Add a start function to activity_actor

src/activity_actor.h

@@ -27,6 +27,13 @@ class activity_actor
          */
         virtual activity_id get_type() const = 0;
 
+        /**
+         * Called once at the start of the activity.
+         * This may be used to preform setup actions and/or set
+         * player_activity::moves_left/moves_total.
+         */
+        virtual void start( player_activity &act, Character &who ) = 0;
+
         /**
          * Called on every turn of the activity
          * It may be used to stop the activity prematurely by setting it to null.
@@ -77,6 +84,8 @@ class move_items_activity_actor : public activity_actor
         activity_id get_type() const override {
             return activity_id( "ACT_MOVE_ITEMS" );
         }
+
+        void start( player_activity &, Character & ) override {};
         void do_turn( player_activity &act, Character &who ) override;
         void finish( player_activity &act, Character &who ) override;
 
@@ -97,6 +106,7 @@ class migration_cancel_activity_actor : public activity_actor
             return activity_id( "ACT_MIGRATION_CANCEL" );
         }
 
+        void start( player_activity &, Character & ) override {};
         void do_turn( player_activity &act, Character &who ) override;
         void finish( player_activity &act, Character &who ) override;
 

src/character.cpp

@@ -9132,9 +9132,8 @@ void Character::assign_activity( const player_activity &act, bool allow_resume )
         activity = act;
     }
 
-    if( activity.rooted() ) {
-        rooted_message();
-    }
+    activity.start( *this );
+
     if( is_npc() ) {
         cancel_stashed_activity();
         npc *guy = dynamic_cast<npc *>( this );

src/player_activity.cpp

@@ -170,6 +170,16 @@ cata::optional<std::string> player_activity::get_progress_message( const avatar
                     get_verb().translated(), extra_info );
 }
 
+void player_activity::start( Character &who )
+{
+    if( actor ) {
+        actor->start( *this, who );
+    }
+    if( rooted() ) {
+        who.rooted_message();
+    }
+}
+
 void player_activity::do_turn( player &p )
 {
     // Should happen before activity or it may fail du to 0 moves

src/player_activity.h

@@ -39,14 +39,15 @@ class player_activity
         int moves_total = 0;
         /** The number of moves remaining in this activity before it is complete. */
         int moves_left = 0;
-        /** An activity specific value. */
+
+        // The members in the following block are deprecated, prefer creating a new
+        // activity_actor.
         int index = 0;
         /**
          *   An activity specific value.
          *   DO NOT USE FOR ITEM INDEX
         */
         int position = 0;
-        /** An activity specific value. */
         std::string name;
         std::vector<item_location> targets;
         std::vector<int> values;
@@ -55,6 +56,7 @@ class player_activity
         std::unordered_set<tripoint> coord_set;
         std::vector<weak_ptr_fast<monster>> monsters;
         tripoint placement;
+
         bool no_drink_nearby_for_auto_consume = false;
         bool no_food_nearby_for_auto_consume = false;
         /** If true, the activity will be auto-resumed next time the player attempts
@@ -122,6 +124,12 @@ class player_activity
         /** Convert from the old enumeration to the new string_id */
         void deserialize_legacy_type( int legacy_type, activity_id &dest );
 
+        /**
+         * Preform necessary initialization to start the activity. Must be
+         * called whenever a Character starts a new activity.
+         */
+        void start( Character &who );
+
         /**
          * Performs the activity for a single turn. If the activity is complete
          * at the end of the turn, do_turn also executes whatever actions, if