For those who, like me, are faced with this task, I can recommend the following 
If you are developing a singleplayer game, you can use the RepNotify for Replication setting of the varible and use generated OnRep_ function to call the event on the varible change.
This method works in single mode because
Standalone: The game is running as a server, that does not accept connections from remote clients. Any players participating in the game are strictly local players. This mode is used for single-player and local multiplayer games. It will run both server-side logic and client-side logic as appropriate for the local players.
However, for those who, like me, are working on a multiplayer project and want to do this only on the client and at the same time not overload the server and network - alas, I don’t have an exact way to solve this yet.
Yes, you can create and use a “set function” instead of a just “Set”, but this option will only work when changing the values (calling the function) in the blueprints, and not when the value of the variable is directly changed (like in maping).
And of course, you can always use some C++ custom class and code, but we’re not here for that 
So for now, I can only suggest a method that is used in the Lyra project - create a second variable that will be every tick/period of time, check if its value differs from the target one and when this happens - call the event and update this variable value.
