Overview

Objects with the CoherenceSync component can be connected to other objects with CoherenceSync components to form a parent-child relationship. For example, an object can be linked to a hand, a hand to an arm, and the arm to a spine.

Usage

Creating an Entity hierarchy is very simple. All you need to do is add a GameObject with a CoherenceSynccomponent as a direct child of another GameObject with a CoherenceSynccomponent. You can add and remove parent-child relationships at runtime (even from the editor).

Destruction or disconnection of the parent object will also destroy and remove all children of this object. Those objects' state needs to be treated on the Client side to be reinstantiated on the next connection.

Hierarchies with intermediary transforms

Sometimes, it is not practical to add CoherenceSyncobjects to all the links in the chain. For example, if a weapon is parented to a hand controlled by an Animator, we do not need to synchronize the entire skeleton over the network. In that case, see CoherenceNode.

Level of detail considerations

If the child object is using LODs, it will base its distance calculations on the world position of its parent. For more details, see the Level of detail documentation.

Handling parent destruction

When the parent CoherenceSync is destroyed, by default its CoherenceSync children get destroyed together with it. This can be changed via the Preserve Children option on the parent:\

When the Preserve Children option is enabled, destroying the parent entity will result in children getting unparented instead of being destroyed together with the parent. Those children will now reside at the root of the scene hierarchy.

While the basic case of direct parent-child relationships between CoherenceSync entities is handled automatically by coherence, more complex hierarchies (with multiple levels) need a little extra work.

An example of such a hierarchy would be a synced Player Prefab with a hierarchical bone structure, where you want to place an item (e.g. a flashlight) in the hand:

Player > Shoulder > Arm > Hand

A Prefab can only have a single CoherenceSync script on it (and only on its root node), so you can't add an additional one to the hand. Instead, you need to add the CoherenceNode component to another Prefab so that it can be parented. Please note that this parenting relationship can only be set up in the scene or at runtime; you can't store it in the parent Prefab since that would break the rule of only one CoherenceSync per Prefab.

To prepare the child Prefab that you want to place in the hierarchy, add the CoherenceNode component to it (it also has to have a CoherenceSync). In the example above, that would be the flashlight you want your player to be able to pick up. You don't need to make any changes to the Player Prefab, just make sure it has a CoherenceSync script in the root.

This setup allows you to place instances of the flashlight Prefab anywhere in the hierarchy of the Player (you could even move it from one hand to the other, and it will work).

To recap, for CoherenceNode to work you need two things:

1. One or more Prefabs with CoherenceSync that have some kind of hierarchy of child transforms (the child transforms can't have CoherenceSyncs on them).

2. Another Prefab with CoherenceSync and CoherenceNode. Instances of this Prefab can now be parented to any transform of the Prefabs with just CoherenceSync (in step 1).

CoherenceNode works using two public fields which are automatically set to sync using the [Sync] attribute.

[Sync] public string path;
[Sync] public int pathDirtyCounter;

The path variable describes where in the parent's hierarchy the child object should be located. It is a string consisting of comma-separated indexes. Every one of these indexes designates a specific child index in the hierarchy. The child object which has the CoherenceNode component will be placed in the resulting place in the hierarchy.

The pathDirtyCounter variable is a helper variable used to keep track of the applied hierarchy changes. In case the object's position in the parent's hierarchy changes, this variable will be used to help settle and properly sync those changes.

Aside from configuring your CoherenceSync bindings from within the Configure window, it's possible to use the [Sync] and [Command] C# attributes directly on your scripts. Your prefabs will get updated to require such bindings.

Sync Attribute

Mark public fields and properties to be synchronized over the network.

[Sync]
public int health;

It's possible to migrate the variable automatically, if you decide to change its definition:

[Sync("health")]
public float hp;

Command Attribute

Mark public methods to be invoked over the network. Method return type must be void.

[Command]
public void Heal(int amount)
{
    ...
}

It's possible to migrate the command automatically, if you decide to change the method signature:

[Command("Heal", typeof(int))]
public void IncreaseHp(float hp)
{
    ...
}

Sometimes you want to synchronize data outside of the current GameObject.

Out of the box, coherence offers you coherence offers you several options to synchronize data from your CoherenceSync objects' hierarchy:

• Child GameObjects: when you need to network data directly from other GameObjects.

• Child CoherenceSyncs: when you create a parent-child relationship of CoherenceSync objects at runtime.

• Deep Child CoherenceSyncs: when you create a complex parent-child relationship of CoherenceSync objects at runtime.

The CoherenceSync component will help you prepare an object for network synchronization. It also exposes an API that allows us to manipulate the object during runtime.

CoherenceSync will query all public variables and methods on any of the attached components, for example Unity components such as Transform, Animator, etc. This will include any custom scripts, including third-party Asset Store packages that you may have downloaded.

