Implement ObjectListener

[renatomassaro]

The idea behind ObjectListener is to create a global listener on game objects that react to certain events, registering a callback when that event occurs.

Sample use case:

• Mission generates file X, which needs to be downloaded. Player accidentally deletes/hides/encrypts file X. File X should be regenerated.

Solution: Add a listener:

listener file_id, [:delete, :hide, :encrypt], &regenerate/1

def regenerate(event) do
  # Regenerate the file, change mission metadata, etc.
end

Also possibly useful:

ObjectBlocker. Similar to Listener, but performs the check before the action, blocking it in some cases. So, on the above example, it would be possible to not allow the file X to be deleted on the first place.

obj_block file_id, [:delete, :hide, :encrypt], "reason"

All macros for illustrative purposes only.

[renatomassaro]

ObjectBlocker cannot be implemented in a similar fashion to ObjectListener, it would be extremely inefficient.

• ObjectBlocker must be synchronous, since it may have to stop the event/request from going on.
• This synchronous step would need to happen before every event is emitted, so at Helix.Event.emit/1.

Contrast this to ObjectListener, which:

• Is asynchronous
• Runs on a different process (does not block the main request/event flow).

So, if a ObjectBlocker-like behavior is desired, it must be implemented at the Henforcer level:

• It is synchronous/blocking, but it must be...
• At least it only blocks when actual verification is required
• So at most a few verifications per HTTP request, as opposed to once everytime an Event.t is emitted.
• It can still be composable / reusable if implemented correctly with a mix of Henforcer and ObjectListener implementation pattern.