Do not refresh stats when opening the stat summary field combo box

Without this change, stats are calculated when users simply
open the combo box to select a new field. With large datasets,
it freezes QGIS for a long time, and the combo box end up
never opening.

Author: nirvn

src/app/qgsstatisticalsummarydockwidget.cpp

@@ -88,7 +88,7 @@ QgsStatisticalSummaryDockWidget::QgsStatisticalSummaryDockWidget( QWidget *paren
   mFieldExpressionWidget->setLayer( mLayerComboBox->layer( 0 ) );
 
   connect( mLayerComboBox, &QgsMapLayerComboBox::layerChanged, this, &QgsStatisticalSummaryDockWidget::layerChanged );
-  connect( mFieldExpressionWidget, static_cast<void ( QgsFieldExpressionWidget::* )( const QString & )>( &QgsFieldExpressionWidget::fieldChanged ), this, &QgsStatisticalSummaryDockWidget::refreshStatistics );
+  connect( mFieldExpressionWidget, static_cast<void ( QgsFieldExpressionWidget::* )( const QString & )>( &QgsFieldExpressionWidget::fieldChanged ), this, &QgsStatisticalSummaryDockWidget::fieldChanged );
   connect( mSelectedOnlyCheckBox, &QAbstractButton::toggled, this, &QgsStatisticalSummaryDockWidget::refreshStatistics );
   connect( mButtonRefresh, &QAbstractButton::clicked, this, &QgsStatisticalSummaryDockWidget::refreshStatistics );
   connect( QgsProject::instance(), static_cast<void ( QgsProject::* )( const QStringList & )>( &QgsProject::layersWillBeRemoved ), this, &QgsStatisticalSummaryDockWidget::layersRemoved );
@@ -101,6 +101,15 @@ QgsStatisticalSummaryDockWidget::QgsStatisticalSummaryDockWidget( QWidget *paren
   refreshStatisticsMenu();
 }
 
+void QgsStatisticalSummaryDockWidget::fieldChanged()
+{
+  if ( mFieldExpressionWidget->expression() != mExpression )
+  {
+    mExpression = mFieldExpressionWidget->expression();
+    refreshStatistics();
+  }
+}
+
 void QgsStatisticalSummaryDockWidget::refreshStatistics()
 {
   if ( !mLayer || ( mFieldExpressionWidget->isExpression() && !mFieldExpressionWidget->isValidExpression() ) )

src/app/qgsstatisticalsummarydockwidget.h

@@ -58,6 +58,7 @@ class APP_EXPORT QgsStatisticalSummaryDockWidget : public QgsDockWidget, private
   private slots:
 
     void layerChanged( QgsMapLayer *layer );
+    void fieldChanged();
     void statActionTriggered( bool checked );
     void layersRemoved( const QStringList &layers );
     void layerSelectionChanged();
@@ -92,6 +93,8 @@ class APP_EXPORT QgsStatisticalSummaryDockWidget : public QgsDockWidget, private
     QMenu *mStatisticsMenu = nullptr;
     DataType mFieldType;
     DataType mPreviousFieldType;
+
+    QString mExpression;
 };
 
 #endif // QGSSTATISTICALSUMMARYDOCKWIDGET_H