Use first feature in layer to generate symbol preview icon in widget

nyalldawson · nyalldawson · commit 23dc1c8f696c · 2018-02-20T07:25:33.000+11:00
Uses the first feature found in a layer in order to create a better preview for the symbol in the symbol selector dialog. This allows data defined settings to be evaluated correctly (at least, for the first feature) and avoids missing previews due to missing attribute values when data defined settings are present. Fixes #17061
diff --git a/src/gui/symbology/qgssymbolselectordialog.cpp b/src/gui/symbology/qgssymbolselectordialog.cpp
@@ -31,6 +31,8 @@
 #include "qgslogger.h"
 #include "qgsapplication.h"
 #include "qgssettings.h"
+#include "qgsfeatureiterator.h"
+#include "qgsvectorlayer.h"
 
 #include <QColorDialog>
 #include <QPainter>
@@ -247,6 +249,19 @@ QgsSymbolSelectorWidget::QgsSymbolSelectorWidget( QgsSymbol *symbol, QgsStyle *s
   layersTree->setModel( model );
   layersTree->setHeaderHidden( true );
 
+  //get first feature from layer for previews
+  if ( mVectorLayer )
+  {
+    QgsFeatureIterator it = mVectorLayer->getFeatures( QgsFeatureRequest().setLimit( 1 ) );
+    it.nextFeature( mPreviewFeature );
+    mPreviewExpressionContext.appendScopes( QgsExpressionContextUtils::globalProjectLayerScopes( mVectorLayer ) );
+    mPreviewExpressionContext.setFeature( mPreviewFeature );
+  }
+  else
+  {
+    mPreviewExpressionContext.appendScopes( QgsExpressionContextUtils::globalProjectLayerScopes( nullptr ) );
+  }
+
   QItemSelectionModel *selModel = layersTree->selectionModel();
   connect( selModel, &QItemSelectionModel::currentChanged, this, &QgsSymbolSelectorWidget::layerChanged );
 
@@ -285,6 +300,15 @@ void QgsSymbolSelectorWidget::setContext( const QgsSymbolWidgetContext &context
 {
   mContext = context;
 
+  if ( mContext.expressionContext() )
+  {
+    mPreviewExpressionContext = *mContext.expressionContext();
+    if ( mVectorLayer )
+      mPreviewExpressionContext.appendScope( QgsExpressionContextUtils::layerScope( mVectorLayer ) );
+
+    mPreviewExpressionContext.setFeature( mPreviewFeature );
+  }
+
   QWidget *widget = stackedWidget->currentWidget();
   QgsLayerPropertiesWidget *layerProp = dynamic_cast< QgsLayerPropertiesWidget * >( widget );
   QgsSymbolsListWidget *listWidget = dynamic_cast< QgsSymbolsListWidget * >( widget );
@@ -362,7 +386,7 @@ void QgsSymbolSelectorWidget::updateUi()
 
 void QgsSymbolSelectorWidget::updatePreview()
 {
-  QImage preview = mSymbol->bigSymbolPreviewImage( mContext.expressionContext() );
+  QImage preview = mSymbol->bigSymbolPreviewImage( &mPreviewExpressionContext );
   lblPreview->setPixmap( QPixmap::fromImage( preview ) );
   // Hope this is a appropriate place
   emit symbolModified();
diff --git a/src/gui/symbology/qgssymbolselectordialog.h b/src/gui/symbology/qgssymbolselectordialog.h
@@ -250,6 +250,9 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
   private:
     std::unique_ptr<DataDefinedRestorer> mDataDefineRestorer;
     QgsSymbolWidgetContext mContext;
+    QgsFeature mPreviewFeature;
+    QgsExpressionContext mPreviewExpressionContext;
+
 };
 
 /**
