feat: add boss bar viewer

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -29,6 +29,7 @@
 import java.util.Optional;
 import java.util.UUID;
 import java.util.function.UnaryOperator;
+import net.kyori.adventure.bossbar.BossBarViewer;
 import net.kyori.adventure.dialog.DialogLike;
 import net.kyori.adventure.identity.Identified;
 import net.kyori.adventure.inventory.Book;
@@ -51,7 +52,8 @@ public interface Player extends
     CommandSource, InboundConnection, ChannelMessageSource, ChannelMessageSink,
     /* Adventure-specific interfaces */
     Identified, HoverEventSource<HoverEvent.ShowEntity>, Keyed, KeyIdentifiable, Sound.Emitter,
-    PlayerHeadObjectContents.SkinSource {
+    PlayerHeadObjectContents.SkinSource,
+    BossBarViewer {
 
   /**
    * Returns the player's current username.

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -145,6 +145,7 @@
 import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.UnmodifiableView;
 
 /**
  * Represents a player that is connected to the proxy.
@@ -185,7 +186,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
   private @Nullable VelocityServerConnection connectionInFlight;
   private @Nullable PlayerSettings settings;
   private @Nullable ModInfo modInfo;
-  private final Set<VelocityBossBarImplementation> bossBars = new HashSet<>();
+  private @Nullable Set<BossBar> bossBars;
   private Component playerListHeader = Component.empty();
   private Component playerListFooter = Component.empty();
   private final InternalTabList tabList;
@@ -237,8 +238,10 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
    * Used for cleaning up resources during a disconnection.
    */
   public void disconnected() {
-    for (final VelocityBossBarImplementation bar : this.bossBars) {
-      bar.viewerDisconnected(this);
+    if (this.bossBars != null) {
+      for (final BossBar bar : this.bossBars) {
+        VelocityBossBarImplementation.get(bar).viewerRemove(this);
+      }
     }
   }
 
@@ -587,25 +590,41 @@ public void resetTitle() {
   }
 
   @Override
-  public void hideBossBar(@NonNull BossBar bar) {
+  public void showBossBar(@NonNull BossBar bar) {
     if (this.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_9)) {
       final VelocityBossBarImplementation impl = VelocityBossBarImplementation.get(bar);
-      if (impl.viewerRemove(this)) {
-        this.bossBars.remove(impl);
+      if (impl.viewerAdd(this)) {
+        if (this.bossBars == null) {
+          this.bossBars = new HashSet<>();
+        }
+        this.bossBars.add(bar);
       }
     }
   }
 
   @Override
-  public void showBossBar(@NonNull BossBar bar) {
+  public void hideBossBar(@NonNull BossBar bar) {
     if (this.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_9)) {
       final VelocityBossBarImplementation impl = VelocityBossBarImplementation.get(bar);
-      if (impl.viewerAdd(this)) {
-        this.bossBars.add(impl);
+      if (impl.viewerRemove(this)) {
+        if (this.bossBars != null) {
+          this.bossBars.remove(bar);
+          if (this.bossBars.isEmpty()) {
+            this.bossBars = null;
+          }
+        }
       }
     }
   }
 
+  @Override
+  public @UnmodifiableView @org.jspecify.annotations.NonNull Iterable<? extends BossBar> activeBossBars() {
+    if (this.bossBars == null) {
+      return Set.of();
+    }
+    return Set.copyOf(this.bossBars);
+  }
+
   @Override
   public ConnectionRequestBuilder createConnectionRequest(RegisteredServer server) {
     return new ConnectionRequestBuilderImpl(server, this.connectedServer);