Unity Events executed out of order

Question

According to this page Events in unity are suppose to execute in a specific order. Specifically, OnCollisionEnter is supposed to be called right after OnTriggerEnter; however, FixedUpdate and Update are both being called at least once in between calls to my OnTriggerEnter2D and OnCollisionEnter2D functions in my Enemy class, why is this the case?

Here is my code:

public class Enemy : Combatant
{
public int kill_points;
public float force;
public List<GameObject> path_verticies;
private int pathi;
private bool on_path;
private bool is_flying;
private bool ignore_collision;          //Used for ignoring collisions with walls, if the enemy has already touched an enemy_controller.

void Start ()
{   Init();
    pathi = 0;
    ignore_collision = false;
    is_flying = false;
    if (gameObject.tag.Contains("Fly")) is_flying = true;//Assuming contains is cheaper than a full string compare.
    on_path = null != path_verticies && 0 != path_verticies.Count;
}

private void FixedUpdate() { ignore_collision = false; }

void Update ()
{   if (!CheckBounds()) Destroy(gameObject);
    if (on_path)
    {   transform.LookAt(path_verticies[pathi].transform, Vector3.up);
        transform.Rotate(new Vector3(0,-90,0));
        phys.velocity = transform.right * speed;
    }
    else dx = Mathf.Sign(Cx) * speed;
}

public void Flip(){ Cx *= -1; }

#pragma warning disable CS0642 // Possible mistaken empty statement
private IEnumerator OnCollisionEnter2D(Collision2D collision)
{   if (on_path && path_verticies[pathi] == collision.gameObject)
    {   if (collision.gameObject == player)
        {   throw new System.NotImplementedException();
            yield return new WaitForSeconds(0.1f);//Used to prevent damage to the player if the player landed on top of the enemy.
            var atk = player.GetComponent<Combatant>().vigor.Attack(2);
            if (Health.Atk.DIES == atk) player.GetComponent<Life>().Die();
        } else pathi = (pathi+1) % path_verticies.Count;
        yield return new WaitForSeconds(0.0f);
    }
    else if (on_path)
    {   if (collision.gameObject == player)
        {   yield return new WaitForSeconds(0.1f);//Used to prevent damage to the player if the player landed on top of the enemy.
            var atk = player.GetComponent<Combatant>().vigor.Attack(2);
            if (Health.Atk.DIES == atk) player.GetComponent<Life>().Die();
        } else if (collision.collider == On(Truebounds)) /*nothing*/;
        else if (collision.gameObject.layer == 0 || collision.gameObject.layer == 8)// TODO
            /*take velocity in the direction opposite of the normal and add it to
             * the perpendicular component the enemy is moving in*/;
        yield return new WaitForSeconds(0.0f);
    }
    else
    {   if (collision.gameObject == player)
        {   yield return new WaitForSeconds(0.1f);//Used to prevent damage to the player if the player landed on top of the enemy.
            var atk = player.GetComponent<Combatant>().vigor.Attack(2);
            if (Health.Atk.DIES == atk) player.GetComponent<Life>().Die();
        } else if (ignore_collision) /*nothing*/;
        else if (collision.collider == On(Truebounds)) { if (is_flying) Flip(); }
        else if (collision.gameObject.layer == 0 || collision.gameObject.layer == 8) Flip();
        yield return new WaitForSeconds(0.0f);
    }
}

private IEnumerator OnTriggerEnter2D(Collider2D collider)
{   var component = collider.GetComponent<MonoBehaviour2>();
    if(!component) yield return null;
    else if (on_path && path_verticies[pathi] == collider.gameObject)
        pathi = (pathi+1) % path_verticies.Count;
    else if (on_path) /*nothing*/;
    else if (component.type == typeof(Enemy_Controller)) { Flip(); ignore_collision = true; }
    yield return null;
}
#pragma warning restore CS0642 // Possible mistaken empty statement

//public void OnDestroy(){  }
}

and these are my timestep settings:

Answer 1

FixedUpdate and Update are not tied together. FixedUpdate is called based on how much time has passed (in your case, 0.02 seconds), while Update is called each frame. Generally (I think) FixedUpdate is used to do physics, collision, and movement, while Update handles input, game state, etc.

If you need something to happen x times every second, use FixedUpdate. If you just need something to happen every frame (but that doesn't rely on the time between frames) use Update. However, you can't rely on either of these to be called before or after the other.

Unity Docs, from the page you cited:

FixedUpdate: FixedUpdate is often called more frequently than Update. It can be called multiple times per frame, if the frame rate is low and it may not be called between frames at all if the frame rate is high. All physics calculations and updates occur immediately after FixedUpdate. When applying movement calculations inside FixedUpdate, you do not need to multiply your values by Time.deltaTime. This is because FixedUpdate is called on a reliable timer, independent of the frame rate.

Update: Update is called once per frame. It is the main workhorse function for frame updates.