Events

Forge uses an event bus that allows mods to intercept events from various vanilla and mod behaviors.

Example: An event can be used to perform an action when a Vanilla stick is right clicked.

The main event bus used for most events is located at MinecraftForge.EVENT_BUS. There are also a few other buses used for specific types of events (like terrain generation) located in the same class.

An event handler is a class that contains one or more public void member methods that are marked with the @SubscribeEvent annotation.

Creating an Event Handler

public class MyForgeEventHandler {
    @SubscribeEvent
    public void pickupItem(EntityItemPickupEvent event) {
        System.out.println("Item picked up!");
    }
}

To register this event handler, use MinecraftForge.EVENT_BUS.register() and pass it an instance of your event handler class.

Static Event Handlers

An event handler may also be static. The handling method is still annotated with @SubscribeEvent and the only difference from an instance handler is that it is also marked static. In order to register a static event handler, an instance of the class won’t do, the Class itself has to be passed in. An example:

public class MyStaticForgeEventHandler {
    @SubscribeEvent
    public static void arrowNocked(ArrowNockEvent event) {
        System.out.println("Arrow nocked!");
    }
}

which must be registered like this: MinecraftForge.EVENT_BUS.register(MyStaticForgeEventHandler.class).

Automatically Registering Static Event Handlers

A class may be annotated with the @Mod.EventBusSubscriber annotation. Such a class is automatically registered to MinecraftForge.EVENT_BUS when the @Mod class itself is constructed. This is essentially equivalent to adding MinecraftForge.EVENT_BUS.register(AnnotatedClass.class); at the end of the @Mod class’s constructor.

Canceling

If an event can be canceled, it will be marked with the @Cancelable annotation, and the method Event#isCancelable() will return true. The cancel state of a cancelable event may be modified by calling Event#setCanceled(boolean canceled), wherin passing the boolean value true is interpreted as canceling the event, and passing the boolean value false is interpreted as “un-canceling” the event. However, if the event cannot be canceled (as defined by Event#isCancelable()), an UnsupportedOperationException will be thrown regardless of the passed boolean value, since the cancel state of a non-cancelable event event is considered immutable.

Results

Some events have an Event.Result, a result can be one of three things, DENY which stops the event, DEFAULT which uses the Vanilla behavior, and ALLOW which forces the action to take place, regardless if it would have originally. The result of an event can be set by calling setResult with an Event.Result on the event. Not all events have results, an event with a result will be annotated with @HasResult.

Priority

Event handler methods (marked with @SubscribeEvent) have a priority. You can set the priority of an event handler method by setting the priority value of the annotation. The priority can be any value of the EventPriority enum (HIGHEST, HIGH, NORMAL, LOW, and LOWEST). Event handlers with priority HIGHEST are executed first and from there in descending order until LOWEST events which are executed last.

Sub Events

Many events have different variations of themselves, these can be different but all based around one common factor (e.g. PlayerEvent) or can be an event that has multiple phases (e.g. PotionBrewEvent). Take note that if you listen to the parent event class, you will receive calls to your method for all subclasses.