Allow annotated event listeners with only one method

[lucko]

I have a mod that (previously, and now) uses @SubscribeEvent annotations in listener classes. The mod has a number of these listener classes, with event subscriptions organised in a way that makes sense for the project. e.g. world events, connection events, etc. split into their own class.

The same registration approach (with the SubscribeEvent annotation) is used for all listeners - it is consistent.

One of the listener classes (currently) only listens to one event. The check below now prevents that listener class from using the annotation-based approach.

EventBus/src/main/java/net/minecraftforge/eventbus/internal/EventListenerFactory.java

Lines 87 to 88 in 0949732

	else if (listeners.size() == 1)
	throw new IllegalArgumentException("Only a single listener found in " + listenerClass + ". You should directly call addListener() on the EventBus of " + firstValidListenerEventType.getSimpleName() + " instead.");

I assume this was done with good intentions to nudge people towards the 'new' way - but I feel in some cases (like the one outlined above) the annotation approach does make sense even with only one event being listened to.

Please could you reconsider.

[LexManos]

It was done because of the performance differences of having to reflectively analyze a class to find all the event handlers is so significant that it would be better if people did things the simpler path.
Having your event handlers split into their own class works fine. You can do it the recommended way:

class Events {
  static register() {
    Event.BUS.register(Events::onEvent);
  }
  private static void onEvent(Event e) {}
}

vs

@EventBusSubscriber
class Events {
  @SubscribeEvent
  public static void onEvent(Event e) {}
}

The cost is still 1 line per event. But the performance is much better. And it allows you to more control over access levels of each method.