Efficient singletons entities and joins in ECS

There's a pattern in used in FPS sample I dislike, but I can't see an obvious alternative.

Code (CSharp):

1. protected override void OnUpdate()
2. {
3.     var gameModeArray = m_gameModeGroup.GetComponentArray<GameMode>();
4.     if (gameModeArray.Length == 0)
5.         return;
6.  
7.     GameDebug.Assert(gameModeArray.Length == 1,
8.         "There should only be one gamemode. Found:{0}",
9.         gameModeArray.Length);
10.  
11.     var localPlayerArray = m_localPlayerGroup.GetComponentArray<LocalPlayer>();
12.     GameDebug.Assert(localPlayerArray.Length == 1,
13.         "There should only be one localplayer. Found:{0}",
14.         localPlayerArray.Length);
15.  
16.     var gameMode = gameModeArray[0];
17.     var localPlayer = localPlayerArray[0];
18.  
19.     Game.game.clientFrontend.UpdateIngame(gameMode, localPlayer);
20. }

source: https://github.com/Unity-Technologi.../Scripts/Game/Frontend/ClientFrontend.cs#L157

The ClientFrontEndUpdate system expects there to be exactly one GameMode and exactly one LocalPlayer entity. It needs to do a join on those two entities in order to call clientFrontend.UpdateIngame.

If these were not restricted to singletons it seems the general form would be the Cartesian product of gameModeGroup and localPlayerGroup. Where you would call update for every gameMode & localPlayer pair.

But even restricting it to singleton entities I don't see how to avoid allocation due to needing to use ToComponentArray() to operate on two queries at once.

 

From another recent post I did discover GetSingleton which seems like it would solve part of the problem but it doesn't support Monobehavior components.

 

I usually denormalize such data. So, keep one Singleton component for keeping a single source of truth and then every frame write to every entity needing this data. I described it here:
https://forum.unity.com/threads/tim...m-and-jobcomponentsystem.663739/#post-4443820

This way I can process everything with just JobForEach and Entities.ForEach

 

You can use Query.GetSingletonEntity() and grab the object from EntityManager (where Query is GetEntityQuery(typeof(Object))