LuaWML

[edit]WML Tags

A: abilities, about, achievement, achievement_group, add_ai_behavior, advanced_preference, advancefrom, advancement, advances, affect_adjacent, ai, allied_with, allow_end_turn, allow_extra_recruit, allow_recruit, allow_undo, and, animate, animate_unit, animation, aspect, attack (replay, weapon), attack_anim, attacks (special, stats), avoid;

B: base_unit, background_layer, berserk, binary_path, break, brush;

C: campaign, cancel_action, candidate_action, capture_village, case, chance_to_hit, change_theme, chat, checkbox, choice, choose, clear_global_variable, clear_menu_item, clear_variable, color_adjust, color_palette, color_range, command (action, replay), continue, core, credits_group, criteria;

D: damage, damage_type, death, deaths, default, defend, defends, defense, delay, deprecated_message, destination, difficulty, disable, disallow_end_turn, disallow_extra_recruit, disallow_recruit, do, do_command, drains, draw_weapon_anim;

E: editor_group, editor_music, editor_times, effect, else (action, animation), elseif, endlevel, end_turn (action, replay), enemy_of, engine, entry (credits, options), era, event, experimental_filter_ability, experimental_filter_ability_active, experimental_filter_specials, extra_anim;

F: facet, facing, fake_unit, false, feedback, female, filter (concept, event), filter_adjacent, filter_adjacent_location, filter_attack, filter_attacker, filter_base_value, filter_condition, filter_defender, filter_enemy, filter_location, filter_opponent, filter_own, filter_owner, filter_radius, filter_recall, filter_second, filter_second_attack, filter_self, filter_side, filter_student, filter_vision, filter_weapon, filter_wml, find_path, fire_event, firststrike, floating_text, fonts, for, foreach, found_item, frame;

G: game_config, get_global_variable, goal, gold, gold_carryover;

H: harm_unit, has_ally, has_attack, has_unit, has_achievement, have_location, have_unit, heal_on_hit, heal_unit, healed_anim, healing_anim, heals, hide_help, hide_unit, hides;

I: idle_anim, if (action, animation, intro), illuminates, image (intro, terrain), init_side, insert_tag, inspect, item, item_group;

J: jamming_costs, join;

K: kill, killed;

L: label, language, leader, leader_goal, leadership, leading_anim, levelin_anim, levelout_anim, lift_fog, limit, literal, load_resource, locale, lock_view, lua;

M: male, menu_item, message, micro_ai, missile_frame, modification, modifications, modify_ai, modify_side, modify_turns, modify_unit, modify_unit_type, move, move_unit, move_unit_fake, move_units_fake, movement_anim, movement costs, movetype, multiplayer, multiplayer_side, music;

N: not, note;

O: object, objective, objectives, on_undo, open_help, option, options, or;

P: part, petrifies, petrify, place_shroud, plague, poison, post_movement_anim, pre_movement_anim, primary_attack, primary_unit, print, progress_achievement, put_to_recall_list;

R: race, random_placement, recall (action, replay), recalls, recruit, recruit_anim, recruiting_anim, recruits, redraw, regenerate, remove_event, remove_item, remove_object, remove_shroud, remove_sound_source, remove_time_area, remove_trait, remove_unit_overlay, repeat, replace_map, replace_schedule, replay, replay_start, reset_fog, resistance (ability, unit), resistance_defaults, resolution, resource, return, role, rule;

S: save, scenario, screen_fade, scroll, scroll_to, scroll_to_unit, secondary_attack, secondary_unit, section, select_unit, sequence, set_achievement, set_extra_recruit, set_global_variable, set_menu_item, set_recruit, set_specials, set_variable, set_variables, sheath_weapon_anim, show_if (message, objective, set_menu_item), show_objectives, side, skirmisher, slider, slow, snapshot, sound, sound_source, source (replay, teleport), special_note, specials, split, stage, standing_anim, statistics, status, store_gold, store_items, store_locations, store_map_dimensions, store_reachable_locations, store_relative_direction, store_side, store_starting_location, store_time_of_day, store_turns, store_unit, store_unit_defense, store_unit_defense_on, store_unit_type, store_unit_type_ids, store_villages, story, swarm, sub_achievement, switch, sync_variable;

T: target, team, teleport (ability, action), teleport_anim, terrain, terrain_defaults, terrain_graphics, terrain_mask, terrain_type, test, test_condition, test_do_attack_by_id, text_input, textdomain, theme, then, tile, time, time_area, topic, toplevel, trait, transform_unit, traveler, true, tunnel;

U: unhide_unit, unit (action, scenario), unit_overlay, unit_type, unit_worth, units, unlock_view, unpetrify, unstore_unit, unsynced;

V: value, variable, variables, variant, variation, victory_anim, village, vision_costs, volume;

W: while, wml_message, wml_schema;

Z: zoom;

The [lua] tag

This tag is a subtag of the [event]. It makes it possible to write actions with the Lua 5.1 language.

The tag supports only the code key, which is a string containing the Lua scripts. Since Lua makes usage of the quotes and the { and } symbols, it is certainly wise to enclose the script between stronger quotes, as they prevent the preprocessor from performing macro expansion and tokenization.

[lua]
    code = << wesnoth.message "Hello World!" >>
[/lua]

The [args] tag can be used to pass a WML object to the script via its variadic local variable.

