NetworkBehaviour synchronization

NetworkBehaviour is an abstract class that derives from MonoBehaviour and is primarily used to create unique netcode or game logic. To replicate any netcode-aware properties or send and receive RPCs, a GameObject must have a NetworkObject component and at least one NetworkBehaviour component.

You can use NetworkBehaviours to synchronize settings before, during, and after spawning NetworkObjects.

For more information about spawning and despawning NetworkBehaviours, refer to the NetworkBehaviour spawning and despawning page.

Pre-spawn, spawn, post-spawn and synchronization

The NetworkObject spawn process can become complicated when there are multiple NetworkBehaviour components attached to the same GameObject. Additionally, there can be times where you want to be able to handle pre- and post-spawn oriented tasks.

• Pre-spawn example: Instantiating a NetworkVariable with owner write permissions and assigning a value to that NetworkVariable on the server or host side.
• Spawn example: Applying a local value or setting that may be used during post spawn by another local NetworkBehaviour component.
• Post-spawn example: Accessing a NetworkVariable or other property that is set during the spawn process.

Below are the available virtual methods you can override within a NetworkBehaviour-derived class:

Method	Scope	Use case	Context
OnNetworkPreSpawn	NetworkObject	Pre-spawn initialization	Client and server
OnNetworkSpawn	NetworkObject	During spawn initialization	Client and server
OnNetworkPostSpawn	NetworkObject	Post-spawn actions	Client and server
OnNetworkSessionSynchronized	All NetworkObjects	New client finished synchronizing	Client-side only
OnInSceneObjectsSpawned	In-scene NetworkObjects	New client finished synchronizing or a scene is loaded	Client and server

OnNetworkSessionSynchronized and OnInSceneObjectsSpawned are two special case convenience methods:

• OnNetworkSessionSynchronized: When scene management is enabled and a new client joins a session, the client starts synchronizing with the network session. During this period of time the client might need to load additional scenes as well as instantiate and spawn NetworkObjects. When a client has finished loading all scenes and all NetworkObjects are spawned, this method gets invoked on all NetworkBehaviours associated with any spawned NetworkObjects. This can be useful if you want to write scripts that might require access to other spawned NetworkObjects and/or their NetworkBehaviour components. When this method is invoked, you are assured that everything is spawned and ready to be accessed and/or to have messages sent from them. Remember that this method is only invoked on clients and isn't invoked on a server or host.
• OnInSceneObjectsSpawned: This method ensures that any in-scene placed NetworkObjects have been spawned prior to a specific set of scripts being invoked. This method is invoked on in-scene placed NetworkObjects when:
  • A server or host first starts up after all in-scene placed NetworkObjects in the currently loaded scene(s) have been spawned.
  • A client finishes synchronizing.
  • On the server and client side after a scene has been loaded and all newly instantiated in-scene placed NetworkObjects have been spawned.

Pre-spawn synchronization with OnSynchronize

There can be scenarios where you need to include additional configuration data or use a NetworkBehaviour to configure some non-netcode related component before a NetworkObject is spawned. This can be particularly critical if you want specific settings applied before NetworkBehaviour.OnNetworkSpawn is invoked. When a client is synchronizing with an existing network session, this can become problematic as messaging requires a client to be fully synchronized before you know "it is safe" to send the message, and even if you send a message there is latency involved that might require additional specialized code to compensate for.

NetworkBehaviour.OnSynchronize allows you to write and read custom serialized data during the NetworkObject serialization process.

There are two cases where NetworkObject synchronization occurs:

• When dynamically spawning a NetworkObject.
• When a client is being synchronized after connection approval (that is, full synchronization of the NetworkObjects and scenes).

info

The INetworkSerializable interface is similar to NetworkBehaviour.OnSynchronize, and can be a useful reference point to understand first.

Order of operations when dynamically spawning

The following provides you with an outline of the order of operations that occur during NetworkObject serialization when dynamically spawned.

Server-side:

• GameObject with NetworkObject component is instantiated.
• The NetworkObject is spawned.
  • For each associated NetworkBehaviour component, NetworkBehaviour.OnNetworkSpawn is invoked.
• The CreateObjectMessage is generated.
  • NetworkObject state is serialized.
  • NetworkVariable state is serialized.
  • NetworkBehaviour.OnSynchronize is invoked for each NetworkBehaviour component.
    • If this method isn't overridden then nothing is written to the serialization buffer.
• The CreateObjectMessage is sent to all clients that are observers of the NetworkObject.

Client-side:

• The CreateObjectMessage is received.
  • GameObject with NetworkObject component is instantiated.
  • NetworkVariable state is deserialized and applied.
  • NetworkBehaviour.OnSynchronize is invoked for each NetworkBehaviour component.
    • If this method isn't overridden then nothing is read from the serialization buffer.
• The NetworkObject is spawned.
  • For each associated NetworkBehaviour component, NetworkBehaviour.OnNetworkSpawn is invoked.

