Behaviour overrides and value events

Status: Partly implemented. Behaviour tables for your own blocks and items (cl_block_desc.behavior: use_without_item, attack, random_tick; cl_item_desc.behavior: use) and the mutable fields of action events work. Overrides of vanilla content’s behaviour and value events are Planned; mixins cover part of that today.

Concepts and examples: Events, behaviours and mixins. Mixins: Mixins.

Behaviour tables (your own content)

Table	Entry	Signature	Page
`cl_block_behavior`	`use_without_item`	`cl_override (const cl_block_ctx, cl_entity player, cl_hand, cl_interaction_result out)`	Blocks and block states
	`attack`	`cl_override (const cl_block_ctx, cl_entity player, int32_t unused)`
	`random_tick`	`cl_override (const cl_block_ctx, int32_t unused)`
`cl_item_behavior`	`use`	`cl_override (const cl_item_ctx, cl_interaction_result out)`	Items and item stacks

Return CL_OVERRIDE_HANDLED (with *out set) or CL_OVERRIDE_CALL_ORIGINAL. C++ takes lambdas (clight::registry::block(desc, {.use_without_item = ...})), Lua takes functions in a behavior table.

Mutable event fields (the value-event subset that exists)

Event	Mutable field
`block.break`	`xp`, `drop_items`
`entity.damage`	`amount`
`player.chat`	`message`
`player.command`	`command`
`client.chat_received`, `client.chat_send`	`message`
`client.sound_play`	`volume`, `pitch`

-- half damage from falling
cl.on("entity.damage", function(event)
  if event.source == "minecraft:fall" then event.amount = event.amount / 2 end
end)

Planned

Overrides of any block, item, entity type or effect, including vanilla’s, as a priority-ordered chain that ends at vanilla’s behaviour, with many more entries (placement, can_survive, neighbour updates, entity contact, redstone, mining speed, tooltips).
Value events: payloads whose result starts as the vanilla value (fall damage, knockback, mining speed, reach, FOV, fog), listed in Value event list.