Event manager

• Event types are defined at compile-time
• Event handlers are pushed back at run-time
• Each event type can be registered with several handlers at run-time
• Each event handler is a callable object
  • It might be with type std::function if needs to capture some info
    • For example: capture self state::CardRef
  • If possible, prefer to use pure function pointer for performance concern
• An event handler cannot be removed by outsiders
  • It can only be removed when the item is triggered, and returns false
    • Pros: quickly clone game state
    • Cons: each event handler needs to know when to remove itself