This event is fired directly before vanilla brewing takes place, and, if canceled, will prevent brewing from taking place. If cancelled but the items are modified, then the PotionBrewPostEvent will be automatically fired. This allows for a simulacrum of "modded" brewing.

You will need to cast the event in the function header as this class:
crafttweaker.event.PotionBrewPreEvent You can, of course, also import the class before and use that name then.

PotionBrewPre Events implement the following interfaces and are able to call all of their methods/getters/setters as well:

• IPotionBrewEvent
• IEventCancelable

The following information can be retrieved/set during the event:

ZenGetter	ZenSetter	Type
ZenGetter length	ZenSetter ​	Type int
ZenGetter canceled	ZenSetter canceled	Type bool

ZenMethods

• event.cancel(); Method, returns void (nothing). Can cancel the event and stop something from happening
• event.getItem(int) returns IItemStack on the given index in the crafting station. Will return an empty IItemStack if the specified index is greater than length.
• event.setItem(int, IItemStack) Replaces or sets IItemStack on given index. If the index is greater than the length of the item array, nothing will happen.

This event is fired right as the calculations for the recipe are taking place, when the "timer" reaches "maximum progress".