provide method for controlling which macro from which template will be used for rendering

composer/schema/field.py

@@ -199,6 +199,9 @@ class FieldInstance(object):
         self.errors.append(error)
         self.severity = max(error.severity, self.severity)
 
+    def getRenderer(self, name):
+        return None
+
 
 class NumberFieldInstance(FieldInstance):
 

composer/schema/interfaces.py

@@ -278,6 +278,14 @@ class IFieldInstance(Interface):
             allow for checking more than one data element.
         """
 
+    def getRenderer(name):
+        """ Return the renderer object (e.g. a ZPT macro) for this field
+            with the name given.
+
+            May return None in which case the calling application should
+            use a default renderer.
+        """
+
 
 class IFormState(Interface):
     """ Represents the state of all fields when editing.