Update layer variables for expression builder (#41417)

Fixes #40255

(cherry picked from commit 76cc050)

src/gui/qgsexpressionbuilderwidget.cpp

@@ -286,11 +286,18 @@ void QgsExpressionBuilderWidget::setLayer( QgsVectorLayer *layer )
   if ( mLayer )
   {
     mExpressionContext << QgsExpressionContextUtils::layerScope( mLayer );
-
+    expressionContextUpdated();
     txtExpressionString->setFields( mLayer->fields() );
   }
 }
 
+void QgsExpressionBuilderWidget::expressionContextUpdated()
+{
+  txtExpressionString->setExpressionContext( mExpressionContext );
+  mExpressionTreeView->setExpressionContext( mExpressionContext );
+  mExpressionPreviewWidget->setExpressionContext( mExpressionContext );
+}
+
 QgsVectorLayer *QgsExpressionBuilderWidget::layer() const
 {
   return mLayer;
@@ -627,9 +634,7 @@ void QgsExpressionBuilderWidget::setExpectedOutputFormat( const QString &expecte
 void QgsExpressionBuilderWidget::setExpressionContext( const QgsExpressionContext &context )
 {
   mExpressionContext = context;
-  txtExpressionString->setExpressionContext( mExpressionContext );
-  mExpressionTreeView->setExpressionContext( context );
-  mExpressionPreviewWidget->setExpressionContext( context );
+  expressionContextUpdated();
 }
 
 void QgsExpressionBuilderWidget::txtExpressionString_textChanged()

src/gui/qgsexpressionbuilderwidget.h

@@ -417,6 +417,9 @@ class GUI_EXPORT QgsExpressionBuilderWidget : public QWidget, private Ui::QgsExp
     QString loadFunctionHelp( QgsExpressionItem *functionName );
     QString helpStylesheet() const;
 
+    // To be called whenever expression context has been updated
+    void expressionContextUpdated();
+
     // Will hold items with
     // * a display string that matches the represented field values
     // * custom data in Qt::UserRole + 1 that contains a ready to use expression literal ('quoted string' or NULL or a plain number )

tests/src/python/test_qgsexpressionbuilderwidget.py

@@ -194,6 +194,26 @@ def testStoredExpressions(self):
         items = w.findExpressions('Stored Expression Number One')
         self.assertEqual(len(items), 0)
 
+    def testLayerVariables(self):
+        """ check through widget model to ensure it is populated with layer variables """
+        w = QgsExpressionBuilderWidget()
+        m = w.model()
+
+        p = QgsProject.instance()
+        layer = QgsVectorLayer("Point", "layer1", "memory")
+        p.addMapLayers([layer])
+
+        w.setLayer(layer)
+
+        items = m.findItems("layer", Qt.MatchRecursive)
+        self.assertEqual(len(items), 1)
+        items = m.findItems("layer_id", Qt.MatchRecursive)
+        self.assertEqual(len(items), 1)
+        items = m.findItems("layer_name", Qt.MatchRecursive)
+        self.assertEqual(len(items), 1)
+
+        p.removeMapLayer(layer)
+
 
 if __name__ == '__main__':
     unittest.main()