Refer to the prefab setup page to learn how to configure your prefab to network state changes.

• bool

• int

• uint

• byte

• char

• short

• ushort

• float

• string

• Vector2

• Vector3

• Quaternion

• GameObject

• Transform

• RectTransform

• CoherenceSync

• SerializeEntityID

• byte[]

• long

• ulong

• Int64

• UInt64

• Color

• double

When you have the Configure window open, it will show the variables, methods and component actions available for synchronization for your currently selected GameObject.

If the Prefab that you are configuring has a hierarchy, you can synchronize variables, methods and component actions for any of the child GameObjects within the hierarchy.

To do so, open the Prefab in Prefab Mode by clicking the Open Prefab option in the inspector. This will allow you to select any of the GameObjects that belong to the hierarchy, the Configure window will be updated automatically, showing you everything that is available to be synchronized.

Overview

Commands are network messages sent from one CoherenceSync to another CoherenceSync. Functionally equivalent to RPCs, commands bind to public methods accessible on the GameObject hierarchy that CoherenceSync sits on.

Design phase

In the design phase, you can expose public methods the same way you select fields for synchronization: through the Configure window on your CoherenceSync component.

By clicking on the method, you bind to it, defining a command. The grid icon on its right lets you configure the routing mode. Commands with a Send to Authority Only mode can be sent only to the authority of the target CoherenceSync, while ones with the Send to All Instances can be broadcasted to all clients that see it. The routing is enforced by the Replication Server as a security measure, so that outdated or malicious clients don't break the game.

Sending a command on a networked object

To send a command, we call the SendCommand method on the target CoherenceSync object. It takes a number of arguments:

• The generic type parameter must be the type of the receiving Component. This ensures that the correct method gets called if the receiving GameObject has components that implement methods that share the same name. Example: sync.SendCommand<Transform>(...)

• The first argument is the name of the method on the component that we want to call. It is good practice to use the C# nameof expression when referring to the method name, since it prevents accidentally misspelling it, or forgetting to update the string if the method changes name.

• Alternatively, if you want to know which Client sent the command, you can add CoherenceSync sender as the first argument of the command, and the correct value will be automatically filled in by the SDK.

• The second argument is an enum that specifies the MessageTarget of the command. The possible values are:

  • MessageTarget.All – sends the command to each Client that has an instance of this Entity.

  • MessageTarget.AuthorityOnly – send the command only to the Client that has authority over the Entity.

  • MessageTarget.Other - sends the command to every Entity other than the one SendCommand is called on.

• The rest of the arguments (if any) vary depending on the command itself. We must supply as many parameters as are defined in the target method and the schema.

Here's an example of how to send a command:

var target = enemy.GetComponent<CoherenceSync>();
                    
target.SendCommand<Character>(nameof(Character.SendDamage), 
                MessageTarget.All,
                damageValue, staminaBlockCost,
                staminaRecoveryDelay, ignoreDefense, 
                hitReaction, hitPosition, 
                ConnectionId, isPlayer);

Sending a command to a specific MonoBehaviour instance

If you have the same command bound more than once in the same Prefab hierarchy, you can target a specific MonoBehaviour when sending a message, you can do so via the SendCommand(Action action) method in CoherenceSync.

Additionally, if you want to target every bound MonoBehaviour, you can do so via the SendCommandToChildren method in CoherenceSync.

Receiving a command

We don't have to do anything special to receive the command. The system will simply call the corresponding method on the target network entity.

If the target is a locally simulated entity, SendCommand will recognize that and not send a network command, but instead simply call the method directly.

Sending a command to multiple CoherenceSyncs

Sometimes you want to inform a bunch of different CoherenceSyncs about a change. For example, an explosion impact on a few players. To do so, we have to go through the instances we want to notify and send commands to each of them.

using Coherence;
using Coherence.Toolkit;
using UnityEngine;

public class MyCommands : MonoBehaviour
{
    public void MulticastCommand()
    {
        var self = GetComponent<CoherenceSync>();
        var listeners = FindObjectsOfType<CoherenceSync>(); 
        
        foreach (var listener in listeners)
        {
            if (!listener || listener == self)
            {
                continue;
            }
            
            listener.SendCommand<MyCommands>(nameof(ReceiveCommand), MessageTarget.AuthorityOnly);
        }
    }

    // bind to this method via the Bindings window
    public void ReceiveCommand()
    {
        Debug.Log("Received!");
    }
}

In this example, a command will get sent to each CoherenceSync under the state authority of this Client. To make it only affect CoherenceSyncs within certain criteria, you need to filter to which CoherenceSync you send the command to, on your own.

