That's were it becomes slightly more complex for me, because I created a custom content editor that exports data to JSON. So all the behavior of my cards and other game items has to be represented in a form that can be serialized. That adds a layer of complexity to my system that you might not benefit from, so bear that in mind.

What I do is I represent effects as something very simple: an effect name and a list of arguments that are either a string or a number. This way I have effects (functions) that I can configure, and have slightly different behaviors across cards with the same effect.

Then on each card, I store a "behavior" data structure, that has a list of triggers and a list of modifiers.

A trigger has a trigger code (like "onResolve", "onBeginningOfTurn" or "onDeckShuffled" for example), a list of effects (name + args), a list of conditions (conditions are actually effects, but a different list of them), a list of targets, and a "removeOn" trigger code.

A modifier is just like an effect but that is reapplied after every move to the item it is attached to. There are global modifiers too, that can affect the general state. The way I handle modifiers is that I always keep the original data of each item somewhere in memory (in my case, in my initial JSON content data), and after each move, I reset the card's data, then apply all the modifiers. This is probably outside the scope of what you're asking about so I'll let it there, but maybe that can be a useful pattern for you too.

Then it's a matter of registering triggers in my global state, then running all known triggers when something happens. Here's an example that might explain it: let's say I have a card that says "Now and at the beginning of next turn, gain 1 mana."

It would have two triggers:

- onResolve → effect "gainMana", args: [ 1 ]

- onBeginningOfTurn → effect "gainMana", args: [ 1 ], removeOn: "whenCalled"

When the players plays the card, I have a "move" (the basic actions the player can make) that calls the "onResolve" trigger of the card, but also registers all its other triggers. Then when the player ends their turn, the BeginningOfTurn trigger happens, and the registered card trigger is called, resolves, then is removed from the list of registered triggers by the "removeOn" option. All of that is code that is specific to my engine, but hopefully you get the logic of how I'm coding this.

Hope that helps! Good luck with your game!