Add CreeperSwellEvent

[shrayus-masanam]

There's currently no event that fires when a creeper's swell level changes, and there's currently no way to stop a creeper from swelling. CreeperIgniteEvent does not fire when a creeper primes due to proximity, EntityExplodeEvent only fires at the end of swelling, and GenericGameEvent of type GameEvent.PRIME_FUSE doesn't allow for cancelling and doesn't fire for individual swell steps.

The proposed CreeperSwellEvent fires whenever a creeper's internal swell level changes, and it also differentiates different reasons for the swell: PRIMED (proximity or ignition), FALL_DAMAGE (falling from a height), or CUSTOM (plugin adjusting swell).

Fixes #12620

[shrayus-masanam]

Example usages:

cancel_fall_damage_swell.mp4

sound_on_swell.mp4

[mbax]

1. Why wouldn't the client reflect the unswelling if cancelled, as it does if you back off from a creeper?

2. I see you're firing a swell event on cancelled explosion. Bizarre edge case question: What happens if someone is always cancelling explosion events and then cancels that specific post-cancel swell event as well? Will the creeper be continuously trying to explode?

[shrayus-masanam]

After reading your reply, I noticed a bug that would let a primed creeper's swell level to continue to change even if the event was cancelled. It happened because cancelling the event set the final swell change to 0, so the vanilla logic kicked in and continued to increment the swell level. Now, the tick function will instead check for a nonzero swellDir to decide whether or not to fire the event and handle its cancellation state.

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1. Why wouldn't the client reflect the unswelling if cancelled, as it does if you back off from a creeper?

The client does render an unswell, but it seems like the client is only programmed to render a full swell in one direction. For example, if I use this code in a plugin to only allow a creeper to swell half-way:

@EventHandler
public void onCreeperSwell(CreeperSwellEvent event) {
    if (event.getCurrentSwell() + event.getFinalSwellChange() > event.getMaxSwell()/2)
        event.setCancelled(true);
}

the client still sees the creeper swelling completely:

stop_swell_halfway.mp4

2. I see you're firing a swell event on cancelled explosion. Bizarre edge case question: What happens if someone is always cancelling explosion events and then cancels that specific post-cancel swell event as well? Will the creeper be continuously trying to explode?

It depends, if the creeper was ignited and is not being primed, it will simply unswell. But if the creeper is actively being primed (e.g. due to proximity), then yes, the creeper is stuck at max swell and tries to explode until it's no longer being primed:

cancel_explosion_and_swell.mp4

Although, I think this is desired behavior rather than a bug? If you cancel a CUSTOM swell then you are cancelling a swell change caused by a plugin, which may or may not cause conflicts