fabric-carpet/docs/scarpet/api/Events.md

Scarpet events system

Scarpet provides the ability to execute specific function whenever an event occurs. The functions to be subscribed for an event need to conform with the arguments to the event specification. There are several built-in events triggered when certain in-game events occur, but app designers can create their own events and trigger them across all loaded apps.

When loading the app, each function that starts with __on_<event> and has the required arguments, will be bound automatically to a corresponding built-in event. 'undef'ying of such function would result in unbinding the app from this event.

In case of player scoped apps, all player action events will be directed to the appropriate player hosts. Global events, like 'tick', that don't have a specific player target will be executed multiple times, once for each player, and once in 'global' scoped apps.

Most built-in events strive to report right before they take an effect in the game. The purpose of that is that this give a choice for the programmer to handle them right away (as it happens, potentially affect the course of action by changing the environment right before it), or decide to handle it after by scheduling another call for the end of the tick. Or both - partially handle the event before it happens and handle the rest after. While in some cases this may lead to programmers confusion (like handling the respawn event still referring to player's original position and dimension), but gives much more control over these events.

Programmers can also define their own events and signal other events, including built-in events, and across all loaded apps.

Event list

Here is a list of events that are handled by default in scarpet. This list includes prefixes for function names, allowing apps to autoload them, but you can always add any function to any event (using /script event command) if it accepts required number of parameters.

Built-in global events

Global events will be handled once per app that is with 'global' scope. With player scoped apps, each player instance is responsible independently from handling their events, so a global event may be executed multiple times for each player.

__on_server_starts()

Event triggers after world is loaded and after all startup apps have started. It won't be triggered with /reload.

__on_server_shuts_down()

Event triggers when the server started the shutdown process, before __on_close() is executed. Unlike __on_close(), it doesn't trigger with /reload.

__on_tick()

Event triggers at the beginning of each tick, located in the overworld. You can use in_dimension() to access other dimensions from there.

__on_tick_nether() (Deprecated)

Duplicate of tick, just automatically located in the nether. Use __on_tick() -> in_dimension('nether', ... instead.

__on_tick_ender() (Deprecated)

Duplicate of tick, just automatically located in the end. Use __on_tick() -> in_dimension('end', ... instead.

__on_chunk_generated(x,z)

Called right after a chunk at a given coordinate is full generated. x and z correspond to the lowest x and z coords in the chunk. Event may (or may not) work with Optifine installed at the same time.

__on_lightning(block, mode)

Triggered right after a lightning strikes. Lightning entity as well as potential horseman trap would already be spawned at that point. mode is true if the lightning did cause a trap to spawn.

__on_carpet_rule_changes(rule, new_value)

Triggered when a carpet mod rule is changed. It includes extension rules, not using default /carpet command, which will then be namespaced as namespace:rule.

entity load event -> check entity_load_handler()

These will trigger every time an entity of a given type is loaded into the game: spawned, added with a chunks, spawned from commands, anything really. Check entity_load_handler() in the entity section for details.

Built-in player events

These are triggered with a player context. For apps with a 'player' scope, they trigger once for the appropriate player. In apps with global scope they trigger once as well as a global event.

__on_player_uses_item(player, item_tuple, hand)

Triggers with a right click action. Event is triggered right after a server receives the packet, before the game manages to do anything about it. Event triggers when player starts eating food, or starts drawing a bow. Use player_finishes_using_item, or player_releases_item to capture the end of these events.

Event is not triggered when a player places a block, for that use player_right_clicks_block or player_places_block event.

__on_player_releases_item(player, item_tuple, hand)

Player stops right-click-holding on an item that can be held. This event is a result of a client request. Example events that may cause it to happen is releasing a bow. The event is triggered after the game processes the request, however the item_tuple is provided representing the item that the player started with. You can use that and compare with the currently held item for a delta.

__on_player_finishes_using_item(player, item_tuple, hand))

Player using of an item is done. This is controlled server side and is responsible for such events as finishing eating. The event is triggered after confirming that the action is valid, and sending the feedback back to the client, but before triggering it and its effects in game.

__on_player_clicks_block(player, block, face)

Representing left-click attack on a block, usually signifying start of breaking of a block. Triggers right after the server receives a client packet, before anything happens on the server side.

__on_player_breaks_block(player, block)

Called when player breaks a block, right before any changes to the world are done, but the decision is made to remove the block.

__on_player_right_clicks_block(player, item_tuple, hand, block, face, hitvec)

Called when player right clicks on a block with anything, or interacts with a block. This event is triggered right before other interaction events, like 'player_interacts_with_block' or 'player_places_block'.

__on_player_interacts_with_block(player, hand, block, face, hitvec)

Called when player successfully interacted with a block, which resulted in activation of said block, right after this happened.

