Events

Overview

Events are what drive Graph execution. Most nodes will have some set of inputs and outputs, but without getting updates from the game state that exists outside the graph, the graph nodes will never execute.

To broadcast events to nodes in a graph, two structures are needed, a NodeEventListener and a NodeEventBroadcaster.

NodeEventBroadcaster

A NodeEventBroadcaster is an object that can be included on any game object that makes sense for your game. They can exist on world objects, players, entities, or anything else. Typically a specific Broadcaster will be associated with the object that holds it. That way you can do things like have an event that triggers when your entity collides with something. In that case, you would broadcast a collision event from the broadcaster which that specific entity owns. That event would then be broadcast to only the graphs that are listening to that broadcaster.

A broadcaster can be registered to any number of listeners, and can broadcast any kind of event.

Events may be broadcast by calling NodeEventBroadcaster::BroadcastEvent(event_id).

NodeEventListener

NodeEventListeners exist on the Node and are not instantiated directly. Instead, during the OnRegistration step, as many listeners as needed may be added to your node. For example:

// ...
static void OnRegister(NodeSignature* signature) {
  signature->AddInput<GameActor>();
  signature->AddListener(kCollisionEventId);
}
virtual void Initialize(NodeArguments* args) {
  GameActor* actor = args->GetInput<GameActor>(0);
  args->BindBroadcaster(0, actor->broadcaster);
}
// ...

In this example this node registers a single NodeEventListener that is listening for the event kCollisionEventId. During initialization, the actor associated with this node has it's broadcaster bound to that listener. Now any time that the actor fires a kCollisionEventId, this node will be marked dirty and have its Execute function (not shown here) called.