Add Tab Completion loosely based on @TheUnnamedDude's work.

api/src/main/java/net/md_5/bungee/api/plugin/PluginManager.java

@@ -11,6 +11,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Stack;
 import java.util.jar.JarEntry;
@@ -75,6 +76,11 @@ public class PluginManager
         commandMap.values().remove( command );
     }
 
+    public boolean dispatchCommand(CommandSender sender, String commandLine)
+    {
+        return dispatchCommand( sender, commandLine, null );
+    }
+
     /**
      * Execute a command if it is registered, else return false.
      *
@@ -83,7 +89,7 @@ public class PluginManager
      * arguments
      * @return whether the command was handled
      */
-    public boolean dispatchCommand(CommandSender sender, String commandLine)
+    public boolean dispatchCommand(CommandSender sender, String commandLine, List<String> tabResults)
     {
         String[] split = argsSplit.split( commandLine );
         // Check for chat that only contains " "
@@ -112,8 +118,14 @@ public class PluginManager
 
         String[] args = Arrays.copyOfRange( split, 1, split.length );
         try
+        {
+            if ( tabResults == null )
             {
                 command.execute( sender, args );
+            } else if ( command instanceof TabExecutor )
+            {
+                tabResults.addAll( ( (TabExecutor) command ).onTabComplete( sender, args ) );
+            }
         } catch ( Exception ex )
         {
             sender.sendMessage( ChatColor.RED + "An internal error occurred whilst executing this command, please check the console log for details." );

api/src/main/java/net/md_5/bungee/api/plugin/TabExecutor.java

@@ -0,0 +1,11 @@
+package net.md_5.bungee.api.plugin;
+
+import net.md_5.bungee.api.CommandSender;
+
+import java.util.List;
+
+public interface TabExecutor
+{
+
+    public List<String> onTabComplete(CommandSender sender, String[] args);
+}

protocol/src/main/java/net/md_5/bungee/protocol/Vanilla.java

@@ -13,6 +13,7 @@ import net.md_5.bungee.protocol.packet.Packet2Handshake;
 import net.md_5.bungee.protocol.packet.Packet3Chat;
 import net.md_5.bungee.protocol.packet.Packet9Respawn;
 import net.md_5.bungee.protocol.packet.PacketC9PlayerListItem;
+import net.md_5.bungee.protocol.packet.PacketCBTabComplete;
 import net.md_5.bungee.protocol.packet.PacketCCSettings;
 import net.md_5.bungee.protocol.packet.PacketCDClientStatus;
 import net.md_5.bungee.protocol.packet.PacketCEScoreboardObjective;
@@ -56,6 +57,7 @@ public class Vanilla implements Protocol
         classes[0xC9] = PacketC9PlayerListItem.class;
         classes[0x2C] = Packet2CEntityProperties.class;
         classes[0xCC] = PacketCCSettings.class;
+        classes[0xCB] = PacketCBTabComplete.class;
         classes[0xCD] = PacketCDClientStatus.class;
         classes[0xCE] = PacketCEScoreboardObjective.class;
         classes[0xCF] = PacketCFScoreboardScore.class;

protocol/src/main/java/net/md_5/bungee/protocol/packet/AbstractPacketHandler.java

@@ -74,4 +74,8 @@ public abstract class AbstractPacketHandler
     public void handle(PacketFFKick kick) throws Exception
     {
     }
+
+    public void handle(PacketCBTabComplete tabComplete) throws Exception
+    {
+    }
 }

protocol/src/main/java/net/md_5/bungee/protocol/packet/PacketCBTabComplete.java

@@ -0,0 +1,56 @@
+package net.md_5.bungee.protocol.packet;
+
+import io.netty.buffer.ByteBuf;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.ToString;
+
+@Getter
+@ToString
+@EqualsAndHashCode(callSuper = false)
+public class PacketCBTabComplete extends DefinedPacket
+{
+
+    private String cursor;
+    private String[] commands;
+
+    private PacketCBTabComplete()
+    {
+        super( 0xCB );
+    }
+
+    public PacketCBTabComplete(String[] alternatives)
+    {
+        this();
+        commands = alternatives;
+    }
+
+    @Override
+    public void read(ByteBuf buf)
+    {
+        cursor = readString( buf );
+    }
+
+    @Override
+    public void write(ByteBuf buf)
+    {
+        String tab = "";
+        for ( String alternative : commands )
+        {
+            if ( tab.isEmpty() )
+            {
+                tab = alternative + " ";
+            } else
+            {
+                tab += "\0" + alternative + " ";
+            }
+        }
+        writeString( tab, buf );
+    }
+
+    @Override
+    public void handle(AbstractPacketHandler handler) throws Exception
+    {
+        handler.handle( this );
+    }
+}

proxy/src/main/java/net/md_5/bungee/connection/UpstreamBridge.java

@@ -13,8 +13,11 @@ import net.md_5.bungee.netty.PacketHandler;
 import net.md_5.bungee.netty.PacketWrapper;
 import net.md_5.bungee.protocol.packet.Packet0KeepAlive;
 import net.md_5.bungee.protocol.packet.Packet3Chat;
+import net.md_5.bungee.protocol.packet.PacketCBTabComplete;
 import net.md_5.bungee.protocol.packet.PacketCCSettings;
 import net.md_5.bungee.protocol.packet.PacketFAPluginMessage;
+import java.util.ArrayList;
+import java.util.List;
 
 public class UpstreamBridge extends PacketHandler
 {
@@ -89,6 +92,18 @@ public class UpstreamBridge extends PacketHandler
         throw new CancelSendSignal();
     }
 
+    @Override
+    public void handle(PacketCBTabComplete tabComplete) throws Exception
+    {
+        if ( tabComplete.getCursor().startsWith( "/" ) )
+        {
+            List<String> results = new ArrayList<>();
+            bungee.getPluginManager().dispatchCommand( con, tabComplete.getCursor(), results );
+            con.unsafe().sendPacket( new PacketCBTabComplete( results.toArray( new String[ results.size() ] ) ) );
+            throw new CancelSendSignal();
+        }
+    }
+
     @Override
     public void handle(PacketCCSettings settings) throws Exception
     {