Events using Simple Event Subsystem

What is the Simple Event Subsystem?

The Simple Event Subsystem is a subsystem that lets different parts of your game talk to each other using gameplay tags with arbitrary struct payloads.
It’s like a message board where game systems can post updates and others can listen for specific types of messages they care about.

Why Use an Event System?

Game systems often need to communicate with each other:

• Your UI needs to know when the player’s health changes
• Your achievement system needs to know when an enemy is defeated
• Your audio system needs to know when to play a sound effect

Rather than creating direct dependencies between these systems, the event subsystem lets them communicate indirectly through events. This creates more flexible, maintainable code.

How It Works

You can access the subsystem from any blueprint that can access the GameInstance (e.g. Actors, ActorComponents, PlayerController, Widgets) using the GetSimpleEventSubsystem node.
Do this by right-clicking in the blueprint editor and searching for GetSimpleEventSubsystem.

Sending Events

• Senders dispatch events with these arguments:
  • Event Tag: describes what happened (e.g., “Player.TookDamage”)
  • Domain Tag: categorizes the event (e.g., “DamageType.Magic”)
  • (optional) Payload: adds additional context
    • This can be any struct type, and is passed in as an instanced struct
  • (optional) Sender: an actor reference representing the source of the event
  • (optional) ListenerFilter: an actor array representing the listeners that should receive the event

Listening for Events

• Listeners subscribe to events with these arguments:
  • Listener: the object that will execute the callback function
  • OnlyTriggerOnce: a boolean that determines if the listener should only trigger once and then stop listening or trigger every time the event is sent
  • (optional) Event Filter: a gameplay tag container that filters the events the listener is interested in. If left empty, the listener will accept any event.
  • (optional) Domain Filter: a gameplay tag container that filters the domains the listener is interested in. If left empty, the listener will accept any domain.
  • Event Received Delegate: the function that will be called when the event is received
  • (optional) Payload Filter: an array of struct types that the listener is interested in. If left empty, the listener will accept any payload.
  • (optional) Sender Filter: an array of actors that the listener is interested in. If left empty, the listener will accept events from all senders.
  • OnlyMatchExactEvent: a boolean that determines if the listener should only trigger if the event tag matches exactly or if it can match any parent tags.
  • OnlyMatchExactDomain: a boolean that determines if the listener should only trigger if the domain tag matches exactly or if it can match any parent tags.
• Calling ListenForEvent will return a GUID that identifies the event subscription. You can use this GUID to stop listening for the event later.

For convenience there is also an async version of the ListenForEvent function:

Stopping Listening

When you’re no longer interested in events, you can stop listening by calling one of these functions:

Tips

• To make creating callback functions in ListenForEvent easier, drag off the EventReceivedDelegate pin and select Create Event to create or select an event function