Draft: migrate individual properties

Since we know how the old class looks like,
we can write code to save the value of the older properties,
and then remove them.

Then we can assign the new class, which will create new properties,
and then we can assign the old values to these.

This can be done for both the proxy object and
for the viewprovider.

src/Mod/Draft/DraftFillet.py

@@ -101,8 +101,90 @@ class Fillet(base.DraftObject):
         _wrn("v0.19, {0}, '{1}' object ".format(obj.Label, _module)
              + "will be migrated to 'draftobjects.fillet.Fillet'")
 
+        # Save the old properties and delete them
+        old_dict = _save_properties0_19_to_0_19(obj)
+
+        # We assign the new class, which could have different properties
+        # from the older class. Since we removed the older properties
+        # we know the new properties will not collide with the old properties.
+        # The new class itself should handle some logic so that it does not
+        # add already existing properties of the same name and type.
         draftobjects.fillet.Fillet(obj)
 
+        # Assign the old properties
+        obj = _assign_properties0_19_to_0_19(obj, old_dict)
+
+        # The same is done for the viewprovider.
         if App.GuiUp:
             vobj = obj.ViewObject
+            old_dict = _save_vproperties0_19_to_0_19(vobj)
             view_fillet.ViewProviderFillet(vobj)
+            _assign_vproperties0_19_to_0_19(vobj, old_dict)
+
+
+def _save_properties0_19_to_0_19(obj):
+    """Save the old property values and remove the old properties.
+
+    Since we know the structure of the older Proxy class,
+    we can take its old values and store them before
+    we remove the property.
+
+    We do not need to save the old properties if these
+    can be recalculated from the new data.
+    """
+    _wrn("Old property values saved, old properties removed.")
+    old_dict = dict()
+    if hasattr(obj, "Length"):
+        old_dict["Length"] = obj.Length
+        obj.removeProperty("Length")
+    if hasattr(obj, "Start"):
+        old_dict["Start"] = obj.Start
+        obj.removeProperty("Start")
+    if hasattr(obj, "End"):
+        old_dict["End"] = obj.End
+        obj.removeProperty("End")
+    if hasattr(obj, "FilletRadius"):
+        old_dict["FilletRadius"] = obj.FilletRadius
+        obj.removeProperty("FilletRadius")
+    return old_dict
+
+
+def _assign_properties0_19_to_0_19(obj, old_dict):
+    """Assign the new properties from the old properties.
+
+    If new properties are named differently than the older properties
+    or if the old values need to be transformed because the class
+    now manages differently the data, this can be done here.
+    Otherwise simple assigning the old values is possible.
+    """
+    _wrn("New property values added.")
+    if hasattr(obj, "Length"):
+        obj.Length = old_dict["Length"]
+    if hasattr(obj, "Start"):
+        obj.Start = old_dict["Start"]
+    if hasattr(obj, "End"):
+        obj.End = old_dict["End"]
+    if hasattr(obj, "FilletRadius"):
+        obj.FilletRadius = old_dict["FilletRadius"]
+    return obj
+
+
+def _save_vproperties0_19_to_0_19(vobj):
+    """Save the old property values and remove the old properties.
+
+    The view provider didn't add new properties so this just returns
+    an empty element.
+    """
+    _wrn("Old view property values saved, old view properties removed. NONE.")
+    old_dict = dict()
+    return old_dict
+
+
+def _assign_vproperties0_19_to_0_19(vobj, old_dict):
+    """Assign the new properties from the old properties.
+
+    The view provider didn't add new properties so this just returns
+    the same viewprovider.
+    """
+    _wrn("New view property values added. NONE.")
+    return vobj