quick frontend(quick widgets): added the ability to use a callable with getOrCreateWidget's on_existing_widget

frontends/src/quick_frontend/quick_widgets.py

@@ -111,6 +111,7 @@ class QuickWidgetsManager(object):
                 C.WIDGET_KEEP  [default]: return the existing widget
                 C.WIDGET_RAISE: raise WidgetAlreadyExistsError
                 C.WIDGET_RECREATE: create a new widget *WITH A NEW HASH*
+                [callable]: this method will be called with existing widget as argument
             if 'force_hash' is present, the hash given in value will be used instead of the one returned by class_.getWidgetHash
             other keys will be used to instanciate class_ if the case happen (e.g. if type_ is present and class_ is a QuickChat subclass,
                 it will be used to create a new QuickChat instance).
@@ -173,6 +174,8 @@ class QuickWidgetsManager(object):
                 assert on_new_widget is None
         else:
             # the widget already exists
+            if on_existing_widget == C.WIDGET_KEEP:
+                pass
             if on_existing_widget == C.WIDGET_RAISE:
                 raise WidgetAlreadyExistsError(hash_)
             elif on_existing_widget == C.WIDGET_RECREATE:
@@ -200,6 +203,11 @@ class QuickWidgetsManager(object):
                     else:
                         log.debug(u"Widget already exists, a new one has been recreated with hash {}".format(new_kwargs['force_hash']))
                         break
+            elif callable(on_existing_widget):
+                on_existing_widget(widget)
+            else:
+                raise exceptions.InternalError("Unexpected on_existing_widget value ({})".format(on_existing_widget))
+
         return widget
 
     def deleteWidget(self, widget_to_delete):