Examples

The following WML event is taken from Wesnoth' tutorial. It will serve as an example to present how Lua scripts are embedded into Wesnoth. The event is fired whenever a unit from side 1 (that is, the hero controlled by the user) moves to a tile that is not the one set in the WML variable target_hex.

# General catch for them moving to the wrong place.
[event]
    name=moveto
    first_time_only=no
    [allow_undo][/allow_undo]
    [filter]
        side=1
    [/filter]

    [if]
        [variable]
            name=target_hex.is_set
            equals=yes
        [/variable]
        [then]
            [if]
                [variable]
                    name=x1
                    equals=$target_hex.x
                [/variable]
                [variable]
                    name=y1
                    equals=$target_hex.y
                [/variable]
                [then]
                [/then]
                [else]
                    [redraw][/redraw]
                    [message]
                        speaker=narrator
                        message=_ "*Oops!
You moved to the wrong place! After this message, you can press 'u' to undo, then try again." +
                        _ "
*Left click or press spacebar to continue..."
                    [/message]
                [/else]
            [/if]
        [/then]
    [/if]
[/event]

A Lua script that performs the same action is presented below.

[event]
    name=moveto
    first_time_only=no
    [allow_undo][/allow_undo]
    [filter]
        side=1
    [/filter]

    [lua]
        code = <<
            local args = ...
            if target_hex.is_set and
               (args.x1 ~= target_hex.x or args.y1 ~= target_hex.y)
            then
                W.redraw()
                narrator_says(_ "*Oops!\nYou moved to the wrong place! After this message, you can press 'u' to undo, then try again.")
            end
        >>
    [/lua]
[/event]

Here is a more detailed explanation of the Lua code. Its first line

local args = ...

loads the parameter of the script into the args local variable. Since it is a moveto event, the args table contains the destination of the unit in the x1 and y1 fields.

The next two lines then test

if target_hex.is_set and
   (args.x1 ~= target_hex.x or args.y1 ~= target_hex.y)

whether the variable target_hex matches the event parameters. Since target_hex is not a local variable, it is taken from the global environment (a table implicitly named _G, so it is actually _G.target_hex). The global environment is not persistent, so it cannot be used to store data. In order to make it useful, it was redirected to the storage of WML variables by the following preload event.

[event]
    name=preload
    first_time_only=no
    [lua]
        code = <<
            H = wesnoth.dofile("lua/helper.lua")
            -- skipping some other initializations
            -- ...
            H.set_wml_var_metatable(_G)
        >>
    [/lua]
[/event]

Without a prelude redirecting _G, the conditional would have been written

if wesnoth.get_variable("target_hex.is_set") and
   (args.x1 ~= wesnoth.get_variable("target_hex.x") or args.y1 ~= wesnoth.get_variable("target_hex.y")

The body of the conditional then performs the redraw action.

W.redraw()

Again, this short syntax is made possible by a line of the prelude that makes W a proxy for performing WML actions.

W = H.set_wml_action_metatable({})

Without this shortcut, the first statement would have been written

wesnoth.fire("redraw")

Finally the script displays a message by

narrator_says(_ "*Oops!\nYou moved to the wrong place! After this message, you can press 'u' to undo, then try again.")

The narrator_says function is defined in the prelude too, since the construct behind it occurs several times in the tutorial. In plain WML, macros would have been used instead. The definition of the function is

function narrator_says(m)
    W.message { speaker="narrator",
                message = m .. _ "\n*Left click or press spacebar to continue..." }
end

The function fires a message and passes a WML object containing the usual two fields to it. The second field is initialized by concatenating the function argument with another string. Both strings are prefixed by the _ symbol to mark them as translatable. (Note that _ is just a unary function, not a keyword.) Again, this is made possible by a specific line of the prelude:

_ = wesnoth.textdomain "wesnoth-tutorial"

A longer translation of the tutorial is available at [1].

Environment

All the Lua scripts of a scenario shares the same global environment (aka Lua state). This environment is not preserved over save/load cycles. Therefore, storing values in the global environment is a generally a bad idea (unless it has been redirected to WML variables, see set_wml_var_metatable). The only time it makes sense to assign global variables is during a preload event, as this event is always run. Therefore, helper functions defined at that time will be available to all the later scripts.

The global environment is preloaded with the following modules: basic (no name), string, table, and math. A wesnoth module is also available, see below.

At the start of the script, the variadic local variable ... (three dots) contains a table. It contains the content of the [args] sub-tag of the [lua] tag. The table also provides (if they make sense for the current event) the fields x1, y1, x2, and y2, containing map locations, and the sub-tables weapon and second_weapon containing attacks.

Helper functions

The following functions are provided by the lua/helper.lua file.

set_wml_action_metatable

Sets the metable of a table so that it can be used to fire WML actions. Returns the table.

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

set_wml_var_metatable

Sets the metable of a table so that it can be used to access WML variables. Returns the table.

helper.set_wml_var_metatable(_G)
my_persistent_variable = 42

modify_unit

Modifies all the units satisfying the given filter (argument 1) with some WML keys/tags (argument 2). This is a Lua implementation of the MODIFY_UNIT macro.

helper.modify_unit({ id="Delfador" }, { moves=0 })

move_unit_fake

Fakes the move of a unit satisfying the given filter (argument 1) to the given position (argument 2). This is a Lua implementation of the MOVE_UNIT macro.

helper.move_unit_fake({ id="Delfador" }, 14, 8)