__on_player_places_block(player, item_tuple, hand, block)

Triggered when player places a block, after block is placed in the world, but before scoreboard is triggered or player inventory adjusted.

__on_player_interacts_with_entity(player, entity, hand)

Triggered when player right clicks (interacts) with an entity, even if the entity has no vanilla interaction with the player or the item they are holding. The event is invoked after receiving a packet from the client, before anything happens server side with that interaction.

__on_player_trades(player, entity, buy_left, buy_right, sell)

Triggered when player trades with a merchant. The event is invoked after the server allow the trade, but before the inventory changes and merchant updates its trade-uses counter. The parameter entity can be null if the merchant is not an entity.

__on_player_collides_with_entity(player, entity)

Triggered every time a player - entity collisions are calculated, before effects of collisions are applied in the game. Useful not only when colliding with living entities, but also to intercept items or XP orbs before they have an effect on the player.

__on_player_chooses_recipe(player, recipe, full_stack)

Triggered when a player clicks a recipe in the crafting window from the crafting book, after server received a client request, but before any items are moved from its inventory to the crafting menu.

__on_player_switches_slot(player, from, to)

Triggered when a player changes their selected hotbar slot. Applied right after the server receives the message to switch the slot.

__on_player_swaps_hands(player)

Triggered when a player sends a command to swap their offhand item. Executed before the effect is applied on the server.

__on_player_swings_hand(player, hand)

Triggered when a player starts swinging their hand. The event typically triggers after a corresponding event that caused it (player_uses_item, player_breaks_block, etc.), but it triggers also after some failed events, like attacking the air. When swinging continues as an effect of an action, no new swinging events will be issued until the swinging is stopped.

__on_player_attacks_entity(player, entity)

Triggered when a player attacks entity, right before it happens server side.

__on_player_takes_damage(player, amount, source, source_entity)

Triggered when a player is taking damage. Event is executed right after potential absorbtion was applied and before the actual damage is applied to the player.

__on_player_deals_damage(player, amount, entity)

Triggered when a player deals damage to another entity. Its applied in the same moment as player_takes_damage if both sides of the event are players, and similar for all other entities, just their absorbtion is taken twice, just noone ever notices that ¯_(ツ)_/¯

__on_player_dies(player)

Triggered when a player dies. Player is already dead, so don't revive them then. Event applied before broadcasting messages about players death and applying external effects (like mob anger etc).

__on_player_respawns(player)

Triggered when a player respawns. This includes spawning after death, or landing in the overworld after leaving the end. When the event is handled, a player is still in its previous location and dimension - will be repositioned right after. In case player died, its previous inventory as already been scattered, and its current inventory will not be copied to the respawned entity, so any manipulation to player data is best to be scheduled at the end of the tick, but you can still use its current reference to query its status as of the respawn event.

__on_player_changes_dimension(player, from_pos, from_dimension, to_pos, to_dimension)

Called when a player moves from one dimension to another. Event is handled still when the player is in its previous dimension and position.

player_changes_dimension returns null as to_pos when player goes back to the overworld from the end , since the respawn location of the player is not controlled by the teleport, or a player can still see the end credits. After the player is eligible to respawn in the overworld, player_respawns will be triggered.

__on_player_rides(player, forward, strafe, jumping, sneaking)

Triggers when a server receives movement controls when riding vehicles. Its handled before the effects are applied server side.

__on_player_jumps(player)

Triggered when a game receives a jump input from the client, and the player is considered standing on the ground.

__on_player_deploys_elytra(player)

Triggered when a server receives a request to deploy elytra, regardless if the flight was agreed upon server side..

__on_player_wakes_up(player)

Player wakes up from the bed mid sleep, but not when it is kicked out of bed because it finished sleeping.

__on_player_escapes_sleep(player)

Same as player_wakes_up but only triggered when pressing the ESC button. Not sure why Mojang decided to send that event twice when pressing escape, but might be interesting to be able to detect that.

__on_player_starts_sneaking(player)

__on_player_stops_sneaking(player)

__on_player_starts_sprinting(player)

__on_player_stops_sprinting(player)

Four events triggered when player controls for sneaking and sprinting toggle.

__on_player_drops_item(player)

__on_player_drops_stack(player)

Triggered when the game receives the request from a player to drop one item or full stack from its inventory. Event happens before anything is changed server side.

__on_player_picks_up_item(player, item)

Triggered AFTER a player successfully ingested an item in its inventory. Item represents the total stack of items ingested by the player. The exact position of these items is unknown as technically these items could be spread all across the inventory.

__on_player_connects(player)

Triggered when the player has successfully logged in and was placed in the game.

