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[1.16.5] Hot Swap fails after registering an event handler with custom event


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Hi, I have an issue that's related to the hot swapping feature on IntelliJ.

From some debugging, I've figured out that after I register an event handler with a custom event, the hot swapping features of IntelliJ will fail with the following errors:

Tutorial [runClient]: schema change not implemented
Tutorial [runClient]: Operation not supported by VM

My event handler is registered something like this, with a simple TestEvent simply just extending Event:

@Mod.EventBusSubscriber(modid = TutorialMod.MOD_ID, bus = Mod.EventBusSubscriber.Bus.FORGE)
public class TestEventHandler {
	public static void onTestEvent(TestEvent event) {
		// Do something

This causes my hot swapping features to fail. I've tried to change the way the event is being registered (i.e. using MinecraftForge.EVENT_BUS.register(...), but to no avail. The only way for the hot swapping feature to work is to remove the registration of the event handler.

Does anyone know what might be the issue? Thanks!


EDIT: I updated my mod to 1.17.1 and the issue seems to have disappeared. The source was most likely what was mentioned below, probably the new update doesn't require that to happen anymore? Not sure myself.

Edited by jumpcutfindo
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1 hour ago, Luis_ST said:

show where you call the event and how do you register it

The problem arises regardless of whether I call the event or not, so I'm not sure if it matters? Just to be clear, the events that I post to the EVENT_BUS are being recognised and picked up by the handler (i.e. everything works fine, my custom event is recognised, my handler works etc.); the only issue is that hot swapping doesn't work when I have the handler implemented.

As for registering the custom event itself, I haven't found any mention of having to register a custom event (not handler) in Forge. I'm not sure if this is something I need to do though. Thanks!


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14 minutes ago, diesieben07 said:

Event classes are class-transformed by Forge to enable a performant event dispatch. IntelliJ probably detects this difference in class structure and tries to hotswap the (unchanged) event class in, which will fail.

Oh, that's most likely it then. Does this mean that if I want to use custom events, then there's no way to circumvent this issue? Or is there some other way I can add my custom event to the game without having this class-transformation occur 😐

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3 minutes ago, diesieben07 said:

The class transformation is necessary for events to work. So probably not.

Ok, that's unfortunate. I've resorted to just removing and adding back the relevant handlers after to fix it for now, it seems to be ok (until I forget to add them back). Appreciate the help.

Edited by jumpcutfindo
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