[processing] Show complete expression context in expression builder

for pre-calculated expressions

Correctly exposes ALL the variables and functions available for use
in pre-calculated expressions so that users actually know they can
use these in their models!

python/gui/auto_generated/processing/qgsprocessingmodelerparameterwidget.sip.in

@@ -12,7 +12,7 @@
 
 
 
-class QgsProcessingModelerParameterWidget : QWidget
+class QgsProcessingModelerParameterWidget : QWidget, QgsExpressionContextGenerator
 {
 %Docstring
 
@@ -117,6 +117,9 @@ Returns the current value of the parameter.
 .. seealso:: :py:func:`setWidgetValue`
 %End
 
+    virtual QgsExpressionContext createExpressionContext() const;
+
+
 };
 
 

src/gui/processing/qgsprocessingmodelerparameterwidget.cpp

@@ -22,6 +22,7 @@
 #include "qgsprocessingguiregistry.h"
 #include "models/qgsprocessingmodelalgorithm.h"
 #include "qgsgui.h"
+#include "qgsexpressioncontext.h"
 #include <QHBoxLayout>
 #include <QToolButton>
 #include <QStackedWidget>
@@ -172,7 +173,23 @@ QgsProcessingModelChildParameterSource QgsProcessingModelerParameterWidget::valu
 
 QgsExpressionContext QgsProcessingModelerParameterWidget::createExpressionContext() const
 {
-  return QgsExpressionContext();
+  QgsExpressionContext c = mContext.expressionContext();
+  if ( mModel )
+  {
+    QgsExpressionContextScope *algorithmScope = QgsExpressionContextUtils::processingAlgorithmScope( mModel->childAlgorithm( mChildId ).algorithm(), QVariantMap(), mContext );
+    c << algorithmScope;
+    QgsExpressionContextScope *childScope = mModel->createExpressionContextScopeForChildAlgorithm( mChildId, mContext, QVariantMap(), QVariantMap() );
+    c << childScope;
+
+    QStringList highlightedVariables = childScope->variableNames();
+    QStringList highlightedFunctions = childScope->functionNames();
+    highlightedVariables += algorithmScope->variableNames();
+    highlightedFunctions += algorithmScope->functionNames();
+    c.setHighlightedVariables( highlightedVariables );
+    c.setHighlightedFunctions( highlightedFunctions );
+  }
+
+  return c;
 }
 
 void QgsProcessingModelerParameterWidget::sourceMenuAboutToShow()

src/gui/processing/qgsprocessingmodelerparameterwidget.h

@@ -143,6 +143,8 @@ class GUI_EXPORT QgsProcessingModelerParameterWidget : public QWidget, public Qg
      */
     virtual QgsProcessingModelChildParameterSource value() const;
 
+    QgsExpressionContext createExpressionContext() const override;
+
   private slots:
 
     void sourceMenuAboutToShow();