__on_player_disconnects(player, reason)

Triggered when a player sends a disconnect package or is forcefully disconnected from the server.

__on_statistic(player, category, event, value)

Triggered when a player statistic changes. Doesn't notify on periodic an rhythmic events, i.e. time_since_death, time_since_rest, and played_one_minute since these are triggered every tick. Event is handled before scoreboard values for these statistics are changed.

Custom events and hacking into scarpet event system

App programmers can define and trigger their own custom events. Unlike built-in events, all custom events pass a single value as an argument, but this doesn't mean that they cannot pass a complex list, map, or nbt tag as a message. Each event signal is either targetting all apps instances for all players, including global apps, if no target player has been identified, or only player scoped apps, if the target player is specified, running once for that player app. You cannot target global apps with player-targeted signals. Built-in events do target global apps, since their first argument is clearly defined and passed. That may change in the future in case there is a compelling argument to be able to target global apps with player scopes.

Programmers can also handle built-in events the same way as custom events, as well as triggering built-in events, which I have have no idea why you would need that. The following snippets have the same effect:

__on_player_breaks_block(player, block) -> print(player+' broke '+block);

and

handle_event('player_breaks_block', _(player, block) -> print(player+' broke '+block));

as well as

undef('__on_player_breaks_block');

and

handle_event('player_breaks_block', null);

And signal_event can be used as a trigger, called twice for player based built-in events

signal_event('player_breaks_block', player, player, block); // to target all player scoped apps
signal_event('player_breaks_block', null  , player, block); // to target all global scoped apps and all player instances

or (for global events)

signal_event('tick') // trigger all apps with a tick event

handle_event(event, callback ...)

Provides a handler for an event identified by the 'event' argument. If the event doesn't exist yet, it will be created. All loaded apps globally can trigger that event, when they call corresponding signal_event(event, ...). Callback can be defined as a function name, function value (or a lambda function), along with optional extra arguments that will be passed to it when the event is triggered. All custom events expect a function that takes one free argument, passed by the event trigger. If extra arguments are provided, they will be appended to the argument list of the callback function.

Returns true if subscription to the event was successful, or false if it failed (for instance wrong scope for built-in event, or incorect number of parameters for the event).

If a callback is specified as null, the given app (or player app instance )stops handling that event.

foo(a) -> print(a);
handle_event('boohoo', 'foo');

bar(a, b, c) -> print([a, b, c]);
handle_event('boohoo', 'bar', 2, 3) // using b = 2, c = 3, a - passed by the caller

handle_event('tick', _() -> foo('tick happened')); // built-in event

handle_event('tick', null)  // nah, ima good, kthxbai

In case you want to pass an event handler that is not defined in your module, please read the tips on "Passing function references to other modules of your application" section in the call(...) section.

signal_event(event, target_player?, ... args?)

Fires a specific event. If the event does not exist (only handle_event creates missing new events), or provided argument list was not matching the callee expected arguments, returns null, otherwise returns number of apps notified. If target_player is specified and not null triggers a player specific event, targetting only player scoped apps for that player. Apps with globals scope will not be notified even if they handle this event. If the target_player is omitted or null, it will target global scoped apps and all instances of player scoped apps. Note that all built-in player events have a player as a first argument, so to trigger these events, you need to provide them twice - once to specify the target player scope and second - to provide as an argument to the handler function.

signal_event('player_breaks_block', player, player, block); // to target all player scoped apps
signal_event('player_breaks_block', null  , player, block); // to target all global scoped apps and all player instances
signal_event('tick') // trigger all apps with a tick event

Custom events example

The following example shows how you can communicate between different instances of the same player scoped app. It important to note that signals can trigger other apps as well, assuming the name of the event matches. In this case the request name is called tp_request and is triggered with a command.

// tpa.sc
global_requester = null;
__config() -> {
	'commands' -> {
		'<player>' -> _(to) -> signal_event('tp_request', to, player()),
      'accept' -> _() -> if(global_requester, 
         run('tp '+global_requester~'command_name'); 
         global_requester = null
      )
	},
   'arguments' -> {
      'player' -> {'type' -> 'players', 'single' -> true}
   }
};
handle_event('tp_request', _(req) -> (
   global_requester = req;
   print(player(), format(
      'w '+req+' requested to teleport to you. Click ',
      'yb here', '^yb here', '!/tpa accept',
      'w  to accept it.'
   ));
));

/script event command

used to display current events and bounded functions. use add_to to register a new event, or remove_from to unbind a specific function from an event. Function to be bounded to an event needs to have the same number of parameters as the action is attempting to bind to (see list above). All calls in modules loaded via /script load that handle specific built-in events will be automatically bounded, and unbounded when script is unloaded.