Remember and restore existing player scoreboards.

This introduces state in the ScoreboardManager for keeping track of the scoreboards that players have before the arena session starts, if any. Upon leaving, the scoreboards are restored.

Ideally, this change should be re-implemented as a Step of the join/leave process, but that would require opening up Arena or ScoreboardManager in ways that would probably be best handled in a rewrite of the scoreboards, so we'll leave it in ScoreboardManager for now.

Fixes #366

src/main/java/com/garbagemule/MobArena/ScoreboardManager.java

@@ -9,12 +9,17 @@ import org.bukkit.scoreboard.Objective;
 import org.bukkit.scoreboard.Score;
 import org.bukkit.scoreboard.Scoreboard;
 
+import java.util.HashMap;
+import java.util.Map;
+
 public class ScoreboardManager {
     private static final String DISPLAY_NAME = ChatColor.GREEN + "Kills       " + ChatColor.AQUA + "Wave ";
 
     private Arena arena;
     private Scoreboard scoreboard;
     private Objective kills;
+
+    private Map<Player, Scoreboard> scoreboards;
     
     /**
      * Create a new scoreboard for the given arena.
@@ -23,6 +28,7 @@ public class ScoreboardManager {
     ScoreboardManager(Arena arena) {
         this.arena = arena;
         scoreboard = Bukkit.getScoreboardManager().getNewScoreboard();
+        scoreboards = new HashMap<>();
     }
     
     /**
@@ -34,6 +40,7 @@ public class ScoreboardManager {
         /* Set the player's scoreboard and give them an initial non-zero
          * score. This is necessary due to either Minecraft or Bukkit
          * not wanting to show non-zero scores initially. */
+        scoreboards.put(player, player.getScoreboard());
         player.setScoreboard(scoreboard);
         kills.getScore(player).setScore(8);
     }
@@ -45,7 +52,12 @@ public class ScoreboardManager {
      */
     void removePlayer(Player player) {
         try {
-            player.setScoreboard(Bukkit.getScoreboardManager().getMainScoreboard());
+            Scoreboard scoreboard = scoreboards.remove(player);
+            if (scoreboard != null) {
+                player.setScoreboard(scoreboard);
+            } else {
+                player.setScoreboard(Bukkit.getScoreboardManager().getMainScoreboard());
+            }
         } catch (IllegalStateException e) {
             // Happens if the player is logging out, just swallow it
         }