observable VS Nfy

Lots of low-level answers here... Any chance you're looking for something higher-level like an observable library? (I'm not advocating for that one in particular, as I've never used it, but its readme demonstrates the idea well.)

For me, the answer is a generic multicast notification system. It utilizes a template based approach that allows you to define prototype messages using a class-based interface that you wish to send to another library. For example, your input library might respond to things like a button down / up, mouse clicks, etc, etc. So you'd create an interface that defines all your input notifications as functions. I won't go into detail, because it's well documented in that link.

In my own code, I make use a generic notification system which allows high-level systems to register themselves with lower-level libraries to help invert dependencies. An example of this is how my game engine's UI library can register itself with the Input library, so it can receive mouse, keyboard, and gamepad events, necessary for manipulating UI controls. But the Game itself also needs to register for some UI events itself, not to mention debug functionality like ImGui for debug windows, etc. So I use this notification system to ensure the low-level Input library can push events out to multiple subscribers without needing to know about them.

See if this is close to what you're looking for: https://github.com/JamesBoer/Nfy