Hello @user168462,

Thank you for reaching out to us. You said,

"I guess there is no way to observe a tag if you are not implementing it."

It depends a bit on how you mean that and how you mean your question in general. @C4DS gave you already a very nice answer (thanks for that). There are basically two scenarios (which effectively will become one):

There is a specific BaseTag instance tag where you did not implement the tag type yourself and you want to track changes for that specific tag instance for a specific parameter. There is a BaseTag type t where you did not implement the tag type yourself and you want to track changes of a parameter for all tags of that type in a scene.

One might be tempted to solve option 1 by simply storing a pointer to that BaseTag and then periodically look up the tag to track changes. But this will not work due to node reallocation. This problem can be solved by either using the Baselink type or by manually traversing a document tree/graph and identifying nodes by their unique id stored under MAXON_CREATOR_ID. This second approach (traversing the whole document tree) is also what is the solution to option 2.

To carry out the periodical checking, the most straight forward option is to the core message EVMSG_CHANGE. You can do this in a MessageData plugin or GeDialog derived GUIs. An alternative approach is to implement a SceneHookData plugin and do the traversing in its Execute() method.

TLDR: While the Cinema 4D classic API does not have a dependency graph or a very granular event system, scene changes can be tracked with EVMSG_CHANGE. It is only that it is up to the user to find out if the change that occurred is of relevance as Cinema 4D does not give any context for the cause of EVMSG_CHANGE.

Cheers,
Ferdinand

Cinema 4D R21 Released