"OnStart" for each PhysicsVelocity entity ?

Questions & Answers
1

I have a bunch of entities that I need to give an initial velocity

physicsVelocity.Linear = velocity

How would I do that for all entities, as OnStartRunning in system is only called once and as soon as there is one entity in scene it stops ?
So basically I would need something like Start() in MonoBehaviour, but for each entity.

2

What you need to do is to add:

  • VelocityDelta & PhysicsVelocity components to an entity

OR

  • VelocityDeltaComponent & PhysicsBody components to a GameObject (that you convert into an entity)

VelocityDeltaSystem looks for entities with VelocityDelta components then:

  1. applies an one-time change in it’s PhysicsVelocity.Linear
  2. removes the VelocityDelta component (so the vel change won’t reapeat)

note: VelocityDeltaSystem executes only when the matching entities exist and is idle otherwise.

VelocityDeltaComponent.cs

using UnityEngine;
using Unity.Entities;
using Unity.Jobs;
using Unity.Mathematics;
using Unity.Transforms;
using Unity.Physics;

[DisallowMultipleComponent]
public class VelocityDeltaComponent : MonoBehaviour, IConvertGameObjectToEntity
{
	[SerializeField] public Vector3 _delta = new Vector3{ z=10 };
	public void Convert ( Entity e , EntityManager cmd , GameObjectConversionSystem cs )
		=> cmd.AddComponentData( e , new VelocityDelta{ Value = _delta } );
	#if UNITY_EDITOR
	void OnDrawGizmosSelected ()
	{
		Gizmos.color = Color.yellow;
		Gizmos.DrawLine( transform.position , transform.position + transform.TransformDirection(_delta) );
	}
	#endif
}

public struct VelocityDelta : IComponentData
{
	public float3 Value;
}

public partial class VelocityDeltaSystem : SystemBase
{
	EndFixedStepSimulationEntityCommandBufferSystem _commandBufferSystem;
	protected override void OnCreate ()
	{
		_commandBufferSystem = World.GetOrCreateSystem<EndFixedStepSimulationEntityCommandBufferSystem>();
	}
	protected override void OnUpdate ()
	{
		var cmd = _commandBufferSystem.CreateCommandBuffer().AsParallelWriter();
		
		Entities
			.ForEach( ( ref PhysicsVelocity velocity , in VelocityDelta delta , in LocalToWorld transform , in Entity e , in int nativeThreadIndex ) =>
			{
				velocity.Linear += math.mul( delta.Value , math.inverse( (float3x3) transform.Value ) );
				cmd.RemoveComponent<VelocityDelta>( nativeThreadIndex , e );
			} )
			.WithBurst()
			.ScheduleParallel();
		
		_commandBufferSystem.AddJobHandleForProducer( this.Dependency );
	}
}