SceneMultiplayer in Godot – Complete Guide

Diving into the world of game development can be both thrilling and overwhelming, especially when it comes to designing games that can be enjoyed by multiple players across the globe in real time. However, thanks to powerful engines like Godot 4, the complexities of creating these interactive experiences become much more manageable. One of the key components of Godot 4’s networking capabilities is the SceneMultiplayer class – a high-level API that simplifies the process of implementing multiplayer functionality in your games. Whether you dream of creating a cooperative dungeon crawler or a competitive racing game, understanding SceneMultiplayer can be a game-changer.

What Is SceneMultiplayer?

SceneMultiplayer is the default high-level multiplayer API implementation offered by the Godot Engine. As a subclass of MultiplayerAPI, it provides an assortment of functionalities that streamline multiplayer game development. With SceneMultiplayer, developers can easily set up remote procedure calls (RPCs), handle scene tree replication, and manage peer-to-peer connections, all with the intent of making multiplayer gaming as smooth as possible.

What Is SceneMultiplayer Used For?

The key purpose of SceneMultiplayer is to facilitate the creation of multiplayer aspects within a game project. It allows game developers to:

  • Perform remote procedure calls (RPCs).
  • Control synchronization of game objects and state across different game sessions.
  • Manage connections and disconnections of players in a networked game.

It supports specialized nodes like MultiplayerSpawner and MultiplayerSynchronizer, which work together with a resource called SceneReplicationConfig to manage the replication of scenes, ensuring that players have a consistent view of the game world.

Why Should You Learn SceneMultiplayer?

For aspiring game developers looking to explore the multiplayer realm, SceneMultiplayer represents an accessible entry-point. Here’s why understanding this class can be highly beneficial:

  • Accessibility: Godot’s SceneMultiplayer simplifies complex networking tasks, making it more accessible for developers of all levels to create multiplayer games.
  • Community: Adding multiplayer features can significantly increase a game’s appeal, fostering a stronger, more engaged community around your creation.
  • Skills Development: Mastering SceneMultiplayer equips you with valuable skills that are in high demand within the gaming industry.

In essence, learning how to manipulate SceneMultiplayer is not just about adding a multiplayer feature to your game; it’s about expanding your capabilities as a game developer and opening up new possibilities for what you can create. So, let’s journey together through the intricacies of Godot’s SceneMultiplayer and start breathing life into the multiplayer experiences you’ve always wanted to build.

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Setting Up a Basic Multiplayer Scene

Before we dive into SceneMultiplayer directly, we need to set up a basic multiplayer scene in Godot 4. Let’s start by initializing the SceneMultiplayer and setting up the network peers.

var scene_multiplayer = SceneMultiplayer.new()

func _ready():
    var peer = NetworkedMultiplayerENet.new()
    # For the server
    peer.create_server(9096, 4) # Port number and max clients
    # For the client
    # peer.create_client("127.0.0.1", 9096) # IP address and port number
    scene_multiplayer.set_network_peer(peer)

Once the network peer is set up, we can define Remote Procedure Calls (RPCs) which allow us to call functions from one instance across the network onto another.

# Marking a function as an RPC
@scene_multiplayer.rpc_sync("_on_Player_moved", rpc_id(1), position)

# This function could be remotely called
func _on_Player_moved(new_position):
    # Move the player to the new position

Handling Player Spawning and Synchronization

In any multiplayer game, spawning players at the start or when they join mid-game is crucial. We utilize MultiplayerSpawner for this purpose.

var spawner = MultiplayerSpawner.new()

func _ready():
    scene_multiplayer.add_child(spawner)
    spawner.set_config(spawn_config)
    spawner.auto_spawn = true

Then, to synchronize objects across the network, we use the MultiplayerSynchronizer:

var synchronizer = MultiplayerSynchronizer.new()

func _ready():
    scene_multiplayer.add_child(synchronizer)
    synchronizer.register_node("player", $Player, true, true, {"position", "rotation"})

Managing State with SceneReplication

A critical component in multiplayer games is ensuring that all players see the same game state. This is where SceneReplicationConfig comes into play.

var replication_config = SceneReplicationConfig.new()

func _ready():
    replication_config.add_class("Player", {"health", "ammo", "position"})
    scene_multiplayer.replication_config = replication_config

The replication_config object declares which properties should be replicated and kept in sync across clients.

