Some beginner EntityComponentSystem questions

Hi folks, just looking at ECS and I have been having trouble figuring out how to structure some really basic patterns in ECS code. Here's an example:

Suppose my game features a tank which drives around and has a turret which it uses to shoot at enemies, and I want to program the "shoot" animation. That means that I want to rotate the turret to face a specific enemy and then spawn a projectile which travels towards that enemy.

Question 1: What is the idiomatic way for an entity to find its child entities? In traditional unity, the Tank prefab might use a direct editor reference or something like GetComponentInChildren to find the Turret GameObject. Would the ECS equivalent be to do something like

Code (CSharp):

1. struct TankData : IComponentData {
2.   Entity TurretEntity;
3. }

? How would the tank go about finding the Entity ID for its turret after it's spawned? What if the turret is a few levels of hierarchy down from the tank?

Question 2: What is the idiomatic way for an entity to query information about other game objects? For example, maybe I have a shoot component that gets added like this:

Code (CSharp):

1. struct ShootData : IComponentData {
2.   Entity Attacker;
3.   Entity Target;
4. }

Given a "Target" entity, how do I determine the current position of that entity so I can send a projectile towards it? What is the equivalent of Target.GetComponent<SomeComponent>() in ECS code? I want to go from the "attacker" entity to "take this tank entity, find its turret child, rotate that to face this position, then shoot a projectile."

 

First of all read about new conversion workflow in ECS:
https://forum.unity.com/threads/new-subscene-converttoentity-workflows.638785/
And read about Parent IComponentData in docs or just look at source code

 

Are you referring to Unity.Transforms.Parent? So I guess that would be a way to look up a parent entity from a child, but not really the other way around. I guess it might be possible to look up child Entity IDs at create time inside

IConvertGameObjectToEntity

, although I'm not sure there's a way to get that information (would the 'turret' child even have an associated entity when the parent's

Convert

runs?).

For question #2, it looks like

ComponentSystem.GetComponentDataFromEntity

is the right way to look up stuff like the position of a random entity, although I'm a little worried about the performance of using that over and over.