Sending null values in command arguments

Some of the primitive types supported are nullable values, this includes:

• Byte[]

• string

• Entity references: CoherenceSync, Transform, and GameObject

In order to send one of these values as a null (or default) we need to use special syntax to ensure the right method signature is resolved.

using Coherence;
using Coherence.Toolkit;
using UnityEngine;
using System;

public class MyCommand : MonoBehaviour
{
    private CoherenceSync sync;

    public void MyNullableCommand(string someString, Byte[] someArray)
    {
        if (!string.IsNullOrEmpty(someString)) //Could be null or string.Empty
        {
            // Do something with the string
        }

        if (someArray != null && someArray.Length > 0) //Could be null or Byte[0]
        {
            // Do something with the array
        }
    }

    public void SendNullableCommand()
    {
        sync.SendCommand<MyCommand>(nameof(MyNullableCommand),
            MessageTarget.All,
            (typeof(string), (string)null),
            (typeof(Byte[]), (Byte[])null));
    }
}

Null-value arguments need to be passed as a ValueTuple<Type, object> so that their type can be correctly resolved. In the example above sending a null value for a string is written as: (typeof(string), (string)null)

and the null Byte[] argument is written as: (typeof(Byte[]), (Byte[])null)

Mis-ordered arguments, type mis-match, or unresolvable types will result in errors logged and the command not being sent.

Limitations

When a Prefab is not using a baked script there are some restrictions for what types can be sent in a single command:

• 4 entity references

• maximum of 511 bytes total of data in other arguments

• a single Byte[] argument can be no longer than 509 bytes because of overhead

Some network primitive types send extra data when serialized (like Byte arrays and string types) so gauging how many bits a command will use is difficult. If a single command is bigger than the supported packet size, it won't work even with baked code. For a good and performant game experience, always try to keep the total command argument sizes low.

Unity Animator's parameters are bindable out of the box, with the exception of triggers.

Triggers

Triggers can be invoked over the network using commands. Here's an example where we inform networked Clients that we have played a jump animation:

using UnityEngine;
using Coherence;
using Coherence.Toolkit;

public class JumpController : MonoBehaviour
{
    CoherenceSync coherenceSync;
    Animator animator;

    void Awake()
    {
        coherenceSync = GetComponent<CoherenceSync>();
        animator = GetComponent<Animator>();
    }

    void Update()
    {
        if (!coherenceSync.HasInputAuthority)
        {
            return;
        }

        if (Input.GetKeyDown(KeyCode.Space))
        {
            MakePlayerJump();
        }
    }

    void MakePlayerJump()
    {
        coherenceSync.SendCommand<JumpController>(nameof(PlayJumpAnimation), MessageTarget.All, coherenceSync);
    }

    // bind to this method via the Bindings window
    public void PlayJumpAnimation(CoherenceSync jumpSync)
    {
        animator.SetTrigger("Jump");
    }
}

Now, bind to the PlayJumpAnimator.

Entity references let you set up references between Entities and have those be synchronized, just like other value types (like integers, vectors, etc.)

To use Entity references, simply select any fields of type GameObject, Transform, or CoherenceSync for syncing in the Configuration window:

The synchronization works both when using reflection and in baked sync scripts.

Entity references can also be used as arguments in Commands.

Limitations

It's important to know about the situations when an Entity reference might become null, even though it seems like it should have a value:

• A client might not have the referenced entity in its LiveQuery. A local reference can only be valid if there's an actual Entity instance to reference. If this becomes a problem, consider switching to using the CoherenceNode component or Parent-Child relationships of prefabs which ensures that another Entity becomes part of the query.

• The owner of the Entity reference might sync the reference to the Replication Server before syncing the referenced Entity. This will lead to the Replication Server storing a null reference. If possible, try setting the Entity references during gameplay when the referenced Entities have already existed for a while.

In any case, it's important to use a defensive coding style when working with Entity references. Make sure that your code can handle missing Entities and nulls in a graceful way.

coherence ships with a Sample UI that can be used to kickstart your project.

One implementation that we often see pretty early on prototypes, is the ability to show a name to identify players within a game session. So we've created a component to help achieve that: CoherencePlayerName.

If you want to network the Player Name set on the Sample UI, add a CoherencePlayerName component to your CoherenceSync:

It is often useful to know when a synchronized variable has changed its value. It can be easily achieved using the OnValueSyncedAttribute. This attribute lets you define a method that will be called each time a value of a synced member (field or property) changes in the non-simulated version of an entity.

Usage

Let's start with a simple example:

Whenever the value of the Health field gets updated (synced with its simulated version) the UpdateHealthLabel will be called automatically, changing the health label text and printing a log with a health difference.

Limitations

The OnValueSynced feature can be used only on members of user-defined types, that is, there's no way to be notified about a change in the value of a Unity type member, like transform.position. This might however change in the future, so stay tuned!

Value sync callbacks are currently only supported for value types. That means the following types are not supported: byte[], CoherenceSync, GameObject, Transform and RectTransform.