Order of operations during full (late-join) client synchronization

Server-side:

• The SceneEventMessage of type SceneEventType.Synchronize is created.
  • All spawned NetworkObjects that are visible to the client, already instantiated, and spawned are serialized.
    • NetworkObject state is serialized.
    • NetworkVariable state is serialized.
    • NetworkBehaviour.OnSynchronize is invoked for each NetworkBehaviour component.
      • If this method isn't overridden then nothing is written to the serialization buffer.
• The SceneEventMessage is sent to the client.

Client-side:

• The SceneEventMessage of type SceneEventType.Synchronize is received.
• Scene information is deserialized and scenes are loaded (if not already).
  • In-scene placed NetworkObjects are instantiated when a scene is loaded.
• All NetworkObject-oriented synchronization information is deserialized.
  • Dynamically spawned NetworkObjects are instantiated and state is synchronized.
  • For each NetworkObject instance:
    • NetworkVariable state is deserialized and applied.
    • NetworkBehaviour.OnSynchronize is invoked.
      • If this method isn't overridden then nothing is read from the serialization buffer.
    • The NetworkObject is spawned.
      • For each associated NetworkBehaviour component, NetworkBehaviour.OnNetworkSpawn is invoked.

OnSynchronize example

Understanding when to use OnSynchronize is key to making the most of the method. NetworkVariables can be useful to synchronize state, but they're only updated every network tick and you might have some form of state that needs to be updated when it happens and not several frames later, so you decide to use RPCs. However, this becomes an issue when you want to synchronize late-joining clients as there is no way to synchronize late-joining clients based on RPC activity over the duration of a network session. This is one of many possible reasons one might want to use NetworkBehaviour.OnSynchronize.

The following example uses NetworkBehaviour.OnSynchronize to synchronize connecting (to-be-synchronized) clients and also uses an RPC to synchronize changes in state for already synchronized and connected clients:

using UnityEngine;
using Unity.Netcode;

/// <summary>
/// Simple RPC driven state that shows one
/// form of NetworkBehaviour.OnSynchronize usage
/// </summary>
public class SimpleRpcState : NetworkBehaviour
{
    private bool m_ToggleState;

    /// <summary>
    /// Late joining clients will be synchronized
    /// to the most current m_ToggleState
    /// </summary>
    protected override void OnSynchronize<T>(ref BufferSerializer<T> serializer)
    {
        serializer.SerializeValue(ref m_ToggleState);
        base.OnSynchronize(ref serializer);
    }

    public void ToggleState(bool stateIsSet)
    {
        m_ToggleState = stateIsSet;
    }

    /// <summary>
    /// Synchronizes connected clients with the
    /// server-side m_ToggleState
    /// </summary>
    /// <param name="stateIsSet"></param>
    [Rpc(SendTo.ClientsAndHost)]
    private void ToggleStateClientRpc(bool stateIsSet)
    {
        m_ToggleState = stateIsSet;
    }
}

caution

NetworkBehaviour.OnSynchronize is only invoked on the server side during the write part of serialization and only invoked on the client side during the read part of serialization. When running a host, NetworkBehaviour.OnSynchronize is still only invoked once (server-side) during the write part of serialization.

Debugging OnSynchronize serialization

If your serialization code has a bug and throws an exception, then NetworkBehaviour.OnSynchronize has additional safety checking to handle a graceful recovery without completely breaking the rest of the synchronization serialization pipeline.

When writing

If user-code throws an exception during NetworkBehaviour.OnSynchronize, it catches the exception and if:

• LogLevel = Normal: A warning message that includes the name of the NetworkBehaviour that threw an exception while writing will be logged and that part of the serialization for the given NetworkBehaviour is skipped.
• LogLevel = Developer: It provides the same warning message as well as it logs an error with the exception message and stack trace.

After generating the log message(s), it rewinds the serialization stream to the point just before it invoked NetworkBehaviour.OnSynchronize and will continue serializing. Any data written before the exception occurred will be overwritten or dropped depending upon whether there are more NetworkBehaviour components to be serialized.

When reading

For exceptions this follows the exact same message logging pattern described above when writing. The difference is that after NetworkBehaviour.OnSynchronize logs one or more messages to the console, it skips over only the serialization data written by the server-side when NetworkBehaviour.OnSynchronize was invoked and continues the deserialization process for any remaining NetworkBehaviour components.

However, there is an additional check to ensure that the total expected bytes to read were actually read from the buffer. If the total number of bytes read does not equal the expected number of bytes to be read it will log a warning that includes the name of the NetworkBehaviour in question, the total bytes read, the expected bytes to be read, and lets you know this NetworkBehaviour is being skipped.

caution

When using NetworkBehaviour.OnSynchronize you should be aware that you are increasing the synchronization payload size per instance. If you have 30 instances that each write 100 bytes of information you will have increased the total full client synchronization size by 3000 bytes.

Serializing NetworkBehaviours

NetworkBehaviours require the use of specialized NetworkBehaviourReference structures to be serialized and used with RPCs and NetworkVariables.