Notification of metatable change?

[Marc Lepage <mlepage@...>]

I'm wondering if there's a good way to know if a metatable for an object is changed.

Suppose the case is that the metatable is not protected. An object wants to wrap its metatable with another, so it can be extended. But if the metatable is changed, it needs to re-wrap it.

Is there a good way the object can know when that occurs, so it can perform the re-wrapping?

I thought of re-writing setmetatable to perform a notification. I guess that would work, but it seems heavy-handed.

I thought of requiring clients to call a different function that does the notification (or even the wrapping), but that requires editing clients.

It would be cool if objects could implement __setmetatable, which would be called if available, when the metatable is set. It would effectively replace __metatable, because you could always just ignore it (do nothing) if you don't want the metatable to be changeable.

Does anyone have advice on this topic? Thanks.