Send additionalData with choices

Currently, this is only implemented in `TakeApproach`.

edgehead/assets/text/Knights/locations/090_start/000_start_tester_build.egb.txt

@@ -6,5 +6,7 @@ DESCRIPTION: $NONE
 // ---
 ROOM: $start_tester_build
 
+POS: 0, 0
+
 FIRST_DESCRIPTION:
 Welcome to the test build of this game. Thank you for taking the time to make the game better.

edgehead/lib/edgehead_lib.dart

@@ -422,6 +422,8 @@ class EdgeheadGame extends Book {
           ..commandPath = ListBuilder<String>(performance.commandPath)
           ..helpMessage = performance.action.helpMessage
           ..successChance = performance.successChance.value
+          ..actionName = performance.action.name
+          ..additionalData = ListBuilder<int>(performance.additionalData)
           ..isImplicit = performance.action.isImplicit);
         callbacks[choice] = () async {
           await _applySelected(

edgehead/lib/egamebook/elements/choice_element.dart

@@ -16,8 +16,11 @@ abstract class Choice extends ElementBase
 
   Choice._();
 
-  /// The chance of success, from `0` to `1`.
-  double get successChance;
+  /// The name of the action being used.
+  String get actionName;
+
+  /// Additional data, as provided by [Action.getAdditionalData].
+  BuiltList<int> get additionalData;
 
   BuiltList<String> get commandPath;
 
@@ -27,4 +30,7 @@ abstract class Choice extends ElementBase
   /// Returns [:true:] when the choice is automatic (presenter picks it
   /// silently). Corresponds to `Action.isImplicit`.
   bool get isImplicit;
+
+  /// The chance of success, from `0` to `1`.
+  double get successChance;
 }

edgehead/lib/fractal_stories/action.dart

@@ -40,6 +40,9 @@ abstract class Action<T> {
   static const Duration defaultPlayerRecoveryDuration =
       Duration(milliseconds: 750);
 
+  /// Allow [Action] subclasses to have `const` constructors.
+  const Action();
+
   /// [OtherActorActionBase] should include the [target] in the result of
   /// [getCommand]. To make it easier to implement, this class will
   /// automatically construct the name given a [Storyline] template.
@@ -58,9 +61,6 @@ abstract class Action<T> {
   /// and, if appropriate, why and when it should be used.
   String get helpMessage;
 
-  /// Allow [Action] subclasses to have `const` constructors.
-  const Action();
-
   /// Whether or not this action is aggressive towards its sufferer. Combat
   /// moves are aggressive, healing moves aren't.
   ///
@@ -135,6 +135,17 @@ abstract class Action<T> {
         'called in the first place.');
   }
 
+  /// Some actions can provide additional data for a give [context] and
+  /// [object]. This data can be used by the presenter.
+  ///
+  /// For example, a travel action can provide a path from the current
+  /// position ([context]) to the destination ([object]). The presenter
+  /// can then draw the path on a map.
+  List<int> getAdditionalData(ApplicabilityContext context, T object) {
+    // By default, there is no additional data.
+    return const [];
+  }
+
   /// The path of sub-commands to be used for the action menu.
   ///
   /// For example, an actions such as "kick <subject> to ground" could have
@@ -369,6 +380,7 @@ abstract class OtherActorActionBase extends Action<Actor> {
 /// as the object. Therefore, assuming there are two different enemies
 /// available for punching, the planner would create two instances
 /// of `Performance`, one for each enemy.
+@immutable
 class Performance<T> {
   /// The action to be performed on the [object]. For example, punch (if
   /// the object is an enemy actor) or take (if the object is an item).
@@ -383,8 +395,20 @@ class Performance<T> {
   /// The object the [action] is performed on.
   final T object;
 
+  /// Additional information about the performance that can be visualized
+  /// by the presenter.
+  ///
+  /// For example, a travel action can provide the path that is to be taken
+  /// by the actor to get to their destination ([object]) given their
+  /// current position ([context]).
+  ///
+  /// This is taken from [Action.getAdditionalData] and defaults to an empty
+  /// list.
+  final List<int> additionalData;
+
   /// Creates a performance object.
-  Performance(this.action, this.context, this.object, this.successChance);
+  const Performance(this.action, this.context, this.object, this.successChance,
+      this.additionalData);
 
   /// The performer of the action.
   Actor get actor => context.actor;
@@ -425,19 +449,17 @@ class Performance<T> {
     }
   }
 
-  void _addWorldRecord(ActionRecordBuilder builder, WorldStateBuilder world) {
-    if (_description == null) {
+  void _addWorldRecord(ActionRecordBuilder builder, WorldStateBuilder world,
+      String description) {
+    if (description == null) {
       throw StateError("No description given when executing $action. You "
           "should return it from your world-modifying function.");
     }
-    builder.description = _description;
+    builder.description = description;
     builder.time = world.time;
     world.recordAction(builder.build());
   }
 
-  /// The description of the performance.
-  String _description;
-
   Storyline _applyToWorldCopy(
       ActorTurn turn,
       Simulation sim,
@@ -457,7 +479,7 @@ class Performance<T> {
         .onBeforeAction(sim, initialWorld, outputStoryline);
     final context = ActionContext(action, turn.actor, sim, initialWorld, output,
         outputStoryline, successChance);
-    _description = applyFunction(context, object);
+    final description = applyFunction(context, object);
 
     // The current situation could have been removed by [applyFunction].
     // If not, let's update its time.
@@ -493,7 +515,7 @@ class Performance<T> {
     // methods.
     output.currentSituation?.onAfterTurn(context);
 
-    _addWorldRecord(builder, output);
+    _addWorldRecord(builder, output, description);
     return outputStoryline;
   }