LuaWML/Events

This page describes the LuaWML functions and helpers for interacting with events and action handlers.

wesnoth.fire

Fires a WML action. Argument 1 is the name of the action. Argument 2 is the WML table describing the action. Note: WML variables are substituted.

wesnoth.fire("message", { speaker="narrator", message=_ "Hello World!" })

wesnoth.register_wml_action

Registers the second argument as a handler for the given action tag. When the game encounters this tag an event, it fires the action handler and passes the content of the WML object as the first argument. If the registered function raises an error with a message starting with ~wml:, it will not be displayed as a Lua error with a backtrace but as a standard WML error. (See also #helper.wml_error.) The following script defines a [freeze_unit] tag that sets to 0 the move points of the unit with the given id.

wesnoth.register_wml_action("freeze_unit",
    function(cfg)
        local unit_id = cfg.id or error("~wml:[freeze_unit] expects an id= attribute.", 0)
        helper.modify_unit({ id = unit_id }, { moves = 0 })
    end
)

# The new tag can now be used in plain WML code.
[freeze_unit]
    id=Delfador
[/freeze_unit]

The function returns the previous action handler. Its metatable appears as "wml action handler". This handler can be called to delegate part of the action, if needed. For instance, the following script modifies the [print] tag so that messages are displayed with a bigger font.

local old_print_handler
old_print_handler = wesnoth.register_wml_action("print",
    function(cfg)
        -- Do not perform variable substitution.
        local new_cfg = cfg.__literal
        -- Modify the size field and call the previous handler.
        new_cfg.size = (cfg.size or 12) + 10
        old_print_handler(cfg)
    end
)

Note that the data coming from the WML tag are stored in a read-only object and variable substitution happens on the fly when accessing attributes and children. The metatable of this proxy object appears as "wml object" See LuaWML#Encoding WML objects into Lua tables for more details.

wesnoth.wml_actions

This is not a function but an associative table indexed by WML action names. It contains functions performing the corresponding actions. Using these functions is similar to calling #wesnoth.fire, while setting entries of the table is similar to calling #wesnoth.register_wml_action.

function wesnoth.wml_actions.freeze_unit(cfg)
    local unit_id = cfg.id or helper.wml_error "[freeze_unit] expects an id= attribute."
    helper.modify_unit({ id = unit_id }, { moves = 0 })
end

wesnoth.fire_event

Fires all the WML events with the given name. Optional parameters allow passing two locations and two tables. These parameters will be matched against the [filter], [filter_second], [filter_attack], and [filter_second_attack] of any event handler, and are used to fill the WML variables "unit", "second_unit", "weapon", and "second_weapon". These parameters can also be read through current.event_context. The function returns a boolean indicating whether the game state was modified.

wesnoth.fire_event("explosion", 17, 42, { damage = "fire" })

wesnoth.eval_conditional

Returns true if the conditional described by the WML table passes. Note: WML variables are substituted.

local result = wesnoth.eval_conditional {
  { "have_unit", { id = "hero" } },
  { "variable", { name = "counter", numerical_equals = "$old_counter" } }
}

helper.set_wml_action_metatable

Sets the metable of a table so that it can be used to fire WML actions. Returns the table. The fields of the table are then simple wrappers around a call to #wesnoth.fire.

local W = helper.set_wml_action_metatable {}
W.message { speaker = "narrator", message = "?" }

helper.wml_error

Interrupts the current execution and displays a chat message that looks like a WML error.

local names = cfg.name or helper.wml_error("[clear_variable] missing required name= attribute.")