From 673d6727ee0b99ea2ff0bd8df498134706341a00 Mon Sep 17 00:00:00 2001
From: Matthias Kuhn <matthias@opengis.ch>
Date: Sun, 20 Sep 2020 13:18:16 +0200
Subject: [PATCH] Improve legend performance

Only recalculate the position of the icons for affected legend nodes.
Avoids brute forcing over complex legends as soon as something is changed.

Fixes #38890
---
 .../layertree/qgslayertreemodel.sip.in        |  1 +
 src/core/layertree/qgslayertreemodel.cpp      | 19 ++++++++++++-------
 src/core/layertree/qgslayertreemodel.h        |  6 ++++++
 3 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/python/core/auto_generated/layertree/qgslayertreemodel.sip.in b/python/core/auto_generated/layertree/qgslayertreemodel.sip.in
index 641ef6cf394..41434334cd6 100644
--- a/python/core/auto_generated/layertree/qgslayertreemodel.sip.in
+++ b/python/core/auto_generated/layertree/qgslayertreemodel.sip.in
@@ -415,6 +415,7 @@ Filter nodes from QgsMapLayerLegend according to the current filtering rules
 
 
 
+
 };
 
 QFlags<QgsLayerTreeModel::Flag> operator|(QgsLayerTreeModel::Flag f1, QFlags<QgsLayerTreeModel::Flag> f2);
diff --git a/src/core/layertree/qgslayertreemodel.cpp b/src/core/layertree/qgslayertreemodel.cpp
index e5967298846..e3080652608 100644
--- a/src/core/layertree/qgslayertreemodel.cpp
+++ b/src/core/layertree/qgslayertreemodel.cpp
@@ -1318,7 +1318,8 @@ void QgsLayerTreeModel::addLegendToLayer( QgsLayerTreeLayer *nodeL )
 
   int count = legendTree ? legendTree->children[nullptr].count() : filteredLstNew.count();
 
-  if ( !filteredLstNew.isEmpty() ) beginInsertRows( node2index( nodeL ), 0, count - 1 );
+  if ( !filteredLstNew.isEmpty() )
+    beginInsertRows( node2index( nodeL ), 0, count - 1 );
 
   LayerLegendData data;
   data.originalNodes = lstNew;
@@ -1328,10 +1329,12 @@ void QgsLayerTreeModel::addLegendToLayer( QgsLayerTreeLayer *nodeL )
 
   mLegend[nodeL] = data;
 
-  if ( !filteredLstNew.isEmpty() ) endInsertRows();
+  if ( !filteredLstNew.isEmpty() )
+    endInsertRows();
 
   // invalidate map based data even if the data is not map-based to make sure
   // the symbol sizes are computed at least once
+  mInvalidatedNodes.insert( nodeL );
   legendInvalidateMapBasedData();
 }
 
@@ -1575,7 +1578,7 @@ void QgsLayerTreeModel::legendInvalidateMapBasedData()
   if ( !testFlag( DeferredLegendInvalidation ) )
     invalidateLegendMapBasedData();
   else
-    mDeferLegendInvalidationTimer.start( 1000 );
+    mDeferLegendInvalidationTimer.start( 10 );
 }
 
 void QgsLayerTreeModel::invalidateLegendMapBasedData()
@@ -1583,15 +1586,17 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
   // we have varying icon sizes, and we want icon to be centered and
   // text to be left aligned, so we have to compute the max width of icons
   //
-  // we do that for nodes who share a common parent
+  // we do that for nodes which share a common parent
   //
   // we do that here because for symbols with size defined in map units
   // the symbol sizes changes depends on the zoom level
 
   std::unique_ptr<QgsRenderContext> context( createTemporaryRenderContext() );
 
-  for ( const LayerLegendData &data : qgis::as_const( mLegend ) )
+  for ( QgsLayerTreeLayer *layerNode : qgis::as_const( mInvalidatedNodes ) )
   {
+    const LayerLegendData &data = mLegend.value( layerNode );
+
     QList<QgsSymbolLegendNode *> symbolNodes;
     QMap<QString, int> widthMax;
     for ( QgsLayerTreeModelLegendNode *legendNode : qgis::as_const( data.originalNodes ) )
@@ -1606,8 +1611,7 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
         symbolNodes.append( n );
       }
     }
-    const auto constSymbolNodes = symbolNodes;
-    for ( QgsSymbolLegendNode *n : constSymbolNodes )
+    for ( QgsSymbolLegendNode *n : qgis::as_const( symbolNodes ) )
     {
       const QString parentKey( n->data( QgsLayerTreeModelLegendNode::ParentRuleKeyRole ).toString() );
       Q_ASSERT( widthMax[parentKey] > 0 );
@@ -1618,6 +1622,7 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
       legendNode->invalidateMapBasedData();
   }
 
+  mInvalidatedNodes.clear();
 }
 
 // Legend nodes routines - end
diff --git a/src/core/layertree/qgslayertreemodel.h b/src/core/layertree/qgslayertreemodel.h
index 385bb6dc43b..2d68c6efe2b 100644
--- a/src/core/layertree/qgslayertreemodel.h
+++ b/src/core/layertree/qgslayertreemodel.h
@@ -435,6 +435,12 @@ class CORE_EXPORT QgsLayerTreeModel : public QAbstractItemModel
     //! Per layer data about layer's legend nodes
     QHash<QgsLayerTreeLayer *, LayerLegendData> mLegend;
 
+    /**
+     * Keep track of layer nodes for which the legend
+     * size needs to be recalculated
+     */
+    QSet<QgsLayerTreeLayer *> mInvalidatedNodes;
+
     QFont mFontLayer;
     QFont mFontGroup;