[processing] better update of expression widget when layer is changed in fields calculator

Author: volaya

python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py

@@ -90,22 +90,21 @@ def manageGui(self):
 
         self.builder.loadRecent('fieldcalc')
 
-        self.updateLayer()
+        self.initContext()
 
-    def updateLayer(self):
-        self.layer = dataobjects.getObject(self.cmbInputLayer.currentText())
-
-        self.builder.setLayer(self.layer)
-        self.builder.loadFieldNames()
-
-        exp_context = QgsExpressionContext()
+    def initContext(self):
+        exp_context = self.builder.expressionContext()
         exp_context.appendScope(QgsExpressionContextUtils.globalScope())
         exp_context.appendScope(QgsExpressionContextUtils.projectScope())
         exp_context.appendScope(QgsExpressionContextUtils.layerScope(self.layer))
         exp_context.lastScope().setVariable("row_number", 1)
         exp_context.setHighlightedVariables(["row_number"])
         self.builder.setExpressionContext(exp_context)
-
+        
+    def updateLayer(self):
+        self.layer = dataobjects.getObject(self.cmbInputLayer.currentText())
+        self.builder.setLayer(self.layer)
+        self.builder.loadFieldNames()
         self.populateFields()
 
     def setupSpinboxes(self, index):