ULTRAKILL Multiplayer Utility

Author: Dolfe

ULTRAKILL Multiplayer Utility is a lightweight, Steam-based peer-to-peer (P2P) utility library for creating multiplayer features in ULTRAKILL mods using BepInEx. It provides a high-level interface for lobby creation, network messaging, data synchronization, and designed to reduce boilerplate and the pain of setting up steamworks.

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Table of Contents

• Features
• Getting Started
• To Do
• Lobby Management
• Data Transmission
• Callbacks & Events
• Best Practices
• Ideas or Additions
• License

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Features

• Lightweight P2P communication built on Steam networking.
• Easy-to-use data sending (reliable, unreliable, etc.).
• Lobby creation, search, join, leave, and chat utils.
• Automatic serialization and deserialization of data.
• Custom type synchronization and event based observation.
• Optional support for using steam 480 (for testing), or mods.
• All LobbyManager methods are documented

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TODO

Set lobby settings method
Add documentation for current_lobby

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Getting Started

Prerequisites

• ULTRAKILL installed via Steam.
• BepInEx (x64) installed and functioning.
• Your project or mod should reference MultiplayerUtil.dll.

Installation

1. Place MultiplayerUtil.dll in the BepInEx/plugins/ directory.
2. Ensure that any mod using this utility declares it as a dependency.
3. Hook into MultiplayerUtil.Callbacks.StartupComplete before invoking any Steam functionality.

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Lobby Management

You can easily manage Steam lobbies via static methods under MultiplayerUtil.LobbyManager:

// Create a new public lobby for 4 players
MU.LobbyManager.CreateLobby(
    lobbyName: "New lobby",
    maxPlayers: 4,
    publicLobby: true,
    cracked: false,
    mods: false,
    modIdentifier: ("YourMod", "Tag")
);

// Search for existing lobbies using your identifier
var lobbies = await LobbyManager.FetchLobbies(("YourMod", "Tag"));

// Join a lobby by its ulong ID
LobbyManager.JoinLobbyWithID(lobbyId);

// Disconnect from current lobby
LobbyManager.Disconnect();

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Data Transmission

Sending Data

MyClass exampleData = new MyClass();
exampleData.playerPos = ...;
LobbyManager.SendData(exampleData, SendMethod.UnreliableNoDelay);
// Make sure your class is System.Serializable

Available send methods:

• Unreliable Fast but no guarantee of delivery or order, Ok at what it does
• UnreliableNoDelay Very fast, no delay, no guarantee of it arriving or being in order, Good for fast changing data such as player positions
• Reliable Guaranteed delivery, but not necessarily ordered, Good for one time updates
• ReliableWithBuffering Reliable and ensures they are sent in the right order, Good for bulk data sending

You can also:

LobbyManager.SendToLobbyOwner(data, SendMethod.Reliable);

Receiving Data

To observe incoming data:

ObserveManager.SubscribeToType(typeof(MyClass), out Callbacks.SenderUnityEvent myEvent);
myEvent.AddListener(payload =>
{
    var obj = Data.Deserialize<MyClass>(payload.Item1);
    var sender = payload.Item2;
});

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Callbacks & Events

Hook into a range of events:

• Callbacks.StartupComplete
• Callbacks.TimeToSendImportantData
• Callbacks.TimeToSendUnimportantData
• Callbacks.OnLobbyMemberJoined
• Callbacks.OnLobbyMemberLeave
• Callbacks.OnChatMessageReceived
• Callbacks.OnLobbyCreated
• Callbacks.OnLobbyEntered
• Callbacks.OnP2PConnectionFailed

Example:

Callbacks.OnChatMessageReceived.AddListener(msg => Debug.Log($"Chat: {msg}"));

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Best Practices

• Use UnreliableNoDelay for real-time data (e.g., positions).
• Use ReliableWithBuffering for sequential state or synced actions.
• Make sure the classes that contain the data you are sending are serialized
• Keep serialized classes small and avoid large strings.
• Use primitives (e.g., byte, short) when possible to reduce size.
• Catch exceptions around network operations to avoid crashes.

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Ideas Or additions

Just message me on discord (@dolfelive) or create a issue/pull request

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License

Dolfe AGPLish License Refer to LICENSE.md