Mirror/docs/Concepts/VisibilityCustom.md

Customizing Network Visibility

The built-in Network Proximity Checker component is the built-in default component for determining a GameObject’s network visibility. However, this only provides you with a distance-based check. Sometimes you might want to use other kinds of visibility check, such as grid-based rules, line-of-sight tests, navigation path tests, or any other type of test that suits your game.

To do this, you can implement your own custom equivalent of the Network Proximity Checker. To do that, you need to understand how the Network Proximity Checker works. See documentation on the in-editor Network Proximity Checker component, and the NetworkProximityChecker API.

The Network Proximity Checker is implemented using the public visibility interface of Mirror’s HLAPI. Using this same interface, you can implement any kind of visibility rules you desire. Each NetworkIdentity
keeps track of the set of players that it is visible to. The players that a NetworkIdentity GameObject is visible to are called the “observers” of the NetworkIdentity.

The Network Proximity Checker calls the RebuildObservers method on the Network Identity component at a fixed interval (set using the “Vis Update Interval” value in the inspector), so that the set of network-visible GameObjects for each player is updated as they move around.

On the NetworkBehaviourclass (which your networked scripts inherit from), there are some virtual functions for determining visibility. These are:

• OnCheckObserver
  This method is called on the server, on each networked GameObject when a new player enters the game. If it returns true, that player is added to the object’s observers. The NetworkProximityChecker method does a simple distance check in its implementation of this function, and uses Physics.OverlapSphere() to find the players that are within the visibility distance for this object.

• OnRebuildObservers
  This method is called on the server when RebuildObservers is invoked. This method expects the set of observers to be populated with the players that can see the object. The NetworkServer then handles sending ObjectHide and ObjectSpawn messages based on the differences between the old and new visibility sets.

You can check whether any given networked GameObject is a player by checking if its NetworkIdentity has a valid connectionToClient. For example:

    var hits = Physics.OverlapSphere(transform.position, visRange);
    foreach (var hit in hits)
    {
        // (if a GameObject has a connectionToClient, it is a player)
        var uv = hit.GetComponent<NetworkIdentity>();
        if (uv != null && uv.connectionToClient != null)
            observers.Add(uv.connectionToClient);
    }