Handling Player Connection and Disconnection

Next, we have to manage the joining and leaving of players. To know when a player connects or disconnects, we can connect to the ‘player_connected’ and ‘player_disconnected’ signals:

func _ready():
    scene_multiplayer.connect("player_connected", self, "_player_connected")
    scene_multiplayer.connect("player_disconnected", self, "_player_disconnected")

func _player_connected(id):
    print("Player connected with ID: ", id)

func _player_disconnected(id):
    print("Player disconnected with ID: ", id)

In these examples, we’ve laid down the foundations for a multiplayer game with basic player spawning, synchronization of objects, managing the game state across sessions, and tracking player connections. With the SceneMultiplayer components set and configured, you’re now on the path to creating the multiplayer experiences you’ve envisioned. Stay tuned as we delve deeper into examples that will showcase more advanced functionality of SceneMultiplayer in the next sections of our tutorial.As we progress further into the intricacies of SceneMultiplayer in Godot 4, we’ll explore how we can effectively leverage RPCs to send and receive important gameplay messages between clients and the server. Additionally, we’ll take a look at how we can use SceneMultiplayer to make gameplay decisions based on the players’ network roles, and manage the overall game state. Remember, clear and concise RPC communication is fundamental for a smooth multiplayer experience.

Expanding on Remote Procedure Calls (RPCs)

Remote Procedure Calls (RPCs) are critical in multiplayer games. They empower you to execute code across different client instances in a networked game. Here’s how you mark a function to be remotely callable in SceneMultiplayer:

# To enable an RPC, you'd use the 'rpc' keyword in a function call as such:
func perform_action(param):
    rpc("perform_action_on_all_clients", param)

@scene_multiplayer.rpc_sync
func perform_action_on_all_clients(param):
    # This function will be called on all connected clients.
    print("Action performed with parameter: ", param)

Determining who receives the RPC calls can be controlled using keywords like `rpc_id` for a specific client, or `rpc_unreliable` for non-critical messages that don’t require guaranteed delivery.

# To call an RPC on a specific client
func send_message_to_client(client_id, message):
    rpc_id(client_id, "receive_message", message)

# This function would be defined on the client to receive the message
@scene_multiplayer.rpc_sync
func receive_message(message):
    print("Message received: ", message)

These RPC call examples show how a function decorated with `@scene_multiplayer.rpc_sync` can be used to sync or send messages between clients.

Using Network Roles to Control Gameplay

Gameplay can often depend on whether a player is a host or a client. It’s crucial to control what each can or cannot do within the game. Identifying the network role is straightforward in Godot:

# Check if we're the server
if scene_multiplayer.is_network_server():
    # Execute server-specific code
    print("This is the server speaking.")

# Check if we're a client
else:
    # Execute client-specific code
    print("This is a client speaking.")

This network role check allows you to branch your code based on whether the code is being executed on the host or a client machine.

Syncing Game State with Scene Replication

Replicating the scene across all network peers is important particularly when a new player joins a game already in progress. Here’s how you replicate an existing game state:

# Assuming 'game_state' is a dictionary with our current game state.
@scene_multiplayer.rpc_sync
func sync_game_state(game_state):
    # Apply the received game state
    apply_game_state(game_state)

# On the client's instance, this function would interpret and apply the state
func apply_game_state(state):
    # Update game state logic here

Using SceneMultiplayer’s replication features, your game can maintain a synchronized game state across all clients.

Handling Network Events and Keeping Track of Players

It’s also essential to handle network events such as connection and disconnection properly:

# Registering player session
func _player_connected(id):
    print("Player ", id, " connected.")
    # Additional logic for when a player connects

func _player_disconnected(id):
    print("Player ", id, " disconnected.")
    # Additional logic for when a player disconnects

In these examples, we have shown how SceneMultiplayer can be utilized to manage both RPC calls and the complexities of network roles, as well as keeping the game state consistent across different game sessions. These are the building blocks to a robust multiplayer system and, with these examples, you’re well on your way to creating an engaging, responsive multiplayer environment in your Godot 4 games.

Remember to keep your functions atomic and manageable, since clear and debuggable code is key in multiplayer game development. With the basics covered, you’ll be ready to implement more complex behaviors and really start bringing your multiplayer game to life.Expanding upon the fundamentals of SceneMultiplayer in Godot 4, we move forward to tackle more nuanced aspects of multiplayer game development. This includes advanced RPCs usage, predictive movements for a smoother gameplay, handling disconnections gracefully, and ensuring security in our multiplayer game.

