The chunk view distance can now be dynamically modified both by the client or with the server using MinecraftServer#setChunkViewDistance, also added UpdateViewDistancePacket

src/main/java/net/minestom/server/MinecraftServer.java

@@ -13,12 +13,14 @@ import net.minestom.server.data.DataType;
 import net.minestom.server.data.SerializableData;
 import net.minestom.server.entity.EntityManager;
 import net.minestom.server.entity.EntityType;
+import net.minestom.server.entity.Player;
 import net.minestom.server.extensions.Extension;
 import net.minestom.server.extensions.ExtensionManager;
 import net.minestom.server.extras.mojangAuth.MojangCrypt;
 import net.minestom.server.fluids.Fluid;
 import net.minestom.server.gamedata.loottables.LootTableManager;
 import net.minestom.server.gamedata.tags.TagManager;
+import net.minestom.server.instance.Chunk;
 import net.minestom.server.instance.InstanceManager;
 import net.minestom.server.instance.block.Block;
 import net.minestom.server.instance.block.BlockManager;
@@ -33,6 +35,7 @@ import net.minestom.server.network.PacketWriterUtils;
 import net.minestom.server.network.netty.NettyServer;
 import net.minestom.server.network.packet.server.play.PluginMessagePacket;
 import net.minestom.server.network.packet.server.play.ServerDifficultyPacket;
+import net.minestom.server.network.packet.server.play.UpdateViewDistancePacket;
 import net.minestom.server.particle.Particle;
 import net.minestom.server.ping.ResponseDataConsumer;
 import net.minestom.server.potion.PotionEffect;
@@ -45,6 +48,7 @@ import net.minestom.server.stat.StatisticType;
 import net.minestom.server.storage.StorageLocation;
 import net.minestom.server.storage.StorageManager;
 import net.minestom.server.timer.SchedulerManager;
+import net.minestom.server.utils.MathUtils;
 import net.minestom.server.utils.thread.MinestomThread;
 import net.minestom.server.utils.validate.Check;
 import net.minestom.server.world.Difficulty;
@@ -57,6 +61,7 @@ import org.slf4j.LoggerFactory;
 import java.io.IOException;
 import java.net.Proxy;
 import java.security.KeyPair;
+import java.util.Collection;
 
 /**
  * The main server class used to start the server and retrieve all the managers.
@@ -432,15 +437,28 @@ public class MinecraftServer {
 
     /**
      * Changes the chunk view distance of the server.
-     * <p>
-     * WARNING: this need to be called before {@link #start(String, int, ResponseDataConsumer)}.
      *
      * @param chunkViewDistance the new chunk view distance
      * @throws IllegalStateException if this is called after the server started
      */
     public static void setChunkViewDistance(int chunkViewDistance) {
-        Check.stateCondition(started, "The chunk view distance cannot be changed after the server has been started.");
+        Check.argCondition(!MathUtils.isBetween(chunkViewDistance, 2, 32), "The chunk view distance needs to be between 2 and 32");
         MinecraftServer.chunkViewDistance = chunkViewDistance;
+        if (started) {
+            UpdateViewDistancePacket updateViewDistancePacket = new UpdateViewDistancePacket();
+            updateViewDistancePacket.viewDistance = chunkViewDistance;
+
+            final Collection<Player> players = connectionManager.getOnlinePlayers();
+
+            PacketWriterUtils.writeAndSend(players, updateViewDistancePacket);
+
+            connectionManager.getOnlinePlayers().forEach(player -> {
+                final Chunk playerChunk = player.getChunk();
+                if (playerChunk != null) {
+                    player.refreshVisibleChunks(playerChunk);
+                }
+            });
+        }
     }
 
     /**

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

@@ -1032,7 +1032,7 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         final boolean isPlayer = this instanceof Player;
 
         if (isPlayer)
-            ((Player) this).onChunkChange(newChunk); // Refresh loaded chunk
+            ((Player) this).refreshVisibleChunks(newChunk); // Refresh loaded chunk
 
         // Refresh entity viewable list
         final int entityViewDistance = MinecraftServer.getEntityViewDistance();

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

@@ -1327,7 +1327,7 @@ public class Player extends LivingEntity implements CommandSender {
             final BlockPosition pos = position.toBlockPosition();
             final Chunk chunk = instance.getChunk(pos.getX() >> 4, pos.getZ() >> 4);
             Check.notNull(chunk, "Tried to interact with an unloaded chunk.");
-            onChunkChange(chunk);
+            refreshVisibleChunks(chunk);
         }
     }
 
@@ -1406,13 +1406,14 @@ public class Player extends LivingEntity implements CommandSender {
 
     /**
      * Called when the player changes chunk (move from one to another).
+     * Can also be used to refresh the list of chunks that the client should see.
      * <p>
      * It does remove and add the player from the chunks viewers list when removed or added.
      * It also calls the events {@link PlayerChunkUnloadEvent} and {@link PlayerChunkLoadEvent}.
      *
      * @param newChunk the current/new player chunk
      */
-    protected void onChunkChange(@NotNull Chunk newChunk) {
+    public void refreshVisibleChunks(@NotNull Chunk newChunk) {
         // Previous chunks indexes
         final long[] lastVisibleChunks = viewableChunks.stream().mapToLong(viewableChunks ->
                 ChunkUtils.getChunkIndex(viewableChunks.getChunkX(), viewableChunks.getChunkZ())
@@ -2389,6 +2390,8 @@ public class Player extends LivingEntity implements CommandSender {
         private byte displayedSkinParts;
         private MainHand mainHand;
 
+        private boolean firstRefresh = true;
+
         /**
          * The player game language.
          *
@@ -2451,6 +2454,9 @@ public class Player extends LivingEntity implements CommandSender {
          * @param mainHand           the player main hand
          */
         public void refresh(String locale, byte viewDistance, ChatMode chatMode, boolean chatColors, byte displayedSkinParts, MainHand mainHand) {
+
+            final boolean viewDistanceChanged = !firstRefresh && this.viewDistance != viewDistance;
+
             this.locale = locale;
             this.viewDistance = viewDistance;
             this.chatMode = chatMode;
@@ -2458,6 +2464,16 @@ public class Player extends LivingEntity implements CommandSender {
             this.displayedSkinParts = displayedSkinParts;
             this.mainHand = mainHand;
             sendMetadataIndex(16);
+
+            this.firstRefresh = false;
+
+            // Client changed his view distance in the settings
+            if (viewDistanceChanged) {
+                final Chunk playerChunk = getChunk();
+                if (playerChunk != null) {
+                    refreshVisibleChunks(playerChunk);
+                }
+            }
         }
 
     }

src/main/java/net/minestom/server/network/packet/server/play/UpdateViewDistancePacket.java

@@ -0,0 +1,21 @@
+package net.minestom.server.network.packet.server.play;
+
+import net.minestom.server.network.packet.server.ServerPacket;
+import net.minestom.server.network.packet.server.ServerPacketIdentifier;
+import net.minestom.server.utils.binary.BinaryWriter;
+import org.jetbrains.annotations.NotNull;
+
+public class UpdateViewDistancePacket implements ServerPacket {
+
+    public int viewDistance;
+
+    @Override
+    public void write(@NotNull BinaryWriter writer) {
+        writer.writeVarInt(viewDistance);
+    }
+
+    @Override
+    public int getId() {
+        return ServerPacketIdentifier.UPDATE_VIEW_DISTANCE;
+    }
+}