Why does forge create a custom class in ASMEventHandler

[codedcosmos]

I read up on this post and found it rather interesting. So I went into the code and started understanding how it all worked together.

However there was something I found rather curious and have no idea what it's purpose is (though If I had to guess I would say performance). The method in question is called "createWrapper" and is inside the ASMEventHandler class on the forge event bus github. What does this code specifically do for forge. If anyone can shed some light I would be quite grateful.

[Cadiboo]

Do you want to know what the code does (what the ASM code does and compiles to) or what it is useful for (why it exists)?

[codedcosmos]

It compiles to a IEventListener right? Does the code just serve as a way to get that class, or is it something else. What is it useful for?

[Cadiboo]

It makes an IEventListener that calls an arbitrary method (specifically one marked with @SubscribeEvent) when invoked. 

This allows events to be piped to event handlers without caring what those event handlers are.

The call hierarchy is EventBus.post -> IEventListener.invoke (a ASMEventHandler) -> IEventListener.invoke (ASMEventHandler.handler) -> @SubscribeEvent method

Edited April 17, 2019 by Cadiboo

[Cadiboo]

The name of the class is in the format ASMEventHandler_ID_callbackClassSimpleName_callbackMethodName_eventSimpleName. I’m using “ExampleClientSubscriber#onModelBake(ModelBakeEvent)” as the callback for the for the example instance event subscriber and “ExampleClientSubscriber.onModelBake(ModelBakeEvent)” as the example callback for the static event subscriber. I’m also assuming the ID is 0. 

 

The generated class looks like

 

1) Instance Event Subscriber

	package net.minecraftforge.eventbus:
	 
	public class ASMEventHandler_0_ExampleClientSubscriber_onModelBake_ModelBakeEvent extends Object implements IEventListener {
	 
	    public Object instance; //this could be made final
	 
	    public ASMEventHandler_0_ExampleClientSubscriber_onModelBake_ModelBakeEvent(Object unnamedParameter) {
	        super();
	        this.instance = unnamedParameter;
	    }
	 
	    @Override //Obviously @Override isn’t actually inserted, I put it here for clarity
	    public void invoke(Event unnamedParameter) {
	        this.instance.onModelBake((ModelBakeEvent) unnamedParameter);
	    }
	 
	}
	

 

2) Static Event Subscriber

(the event parameter isn’t named, but I can’t edit the code block because I’m on mobile)

	package net.minecraftforge.eventbus;
	public class ASMEventHandler_0_ExampleClientSubscriber_onModelBake_ModelBakeEvent extends Object implements IEventListener {
	 
	    public ASMEventHandler_0_ExampleClientSubscriber_onModelBake_ModelBakeEvent() {
	        super();
	    }
	 
	    @Override //Obviously @Override isn’t actually inserted, I put it here for clarity
	    public void invoke(Event event) {
	        // there seems to be an ALOAD 0 here that should only be in the non-static event subscriber? Doesn’t really effect anything though.
	        ExampleClientSubscriber.onModelBake((ModelBakeEvent) event);
	    }
	 
	}
	

Edited April 17, 2019 by Cadiboo

[codedcosmos]

Ah so its a common interface, cheers

[Draco18s]

5 hours ago, codedcosmos said:

Ah so its a common interface, cheers

Its a common interface so that event handler methods don't need to be called with Reflection (which is slow).

Edited April 17, 2019 by Draco18s