First, let’s consider the unlikely, yet possible scenario where a message might get lost. For non-critical updates where occassional loss is acceptable, you can use an unreliable RPC:

# An unreliable RPC call that might occasionally drop messages
@scene_multiplayer.rpc_unreliable
func update_player_position(new_position):
    # This code updates the player’s position
    update_position(new_position)

Now, let’s imagine we want to implement lag compensation for character movement. We can use an authoritative server model, where clients send inputs to the server, and the server updates the game state and returns the result:

# Client-side function sends the movement input to the server
func send_movement_input(movement_vec):
    rpc_id(1, "process_movement", movement_vec)

# Server-side function that processes the movement input
@scene_multiplayer.rpc_master
func process_movement(movement_vec):
    # Perform movement logic here
    # Then sync the position back to the clients
    rpc("update_player_position_on_clients", get_position())

Next, consider a function that should only be executable by the client that owns the object. For instance, a player can only request to open their own inventory:

# The client requests to open its inventory
func request_open_inventory():
    rpc_id(1, "open_inventory", get_tree().get_network_unique_id())

# Server-side checks the request comes from the client that owns the player node
@scene_multiplayer.rpc_master
func open_inventory(requesting_id):
    if requesting_id == get_tree().get_network_unique_id():
        # Open the inventory for the player

Graceful disconnection handling is a crucial part of maintaining a pleasant multiplayer experience. Here’s how you might handle an unexpected disconnection:

func _player_disconnected(id):
    print("Player ", id, " disconnected unexpectedly.")
    # Execute any cleanup or game state updates required
    handle_disconnect(id)

Security is also a major concern in multiplayer games. You must validate all client inputs on the server to prevent cheating. One way to do this is by only allowing the server to make significant changes to the game state:

# This function should only be allowed to be called by the server
@scene_multiplayer.rpc_master
func update_game_state(new_state):
    if scene_multiplayer.is_network_server():
        # Update the authoritative game state
        game_state = new_state
    else:
        print("Unauthorised attempt to change game state")

With robust systems in place for network communication, movement prediction, handling client disconnections, and security, your multiplayer game is shaping up to be both dynamic and secure. These examples highlight just a small slice of what’s possible with Godot’s SceneMultiplayer. The network multiplayer API is a powerful tool, and as you continue to experiment and learn, you’ll discover even more ways to optimize and enhance multiplayer experiences in your games.

Continue Your Game Development Journey with Godot

You’ve taken the first crucial steps into multiplayer game development with Godot 4, and your journey has only just begun. There’s a whole universe of knowledge out there waiting for you to master it, and we’re here to support you along the way. At Zenva, we offer courses that cater to both beginners and more experienced developers, arming you with the skills necessary to take your game creation dreams from concept to reality.

The Godot Game Development Mini-Degree is your next destination. This comprehensive learning path is specially designed to deepen your understanding of the Godot 4 engine and expand your skills in creating various game genres. Dive into topics like 2D and 3D game development, GDScript, game mechanics, and more, all at your own pace and on your preferred device.

Furthermore, for those who want to explore a variety of Godot tutorials and courses, visit our full collection of Godot courses. With over 250 supported courses, Zenva is the perfect platform to bolster your career, learn new coding techniques, craft amazing games, and earn certificates. Continue to learn and grow with us – your potential is unlimited, and with Zenva, you can transform your passion into profession.

Conclusion

As you forge ahead in the world of game development, remember that every line of code, every successful compile, and every multiplayer session you initiate brings you closer to realizing your visions. Harnessing the power of Godot 4 and its SceneMultiplayer API empowers you to create engaging multiplayer experiences that could resonate with players around the world. The path of a game developer is one of lifelong learning and creativity, and you’re not walking it alone. At Zenva, we’re committed to providing you with the education you need to keep creating, innovating, and sharing your passion for game development with a community that celebrates your achievements.

Dive deeper into game development, with our Godot Game Development Mini-Degree, and transform your ability to bring multiplayer dreams to life. Whether you’re just starting out or looking to polish your skills and learn new techniques, we’re here to support your growth every step of the way. Let your creativity run wild, and may your games be played and loved by many!

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