Events

One of NeoForge's main features is the event system. Events are fired for various things that happen in the game. For example, there are events for when the player right clicks, when a player or another entity jumps, when blocks are rendered, when the game is loaded, etc. A modder can subscribe event handlers to each of these events, and then perform their desired behavior inside these event handlers.

Events are fired on their respective event bus. The most important bus is NeoForge.EVENT_BUS, also known as the game bus. Besides that, during startup, a mod bus is spawned for each loaded mod and passed into the mod's constructor. Many mod bus events are fired in parallel (as opposed to main bus events that always run on the same thread), dramatically increasing startup speed. See below for more information.

Registering an Event Handler

There are multiple ways to register event handlers. Common for all of those ways is that every event handler is a method with a single event parameter and no result (i.e. return type void).

IEventBus#addListener

The simplest way to register method handlers is by registering their method reference, like so:

@Mod("yourmodid")
public class YourMod {
    public YourMod(IEventBus modBus) {
        NeoForge.EVENT_BUS.addListener(YourMod::onLivingJump);
    }

    // Heals an entity by half a heart every time they jump.
    private static void onLivingJump(LivingJumpEvent event) {
        Entity entity = event.getEntity();
        // Only heal on the server side
        if (!entity.level().isClientSide()) {
            entity.heal(1);
        }
    }
}

@SubscribeEvent

Alternatively, event handlers can be annotation-driven by creating an event handler method and annotating it with @SubscribeEvent. Then, you can pass an instance of the encompassing class to the event bus, registering all @SubscribeEvent-annotated event handlers of that instance:

public class EventHandler {
    @SubscribeEvent
    public void onLivingJump(LivingJumpEvent event) {
        Entity entity = event.getEntity();
        if (!entity.level().isClientSide()) {
            entity.heal(1);
        }
    }
}

@Mod("yourmodid")
public class YourMod {
    public YourMod(IEventBus modBus) {
        NeoForge.EVENT_BUS.register(new EventHandler());
    }
}

You can also do it statically. Simply make all event handlers static, and instead of a class instance, pass in the class itself:

public class EventHandler {
	@SubscribeEvent
    public static void onLivingJump(LivingJumpEvent event) {
        Entity entity = event.getEntity();
        if (!entity.level().isClientSide()) {
            entity.heal(1);
        }
    }
}

@Mod("yourmodid")
public class YourMod {
    public YourMod(IEventBus modBus) {
        NeoForge.EVENT_BUS.register(EventHandler.class);
    }
}

@EventBusSubscriber

We can go one step further and also annotate the event handler class with @EventBusSubscriber. This annotation is discovered automatically by NeoForge, allowing you to remove all event-related code from the mod constructor. In essence, it is equivalent to calling NeoForge.EVENT_BUS.register(EventHandler.class) at the end of the mod constructor. This means that all handlers must be static, too.

While not required, it is highly recommended to specify the modid parameter in the annotation, in order to make debugging easier (especially when it comes to mod conflicts).

@EventBusSubscriber(modid = "yourmodid")
public class EventHandler {
    @SubscribeEvent
    public static void onLivingJump(LivingJumpEvent event) {
        Entity entity = event.getEntity();
        if (!entity.level().isClientSide()) {
            entity.heal(1);
        }
    }
}

Event Options

Fields and Methods

Fields and methods are probably the most obvious part of an event. Most events contain context for the event handler to use, such as an entity causing the event or a level the event occurs in.

Hierarchy

In order to use the advantages of inheritance, some events do not directly extend Event, but one of its subclasses, for example BlockEvent (which contains block context for block-related events) or EntityEvent (which similarly contains entity context) and its subclasses LivingEvent (for LivingEntity-specific context) and PlayerEvent (for Player-specific context). These context-providing super events are abstract and cannot be listened to.

danger

If you listen to an abstract event, your game will crash, as this is never what you want. You always want to listen to one of the subevents instead.

Cancellable Events

Some events implement the ICancellableEvent interface. These events can be cancelled using #setCanceled(boolean canceled), and the cancellation status can be checked using #isCanceled(). If an event is cancelled, other event handlers for this event will not run, and some kind of behavior that is associated with "cancelling" is enabled. For example, cancelling LivingJumpEvent will prevent the jump.

Event handlers can opt to explicitly receive cancelled events. This is done by setting the receiveCanceled boolean parameter in IEventBus#addListener (or @SubscribeEvent, depending on your way of attaching the event handlers) to true.

