Add result variable to [message] tag

If [message] includes one or more [option] choices, store the choice index in the result variable.

changelog

@@ -18,6 +18,11 @@ Version 1.13.5+dev:
    * Fix game map sometimes showing and buttons sometimes not rendered properly in story screen (bug #24553)
    * Improved font rendering on Windows.
  * WML Engine:
+   * New attributes for [message] with [option]
+     - Added variable= to [message]: if specified, gives variable name to receive the [option] index (1..n) selected
+       only used if any [option] appear
+     - Added value= to [option]: if specified, gives value to store in variable instead of index number, only used
+       if variable= appears in [message]
    * New attributes for [role]:
      - search_recall_list=yes|no|only(default yes) controls where to look
      - reassign=yes|no(default yes) if no, check for a unit and do not assign to another

data/lua/wml/message.lua

@@ -287,6 +287,7 @@ function wesnoth.wml_actions.message(cfg)
 					description = option.description,
 					image = option.image,
 					default = option.default,
+					value = option.value
 				}
 				if option.message then
 					if not option.label then
@@ -391,6 +392,14 @@ function wesnoth.wml_actions.message(cfg)
 			return
 		end
 
+		if cfg.variable ~= nil then
+			if options[option_chosen].value == nil then
+				wesnoth.set_variable(cfg.variable, option_chosen)
+			else
+				wesnoth.set_variable(cfg.variable, options[option_chosen].value)
+			end
+		end
+
 		for i, cmd in ipairs(option_events[option_chosen]) do
 			local action = utils.handle_event_commands(cmd, "plain")
 			if action ~= "none" then break end