API tweaks

Author: nyalldawson

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

@@ -291,6 +291,17 @@ current value is associated with.
 Returns the current value of the parameter.
 
 .. seealso:: :py:func:`setWidgetValue`
+%End
+
+    virtual const QgsVectorLayer *linkedVectorLayer() const;
+%Docstring
+Returns the optional vector layer associated with this widget wrapper, or None if no vector
+layer is applicable.
+
+This is used to correctly generate expression contexts within the GUI, e.g. to allow expression
+buttons and property override buttons to correctly show the appropriate vector layer fields.
+
+.. versionadded:: 3.6
 %End
 
   protected:

src/gui/processing/qgsprocessingwidgetwrapper.cpp

@@ -187,6 +187,13 @@ QLabel *QgsAbstractProcessingParameterWidgetWrapper::createLabel()
   return nullptr;
 }
 
+const QgsVectorLayer *QgsAbstractProcessingParameterWidgetWrapper::linkedVectorLayer() const
+{
+  if ( mPropertyButton )
+    return mPropertyButton->vectorLayer();
+  return nullptr;
+}
+
 void QgsAbstractProcessingParameterWidgetWrapper::postInitialize( const QList<QgsAbstractProcessingParameterWidgetWrapper *> &wrappers )
 {
   switch ( mType )
@@ -230,8 +237,8 @@ QgsExpressionContext QgsAbstractProcessingParameterWidgetWrapper::createExpressi
 
   QgsExpressionContext c = context->expressionContext();
 
-  if ( mPropertyButton->vectorLayer() )
-    c << QgsExpressionContextUtils::layerScope( mPropertyButton->vectorLayer() );
+  if ( linkedVectorLayer() )
+    c << QgsExpressionContextUtils::layerScope( linkedVectorLayer() );
 
   if ( mWidgetContext.model() )
   {

src/gui/processing/qgsprocessingwidgetwrapper.h

@@ -320,6 +320,17 @@ class GUI_EXPORT QgsAbstractProcessingParameterWidgetWrapper : public QObject, p
      */
     virtual QVariant widgetValue() const = 0;
 
+    /**
+     * Returns the optional vector layer associated with this widget wrapper, or nullptr if no vector
+     * layer is applicable.
+     *
+     * This is used to correctly generate expression contexts within the GUI, e.g. to allow expression
+     * buttons and property override buttons to correctly show the appropriate vector layer fields.
+     *
+     * \since QGIS 3.6
+     */
+    virtual const QgsVectorLayer *linkedVectorLayer() const;
+
   protected:
 
     QgsProcessingContextGenerator *mProcessingContextGenerator = nullptr;