Avoid unnecessary handling of active layer changed when we are

adding many layers at once, or removing many (e.g. due to project
clear)

Saves a lot of unnecessary and potentially expensive processing

src/app/gps/qgsgpsinformationwidget.cpp

@@ -260,7 +260,7 @@ QgsGpsInformationWidget::QgsGpsInformationWidget( QgsMapCanvas *thepCanvas, QWid
   //SLM - added functionality
   mLogFile = nullptr;
 
-  connect( QgisApp::instance()->layerTreeView(), &QgsLayerTreeView::currentLayerChanged,
+  connect( QgisApp::instance(), &QgisApp::activeLayerChanged,
            this, &QgsGpsInformationWidget::updateCloseFeatureButton );
 
   mStackedWidget->setCurrentIndex( 3 ); // force to Options

src/app/qgisapp.cpp

@@ -567,9 +567,13 @@ void QgisApp::layerTreeViewDoubleClicked( const QModelIndex &index )
   }
 }
 
-void QgisApp::activeLayerChanged( QgsMapLayer *layer )
+void QgisApp::onActiveLayerChanged( QgsMapLayer *layer )
 {
-  Q_FOREACH ( QgsMapCanvas *canvas, mapCanvases() )
+  if ( mBlockActiveLayerChanged )
+    return;
+
+  const QList< QgsMapCanvas * > canvases = mapCanvases();
+  for ( QgsMapCanvas *canvas : canvases )
     canvas->setCurrentLayer( layer );
 
   if ( mUndoWidget )
@@ -584,6 +588,8 @@ void QgisApp::activeLayerChanged( QgsMapLayer *layer )
     }
     updateUndoActions();
   }
+
+  emit activeLayerChanged( layer );
 }
 
 /*
@@ -1702,6 +1708,10 @@ QVector<QPointer<QgsLayoutCustomDropHandler> > QgisApp::customLayoutDropHandlers
 
 void QgisApp::handleDropUriList( const QgsMimeDataUtils::UriList &lst )
 {
+  // avoid unnecessary work when adding lots of layers at once - defer emitting the active layer changed signal until we've
+  // added all layers, and only emit the signal once for the final layer added
+  mBlockActiveLayerChanged = true;
+
   // insert items in reverse order as each one is inserted on top of previous one
   for ( int i = lst.size() - 1 ; i >= 0 ; i-- )
   {
@@ -1742,8 +1752,10 @@ void QgisApp::handleDropUriList( const QgsMimeDataUtils::UriList &lst )
       openFile( u.uri, QStringLiteral( "project" ) );
     }
   }
-}
 
+  mBlockActiveLayerChanged = false;
+  emit activeLayerChanged( activeLayer() );
+}
 
 bool QgisApp::event( QEvent *event )
 {
@@ -3339,9 +3351,9 @@ void QgisApp::setupConnections()
   } );
 
   // connect legend signals
-  connect( mLayerTreeView, &QgsLayerTreeView::currentLayerChanged,
+  connect( this, &QgisApp::activeLayerChanged,
            this, &QgisApp::activateDeactivateLayerRelatedActions );
-  connect( mLayerTreeView, &QgsLayerTreeView::currentLayerChanged,
+  connect( this, &QgisApp::activeLayerChanged,
            this, &QgisApp::setMapStyleDockLayer );
 
   connect( mLayerTreeView->selectionModel(), &QItemSelectionModel::selectionChanged,
@@ -3837,7 +3849,7 @@ void QgisApp::initLayerTreeView()
   setupLayerTreeViewFromSettings();
 
   connect( mLayerTreeView, &QAbstractItemView::doubleClicked, this, &QgisApp::layerTreeViewDoubleClicked );
-  connect( mLayerTreeView, &QgsLayerTreeView::currentLayerChanged, this, &QgisApp::activeLayerChanged );
+  connect( mLayerTreeView, &QgsLayerTreeView::currentLayerChanged, this, &QgisApp::onActiveLayerChanged );
   connect( mLayerTreeView->selectionModel(), &QItemSelectionModel::currentChanged, this, &QgisApp::updateNewLayerInsertionPoint );
   connect( QgsProject::instance()->layerTreeRegistryBridge(), &QgsLayerTreeRegistryBridge::addedLayersToLayerTree,
            this, &QgisApp::autoSelectAddedLayer );
@@ -11172,7 +11184,14 @@ void QgisApp::closeProject()
   mMapCanvas->clearCache();
   mOverviewCanvas->setLayers( QList<QgsMapLayer *>() );
   mMapCanvas->freeze( false );
+
+  // Avoid unnecessary layer changed handling for each layer removed - instead,
+  // defer the handling until we've removed all layers
+  mBlockActiveLayerChanged = true;
   QgsProject::instance()->clear();
+  mBlockActiveLayerChanged = false;
+
+  emit activeLayerChanged( activeLayer() );
 }
 
 

src/app/qgisapp.h

@@ -719,7 +719,7 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
     void updateNewLayerInsertionPoint();
     //! connected to layer tree registry bridge, selects first of the newly added map layers
     void autoSelectAddedLayer( QList<QgsMapLayer *> layers );
-    void activeLayerChanged( QgsMapLayer *layer );
+
     //! Zoom to full extent
     void zoomFull();
     //! Zoom to the previous extent
@@ -1664,6 +1664,8 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
     //! handle project crs changes
     void projectCrsChanged();
 
+    void onActiveLayerChanged( QgsMapLayer *layer );
+
   signals:
 
     /**
@@ -1737,6 +1739,11 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
      */
     void layerSavedAs( QgsMapLayer *l, const QString &path );
 
+    /**
+     * Emitted when the active layer is changed.
+     */
+    void activeLayerChanged( QgsMapLayer *layer );
+
   private:
     void startProfile( const QString &name );
     void endProfile();
@@ -2285,6 +2292,9 @@ class APP_EXPORT QgisApp : public QMainWindow, private Ui::MainWindow
 
     QgsProxyProgressTask *mProjectLoadingProxyTask = nullptr;
 
+    //! True if we are blocking the activeLayerChanged signal from being emitted
+    bool mBlockActiveLayerChanged = false;
+
     friend class TestQgisAppPython;
 };
 

src/app/qgisappinterface.cpp

@@ -53,7 +53,7 @@ QgisAppInterface::QgisAppInterface( QgisApp *_qgis )
   , pluginManagerIface( _qgis->pluginManager() )
 {
   // connect signals
-  connect( qgis->layerTreeView(), &QgsLayerTreeView::currentLayerChanged,
+  connect( qgis, &QgisApp::activeLayerChanged,
            this, &QgisInterface::currentLayerChanged );
   connect( qgis, &QgisApp::currentThemeChanged,
            this, &QgisAppInterface::currentThemeChanged );