Use feature from expression context in expression builder preview

nyalldawson · nyalldawson · commit a13ef7bdc121 · 2017-01-12T16:09:40.000+10:00
instead of always getting first feature from the layer

This means that e.g. expression builder previews in composer atlas
will use the current atlas preview feature

Also remove an unnecessary restriction that blocks the preview if
a layer has no features
diff --git a/src/gui/qgsexpressionbuilderwidget.cpp b/src/gui/qgsexpressionbuilderwidget.cpp
@@ -543,33 +543,19 @@ void QgsExpressionBuilderWidget::on_txtExpressionString_textChanged()
     // Only set calculator if we have layer, else use default.
     exp.setGeomCalculator( &mDa );
 
-    if ( !mFeature.isValid() )
+    if ( !mExpressionContext.feature().isValid() )
     {
-      mLayer->getFeatures().nextFeature( mFeature );
-    }
-
-    if ( mFeature.isValid() )
-    {
-      mExpressionContext.setFeature( mFeature );
-      QVariant value = exp.evaluate( &mExpressionContext );
-      if ( !exp.hasEvalError() )
-        lblPreview->setText( QgsExpression::formatPreviewString( value ) );
-    }
-    else
-    {
-      // The feature is invalid because we don't have one but that doesn't mean user can't
-      // build a expression string.  They just get no preview.
-      lblPreview->setText( QLatin1String( "" ) );
+      // no feature passed yet, try to get from layer
+      QgsFeature f;
+      mLayer->getFeatures().nextFeature( f );
+      mExpressionContext.setFeature( f );
     }
   }
-  else
+
+  QVariant value = exp.evaluate( &mExpressionContext );
+  if ( !exp.hasEvalError() )
   {
-    // No layer defined
-    QVariant value = exp.evaluate( &mExpressionContext );
-    if ( !exp.hasEvalError() )
-    {
-      lblPreview->setText( QgsExpression::formatPreviewString( value ) );
-    }
+    lblPreview->setText( QgsExpression::formatPreviewString( value ) );
   }
 
   if ( exp.hasParserError() || exp.hasEvalError() )
diff --git a/src/gui/qgsexpressionbuilderwidget.h b/src/gui/qgsexpressionbuilderwidget.h
@@ -307,7 +307,6 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
     QSortFilterProxyModel *mProxyValues;
     QgsExpressionItemSearchProxy *mProxyModel;
     QMap<QString, QgsExpressionItem*> mExpressionGroups;
-    QgsFeature mFeature;
     QgsExpressionHighlighter* highlighter;
     bool mExpressionValid;
     QgsDistanceArea mDa;