TriStates and Results

Some events have three potential return states represented by TriState, or a Result enum directly on the event class. The return states can typically either cancel the action the event is handling (TriState#FALSE), force the action to run (TriState#TRUE), or execute default Vanilla behavior (TriState#DEFAULT).

An event with three potential return states has some set* method to set the desired outcome.

// In some class where the listeners are subscribed to the game event bus

@SubscribeEvent
public void renderNameTag(RenderNameTagEvent event) {
    // Uses TriState to set the return state
    event.setCanRender(TriState.FALSE);
}

@SubscribeEvent
public void mobDespawn(MobDespawnEvent event) {
    // Uses a Result enum to set the return state
    event.setResult(MobDespawnEvent.Result.DENY);
}

Priority

Event handlers can optionally get assigned a priority. The EventPriority enum contains five values: HIGHEST, HIGH, NORMAL (default), LOW and LOWEST. Event handlers are executed from highest to lowest priority. If they have the same priority, they fire in registration order on the main bus, which is roughly related to mod load order, and in exact mod load order on the mod bus (see below).

Priorities can be defined by setting the priority parameter in IEventBus#addListener or @SubscribeEvent, depending on how you attach event handlers. Note that priorities are ignored for events that are fired in parallel.

Sided Events

Some events are only fired on one side. Common examples include the various render events, which are only fired on the client. Since client-only events generally need to access other client-only parts of the Minecraft codebase, they need to be registered accordingly.

Event handlers that use IEventBus#addListener() should check the current physical side via FMLEnvironment.dist or the Dist parameter in your made mod constructor and add the listener in a separate client-only class, as outlined in the article on sides.

Event handlers that use @EventBusSubscriber can specify the side as the value parameter of the annotation, for example @EventBusSubscriber(value = Dist.CLIENT, modid = "yourmodid").

Event Buses

While most events are posted on the NeoForge.EVENT_BUS, some events are posted on the mod event bus instead. These are generally called mod bus events. Mod bus events can be distinguished from regular events by their superinterface IModBusEvent.

The mod event bus is passed to you as a parameter in the mod constructor, and you can then subscribe mod bus events to it. If you use @EventBusSubscriber, you can also set the bus as an annotation parameter, like so: @EventBusSubscriber(bus = Bus.MOD, modid = "yourmodid"). The default bus is Bus.GAME.

The Mod Lifecycle

Most mod bus events are what is known as lifecycle events. Lifecycle events run once in every mod's lifecycle during startup. Many of them are fired in parallel by subclassing ParallelDispatchEvent; if you want to run code from one of these events on the main thread, enqueue them using #enqueueWork(Runnable runnable).

The lifecycle generally follows the following order:

• The mod constructor is called. Register your event handlers here, or in the next step.
• All @EventBusSubscribers are called.
• FMLConstructModEvent is fired.
• The registry events are fired, these include NewRegistryEvent, DataPackRegistryEvent.NewRegistry and, for each registry, RegisterEvent.
• FMLCommonSetupEvent is fired. This is where various miscellaneous setup happens.
• The sided setup is fired: FMLClientSetupEvent if on a physical client, and FMLDedicatedServerSetupEvent if on a physical server.
• InterModComms are handled (see below).
• FMLLoadCompleteEvent is fired.

InterModComms

InterModComms is a system that allows modders to send messages to other mods for compatibility features. The class holds the messages for mods, all methods are thread-safe to call. The system is mainly driven by two events: InterModEnqueueEvent and InterModProcessEvent.

During InterModEnqueueEvent, you can use InterModComms#sendTo to send messages to other mods. These methods accept the id of the mod to send the message to, the key associated with the message data (to distinguish between different messages), and a Supplier holding the message data. The sender can be optionally specified as well.

Then, during InterModProcessEvent, you can use InterModComms#getMessages to get a stream of all received messages as IMCMessage objects. These hold the sender of the data, the intended receiver of the data, the data key, and the supplier for the actual data.

Other Mod Bus Events

Next to the lifecycle events, there are a few miscellaneous events that are fired on the mod event bus, mostly for legacy reasons. These are generally events where you can register, set up, or initialize various things. Most of these events are not ran in parallel in contrast to the lifecycle events. A few examples:

• RegisterColorHandlersEvent
• ModelEvent.BakingCompleted
• TextureAtlasStitchedEvent

warning

Most of these events are planned to be moved to the main event bus in a future version.