In may games that I have played, I have seen events being triggered, such as when you walk into a certain land area while holding a specific object, it will trigger a special creature to spawn.

I was wondering, how do games deal with events such as this? Not in a specific game, but in general among games.

The first thought I had was that each place has a hard-coded set of events that it will call when something happens there. However, that would be too inefficient to maintain, as when something new is added, that would require modification of every part of the game that would potentially cause the event to be called.

Next up, I had the idea of maybe how GUI programming works. In all of the GUI programming I've done, you create a component and a callback function, or a listener. Then, when the user interacts when the button, the callback function is called, allowing you to do something with it. So, I was thinking that in terms of a game, when a land area gets loaded the game loops over a list of all events, creating instances of them and calling public methods to bind them to the current scene. The events themselves then handle what scene it is, and if it is a scene that pertains to the event, will call the public method of the scene to bind the event to an action. Then, when the action takes place, the scene would call all events that are bound to that action.

However, I'm sure that's not how games would operate either, as that would require a lot of creating of events all the time.

So how to video games handle events, are either of those methods correct, or is it something completely different?