[processing] When running in toolbox mode, set the source layer for

the field expression widgets in the refactor fields widget so that
the expression editor can generate feature based previews

Fixes #37912

python/gui/auto_generated/qgsfieldmappingwidget.sip.in

@@ -79,6 +79,22 @@ Returns the selection model
     void setSourceFields( const QgsFields &sourceFields );
 %Docstring
 Set source fields of the underlying mapping model to ``sourceFields``
+%End
+
+    void setSourceLayer( QgsVectorLayer *layer );
+%Docstring
+Sets a source ``layer`` to use when generating expression previews in the widget.
+
+.. versionadded:: 3.16
+%End
+
+    QgsVectorLayer *sourceLayer();
+%Docstring
+Returns the source layer for use when generating expression previews.
+
+Returned value may be ``None``.
+
+.. versionadded:: 3.16
 %End
 
     void setDestinationFields( const QgsFields &destinationFields,

src/gui/processing/qgsprocessingfieldmapwidgetwrapper.cpp

@@ -69,6 +69,7 @@ void QgsProcessingFieldMapPanelWidget::setLayer( QgsVectorLayer *layer )
     return;
 
   mLayer = layer;
+  mFieldsView->setSourceLayer( mLayer );
   if ( mModel->rowCount() == 0 )
   {
     loadFieldsFromLayer();

src/gui/qgsfieldmappingwidget.cpp

@@ -18,6 +18,7 @@
 #include "qgsfieldexpressionwidget.h"
 #include "qgsexpression.h"
 #include "qgsprocessingaggregatewidgets.h"
+#include "qgsvectorlayer.h"
 
 #include <QTableView>
 #include <QVBoxLayout>
@@ -45,7 +46,7 @@ QgsFieldMappingWidget::QgsFieldMappingWidget( QWidget *parent,
 #endif
 
   mTableView->setModel( mModel );
-  mTableView->setItemDelegateForColumn( static_cast<int>( QgsFieldMappingModel::ColumnDataIndex::SourceExpression ), new ExpressionDelegate( mTableView ) );
+  mTableView->setItemDelegateForColumn( static_cast<int>( QgsFieldMappingModel::ColumnDataIndex::SourceExpression ), new ExpressionDelegate( this ) );
   mTableView->setItemDelegateForColumn( static_cast<int>( QgsFieldMappingModel::ColumnDataIndex::DestinationType ), new TypeDelegate( mTableView ) );
   updateColumns();
   // Make sure columns are updated when rows are added
@@ -98,6 +99,16 @@ void QgsFieldMappingWidget::setSourceFields( const QgsFields &sourceFields )
   model()->setSourceFields( sourceFields );
 }
 
+void QgsFieldMappingWidget::setSourceLayer( QgsVectorLayer *layer )
+{
+  mSourceLayer = layer;
+}
+
+QgsVectorLayer *QgsFieldMappingWidget::sourceLayer()
+{
+  return mSourceLayer;
+}
+
 void QgsFieldMappingWidget::setDestinationFields( const QgsFields &destinationFields, const QMap<QString, QString> &expressions )
 {
   model()->setDestinationFields( destinationFields, expressions );
@@ -257,6 +268,13 @@ QWidget *QgsFieldMappingWidget::ExpressionDelegate::createEditor( QWidget *paren
   {
     Q_ASSERT( false );
   }
+
+  if ( QgsFieldMappingWidget *mappingWidget = qobject_cast< QgsFieldMappingWidget *>( ExpressionDelegate::parent() ) )
+  {
+    if ( mappingWidget->sourceLayer() )
+      editor->setLayer( mappingWidget->sourceLayer() );
+  }
+
   editor->setField( index.model()->data( index, Qt::DisplayRole ).toString() );
   connect( editor,
            qgis::overload<const  QString &, bool >::of( &QgsFieldExpressionWidget::fieldChanged ),

src/gui/qgsfieldmappingwidget.h

@@ -19,6 +19,7 @@
 #include <QWidget>
 #include <QAbstractTableModel>
 #include <QStyledItemDelegate>
+#include <QPointer>
 
 #include "qgis_gui.h"
 #include "qgsfieldmappingmodel.h"
@@ -26,6 +27,7 @@
 
 class QTableView;
 class QItemSelectionModel;
+class QgsVectorLayer;
 
 /**
  * \ingroup gui
@@ -85,6 +87,22 @@ class GUI_EXPORT QgsFieldMappingWidget : public QgsPanelWidget
     //! Set source fields of the underlying mapping model to \a sourceFields
     void setSourceFields( const QgsFields &sourceFields );
 
+    /**
+     * Sets a source \a layer to use when generating expression previews in the widget.
+     *
+     * \since QGIS 3.16
+     */
+    void setSourceLayer( QgsVectorLayer *layer );
+
+    /**
+     * Returns the source layer for use when generating expression previews.
+     *
+     * Returned value may be NULLPTR.
+     *
+     * \since QGIS 3.16
+     */
+    QgsVectorLayer *sourceLayer();
+
     /**
      * Set destination fields to \a destinationFields in the underlying model,
      * initial values for the expressions can be optionally specified through
@@ -130,6 +148,8 @@ class GUI_EXPORT QgsFieldMappingWidget : public QgsPanelWidget
 
     QTableView *mTableView = nullptr;
     QAbstractTableModel *mModel = nullptr;
+
+    QPointer< QgsVectorLayer > mSourceLayer;
     void updateColumns();
     //! Returns selected row indexes in ascending order
     std::list<int> selectedRows( );