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Custom Interest Management
Custom Interest Management
Mirror allows you to implement custom Interest Management solutions. For example:
Raycast based so DotA players don't see other players behind a wall or in the bushes
Predictive Raycasting for Counter-Strike like games. Wallhacks show players behind walls. You could create a custom interest management system to only send enemies to the player shortly before they would pop out behind a wall. In first person shooters, players move fast so you would still have to send them a couple of milliseconds before they become visible. 
All of the above custom solutions are possible in Mirror. To understand how interest management can be implemented, let's walk through it step by step.
Script Template
Mirror includes a Script Template for custom interest management.  It is fully commented with all the virtual method overrides already stubbed out for you. If you used our legacy Interest Management system before, then these should look familiar.
OnCheckObserver is called when someone spawns. Returns true if 'identity' can be seen by 'newObserver'
OnRebuildObservers rebuilds observers for the given Network Identity. The result is stored in newObservers. 
Mirror will automatically put newObservers into identity.observers internally. We don't do this directly because it's a bit more complicated than adding/removing. Mirror takes care of it. Nothing to worry about :)
RebuildAll is a helper function to rebuild every spawned Network Identity's observers.
Implementations probably want to call this every interval.
Distance Example
Distance Interest Management is the easiest, straight forward implementation. Let's walk through it to see how to inherit from the abstract InterestManagement class.
public class DistanceInterestManagement : InterestManagement
{
    [Tooltip("The maximum range that objects will be visible at.")]
    public int visRange = 10;
​
    [Tooltip("Rebuild all every 'rebuildInterval' seconds.")]
    public float rebuildInterval = 1;
    double lastRebuildTime;
​
    public override bool OnCheckObserver(NetworkIdentity identity, NetworkConnection newObserver)
    {
        return Vector3.Distance(identity.transform.position, newObserver.identity.transform.position) <= visRange;
    }
​
    public override void OnRebuildObservers(NetworkIdentity identity, HashSet<NetworkConnection> newObservers, bool initialize)
    {
        Vector3 position = identity.transform.position;
        
        // for each connection
        foreach (NetworkConnectionToClient conn in NetworkServer.connections.Values)
            // if authenticated and joined the world
            if (conn != null && conn.isAuthenticated && conn.identity != null)
                // check distance to our 'identity'
                if (Vector3.Distance(conn.identity.transform.position, position) < visRange)
                    // add to result
                    newObservers.Add(conn);
    }
​
    [ServerCallback]
    void Update()
    {
        // rebuild all spawned NetworkIdentity's observers every interval
        if (NetworkTime.time >= lastRebuildTime + rebuildInterval)
        {
            RebuildAll();
            lastRebuildTime = NetworkTime.time;
        }
    }
}
This isn't too complicated, is it?
OnCheckObserver simply compares the distance between the newly spawned identity and a connection. A connection has a main player, which is what we use for the distance check here.
Note that you could also check against every object that the connection owns. For example, if Bob spawns and Alice isn't close enough, Alice's pet might be close enough and so Alice and Bob should still see each other, despite being a bit out of normal range.
OnRebuildObservers's job is to return that HashSet of Network Connections that can see our Network Identity. So obviously, we just iterate all NetworkServer.connections and check their main player's distances to our Network Identity.
Note that we only check the ones that are authenticated and have a player in the world. Connections that are still logging in or choosing characters shouldn't observe anything.
Update's job is to actually call RebuildAll() every now and then. If we don't call RebuildAll(), then Mirror would never rebuild observers. 
Host Mode Visibility
In host mode, someone runs the server while also playing on it themselves, so you might think:
I am the server. The server sees everyone. Therefore, I should see everyone.
This is technically true, but if you were fortunate enough to ever be on a LAN party then you'll remember it differently:
The best of days.
 For example, someone on a LAN hosts a Counter-Strike or DotA game. Let's think about that case for a moment:
The host runs the server. The server holds the whole world state in memory, yet the host player only sees the world around him.
The idea is for the host player to be a regular player in the game. LAN parties wouldn't be much fun if you play DotA / Counter Strike and the host always sees everyone else's position, right?
Obviously, the host can cheat. If you cheat on LAN then you need professional help.
Mirror has a virtual method SetHostVisibility(NetworkIdentity, bool) that enables / disables renderers in host mode. In other words, the world state is still there - the host player just doesn't see it.  You can override this in your custom system to suit your needs.
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Custom Interest Management
Script Template
Distance Example
Host Mode Visibility