DrawLineSystem.cs

using UnityEngine;
using Unity.Entities;
using Unity.Transforms;

public class DrawLineSystem : ComponentSystem {

    private ComponentGroup group;

    protected override void OnCreateManager()
    {
        group = GetComponentGroup(ComponentType.ReadOnly<EntityBuffer>());
    }

    protected override void OnUpdate()
    {
        BufferArray<EntityBuffer> entityBuffer = group.GetBufferArray<EntityBuffer>();
        EntityArray entities = group.GetEntityArray();

        for(int entityIndex=0; entityIndex<entities.Length; entityIndex++)
        {
            DrawLine(entities[entityIndex], entityBuffer[entityIndex].Reinterpret<Entity>());
        }
    }

    void DrawLine(Entity entity, DynamicBuffer<Entity> buffer)
    {
        Position entityOwnerPos = EntityManager.GetComponentData<Position>(entity);
        for (int i = 0; i < buffer.Length; i++)
        {
            Position targetPos = EntityManager.GetComponentData<Position>(buffer[i]);
            Debug.DrawLine(entityOwnerPos.Value, targetPos.Value);
        }
    }
}