Implement client spectate packet

src/main/java/net/minestom/server/entity/Entity.java

@@ -53,6 +53,7 @@ import java.util.function.Consumer;
 public abstract class Entity implements Viewable, EventHandler, DataContainer, PermissionHandler {
 
     private static final Map<Integer, Entity> entityById = new ConcurrentHashMap<>();
+    private static final Map<UUID, Entity> entityByUuid = new ConcurrentHashMap<>();
     private static final AtomicInteger lastEntityId = new AtomicInteger();
 
     // Metadata
@@ -137,10 +138,10 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer, P
     private long ticks;
     private final EntityTickEvent tickEvent = new EntityTickEvent(this);
 
-    public Entity(@NotNull EntityType entityType, @NotNull Position spawnPosition) {
+    public Entity(@NotNull EntityType entityType, @NotNull UUID uuid, @NotNull Position spawnPosition) {
         this.id = generateId();
         this.entityType = entityType;
-        this.uuid = UUID.randomUUID();
+        this.uuid = uuid;
         this.position = spawnPosition.clone();
 
         setBoundingBox(0, 0, 0);
@@ -148,6 +149,11 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer, P
         setAutoViewable(true);
 
         entityById.put(id, this);
+        entityByUuid.put(uuid, this);
+    }
+
+    public Entity(@NotNull EntityType entityType, @NotNull Position spawnPosition) {
+        this(entityType, UUID.randomUUID(), spawnPosition);
     }
 
     public Entity(@NotNull EntityType entityType) {
@@ -177,6 +183,18 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer, P
         return entityById.getOrDefault(id, null);
     }
 
+    /**
+     * Gets an entity based on its UUID (from {@link #getUuid()}).
+     *
+     * @param uuid the entity UUID
+     * @return the entity having the specified uuid, null if not found
+     */
+    @Nullable
+    public static Entity getEntity(@NotNull UUID uuid) {
+        return entityByUuid.getOrDefault(uuid, null);
+    }
+
+
     /**
      * Generate and return a new unique entity id.
      * <p>

src/main/java/net/minestom/server/listener/SettingsListener.java

@@ -12,7 +12,6 @@ public class SettingsListener {
 
         PlayerSettingsChangeEvent playerSettingsChangeEvent = new PlayerSettingsChangeEvent(player);
         player.callEvent(PlayerSettingsChangeEvent.class, playerSettingsChangeEvent);
-
     }
 
 }

src/main/java/net/minestom/server/listener/SpectateListener.java

@@ -0,0 +1,23 @@
+package net.minestom.server.listener;
+
+import net.minestom.server.entity.Entity;
+import net.minestom.server.entity.Player;
+import net.minestom.server.network.packet.client.play.ClientSpectatePacket;
+
+import java.util.UUID;
+
+public class SpectateListener {
+
+    public static void listener(ClientSpectatePacket packet, Player player) {
+        final UUID targetUuid = packet.targetUuid;
+        final Entity target = Entity.getEntity(targetUuid);
+
+        // Check if the target is valid
+        if (target == null || target == player)
+            return;
+
+        // TODO check if 'target' is in a different instance
+        player.spectate(target);
+    }
+
+}

src/main/java/net/minestom/server/listener/manager/PacketListenerManager.java

@@ -53,6 +53,7 @@ public final class PacketListenerManager {
         setListener(ClientTeleportConfirmPacket.class, TeleportListener::listener);
         setListener(ClientResourcePackStatusPacket.class, ResourcePackListener::listener);
         setListener(ClientAdvancementTabPacket.class, AdvancementTabListener::listener);
+        setListener(ClientSpectatePacket.class, SpectateListener::listener);
     }
 
     /**