgsRenderContext> context( new QgsRenderContext );
+  context->setScaleFactor( dpi / 25.4 );
+  context->setRendererScale( scale );
+  context->setMapToPixel( QgsMapToPixel( mupp ) );
+  return validData ? context.take() : nullptr;
+}
+
 
 QgsLayerTreeModelLegendNode* QgsLayerTreeModel::index2legendNode( const QModelIndex& index )
 {
@@ -1470,6 +1486,8 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
   // we do that here because for symbols with size defined in map units
   // the symbol sizes changes depends on the zoom level
 
+  QScopedPointer<QgsRenderContext> context( createTemporaryRenderContext() );
+
   Q_FOREACH ( const LayerLegendData& data, mLegend )
   {
     QList<QgsSymbolLegendNode*> symbolNodes;
@@ -1479,7 +1497,7 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
       QgsSymbolLegendNode* n = dynamic_cast<QgsSymbolLegendNode*>( legendNode );
       if ( n )
       {
-        const QSize sz( n->minimumIconSize() );
+        const QSize sz( n->minimumIconSize( *context ) );
         const QString parentKey( n->data( QgsLayerTreeModelLegendNode::ParentRuleKeyRole ).toString() );
         widthMax[parentKey] = qMax( sz.width(), widthMax.contains( parentKey ) ? widthMax[parentKey] : 0 );
         n->setIconSize( sz );

diff --git a/src/core/layertree/qgslayertreemodel.h b/src/core/layertree/qgslayertreemodel.h
@@ -31,6 +31,7 @@ class QgsMapHitTest;
 class QgsMapLayer;
 class QgsMapSettings;
 class QgsExpression;
+class QgsRenderContext;
 
 /** \ingroup core
  * The QgsLayerTreeModel class is model implementation for Qt item views framework.
@@ -191,7 +192,7 @@ class CORE_EXPORT QgsLayerTreeModel : public QAbstractItemModel
     //! Get hints about map view - to be used in legend nodes. Arguments that are not null will receive values.
     //! If there are no valid map view data (from previous call to setLegendMapViewData()), returned values are zeros.
     //! @note added in 2.6
-    void legendMapViewData( double *mapUnitsPerPixel, int *dpi, double *scale );
+    void legendMapViewData( double *mapUnitsPerPixel, int *dpi, double *scale ) const;
 
     //! Get map of map layer style overrides (key: layer ID, value: style name) where a different style should be used instead of the current one
     //! @note added in 2.10
@@ -328,6 +329,11 @@ class CORE_EXPORT QgsLayerTreeModel : public QAbstractItemModel
     int mLegendMapViewDpi;
     double mLegendMapViewScale;
     QTimer mDeferLegendInvalidationTimer;
+
+  private:
+
+    //! Returns a temporary render context
+    QgsRenderContext* createTemporaryRenderContext() const;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS( QgsLayerTreeModel::Flags )

diff --git a/src/core/layertree/qgslayertreemodellegendnode.cpp b/src/core/layertree/qgslayertreemodellegendnode.cpp
@@ -157,23 +157,27 @@ Qt::ItemFlags QgsSymbolLegendNode::flags() const
 
 
 QSize QgsSymbolLegendNode::minimumIconSize() const
+{
+  QScopedPointer<QgsRenderContext> context( createTemporaryRenderContext() );
+  return minimumIconSize( *context );
+}
+
+QSize QgsSymbolLegendNode::minimumIconSize( QgsRenderContext& context ) const
 {
   QSize minSz( 16, 16 );
   if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Marker )
   {
-    QScopedPointer<QgsRenderContext> context( createTemporaryRenderContext() );
     minSz = QgsImageOperation::nonTransparentImageRect(
               QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( 512, 512 ),
-                  context.data() ).toImage(),
+                  &context ).toImage(),
               minSz,
               true ).size();
   }
   else if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Line )
   {
-    QScopedPointer<QgsRenderContext> context( createTemporaryRenderContext() );
     minSz = QgsImageOperation::nonTransparentImageRect(
               QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( minSz.width(), 512 ),
-                  context.data() ).toImage(),
+                  &context ).toImage(),
               minSz,
               true ).size();
   }

diff --git a/src/core/layertree/qgslayertreemodellegendnode.h b/src/core/layertree/qgslayertreemodellegendnode.h
@@ -171,10 +171,23 @@ class CORE_EXPORT QgsSymbolLegendNode : public QgsLayerTreeModelLegendNode
     //! @note added in 2.10
     QSize iconSize() const { return mIconSize; }
 
-    //! Get the minimum icon size to prevent cropping
-    //! @note added in 2.10
+    /**
+     * Calculates the minimum icon size to prevent cropping. When evaluating
+     * the size for multiple icons it is more efficient to create a single
+     * render context in advance and use the variant which accepts a QgsRenderContext
+     * argument.
+     * @note added in 2.10
+     */
     QSize minimumIconSize() const;
 
+    /**
+     * Calculates the minimum icon size to prevent cropping. When evaluating
+     * the size for multiple icons it is more efficient to create a single
+     * render context in advance and call this method instead of minimumIconSize().
+     * @note added in QGIS 2.18
+     */
+    QSize minimumIconSize( QgsRenderContext &context ) const;
+
     /** Returns the symbol used by the legend node.
      * @see setSymbol()
      * @note added in QGIS 2.14