You might want to give a messaging/callback system a chance

e.g.: you got a game API and want to talk to the rest of the engine

a) either send messages, those however might arrive delayed

b) use callbacks, you register them one level above your engine & API layer at startup, this is pretty flexible, and the callbacks should do the data transformation to fit the data in the engine/API independent data structures.

both systems need to be designed in a way that the rough control flow isn t depending on each other.