Proper way to destroy systems

At the level unloading I would like to remove level only systems from the world, something like this:

Code (CSharp):

1.         private void OnDestroy()
2.         {
3.             foreach (var system in systems)
4.             {
5.                 world.DestroyManager(system);
6.             }
7.  
8.             ScriptBehaviourUpdateOrder.UpdatePlayerLoop(world);
9.         }

But I am getting this exception

Code (CSharp):

1. NullReferenceException: Object reference not set to an instance of an object
2. Unity.Entities.ComponentSystemBase.BeforeUpdateVersioning () (at C:/.../com.unity.entities@0.0.12-preview.8/Unity.Entities/ComponentSystem.cs:140)
3. Unity.Entities.ComponentSystem.BeforeOnUpdate () (at C:/.../com.unity.entities@0.0.12-preview.8/Unity.Entities/ComponentSystem.cs:263)
4. Unity.Entities.ComponentSystem.InternalUpdate () (at C:/.../com.unity.entities@0.0.12-preview.8/Unity.Entities/ComponentSystem.cs:290)
5. Unity.Entities.ScriptBehaviourManager.Update () (at C:/.../com.unity.entities@0.0.12-preview.8/Unity.Entities/ScriptBehaviourManager.cs:82)
6. Unity.Entities.ScriptBehaviourUpdateOrder+DummyDelagateWrapper.TriggerUpdate () (at C:/.../com.unity.entities@0.0.12-preview.8/Unity.Entities/ScriptBehaviourUpdateOrder.cs:734)

It looks like Unity tries to run the removed systems once again.

So what is the proper way to remove systems from the world?

 

What is very unclear to me in ECS is how to handle things like for example level settings and how to pass them to the systems. You know, all the "singleton" objects.

I have implemented a factory class that creates the systems and injects (through the contructor) dependencies like level settings to them. But naturally such a system should live only through the level and no longer, that is why I am trying to destroy it. (Well, on second thought this probably isn't the correct way.)

The other way could be to have an entity with the LevelSettings component, but it means writing much more code than what I was used to:

Code (CSharp):

1. [Zenject.Inject] LevelSetting levelSettings;

In your TwoStickShooter example you use a static class for settings, but static class is not probably something you would like to use in a real game.

So how would you "inject" things like level settings to the system in some scalable and maintenable way?

 

I wouldn't say my answer is the best approach, but I would simply have system class, with static properties, of which you assign relevant values, when level is started/restarted/ended.

 

I've talked about this before and wrote a solution to allow custom injections (I use Zenject): https://forum.unity.com/threads/request-for-world-addmanager.539271/#post-3558224

I really wish they'd just add an the AddManager method themselves to World so we can control how to instantiate systems however we want without having to hack at it.

 

I am not going to use static properties.

Yes, I am using similar approach, but now I am not sure about it. As I wrote above, with constructor injection I would need to create/destroy systems with level loading and it doesn't seem too effective.

Well, I will probably do so. I generally like the idea of ECS to "let the systems pick what they need", it is just that for such a simple (single) things it feels like a hack.

 

Problem is still here in 21 preview. Deleting a system causes an exception in the next player loop update, when this system is accessed from the native. System is disposed but looks like native had not cleared an update hook registration for it. Player loop updates correctly inside the `
ScriptBehaviourUpdateOrder.UpdatePlayerLoop(world);`.

 

I had a similar problem when destroying a world during the PlayerLoop resuling in some systems still getting updates for the current frame. My workaround for this was to set all the systems to Enabled=false before deleting the world.

 

But how do I do this?
I'm writing a plugin that support both entities and gameobjects and I'm creating the system in a way that its easy to use and works similar to the gameobjects. Under the hood it does ecs stuff so for example settings are added outside the ecs componensystem component.

so if I call.

Code (CSharp):

1. plugin.EntityWorld.DestroySystem((ComponentSystemBase) targetinginstruction.TargetingSystemEntities);

How do I use the:

Code (CSharp):

1. ComponentSystemGroup.RemoveSystemFromUpdateList()

Because it say I cannot use it outside the static context meaning I guess that I should get reference to the object else where.

 

World.GetOrCreateSystem

will give you a reference.

For example, you could use

initializationSystemGroup = World.GetOrCreateSystem<InitializationSystemGroup>();

in the

OnCreate

method, and then call

initializationSystemGroup.RemoveSystemFromUpdateList(systemToBeDestroyed);

and then

World.DestroySystem(systemToBeDestroyed);

GetOrCreateSystem

works because: