4.1 — The Enemy: Patrol

Compare against your own file rather than copying it. The gap is the turn, and it is the one line in this lesson with nothing earlier to copy: the field remembers which end of the beat the enemy is heading for, and the turn is that field changing its mind. This file writes it as a small method with an if/else, and turns before it walks where the walkthrough turns after — on every frame but the one where the enemy arrives, those two orders are the same file, and any form that hands the field the other waypoint is correct. Your waypoint placement, your speed, your threshold and your movement method from 2.2 are yours; the shape is what has to match.

// ------------------------------------------------------------------
//  REFERENCE FILE — Assets/Scripts/EnemyController.cs
//  Written to be read, not pasted. Build it with the walkthrough
//  above, one step at a time, then open this and compare.
//  If it is sitting in your project unchanged, that is the thing
//  to go and do now — the walkthrough is still above you.
// ------------------------------------------------------------------
using UnityEngine;

// EnemyController.cs — one guard, one beat, forever.
// The two ends of the beat are objects in the scene. This script keeps exactly
// one thing about the walk: which end it is heading for. The turn is that
// field changing its mind.
public class EnemyController : MonoBehaviour
{
    [SerializeField] private Transform waypointA;           // one end of the beat
    [SerializeField] private Transform waypointB;           // the other end
    [SerializeField] private float moveSpeed = 2f;          // units per SECOND
    [SerializeField] private float arriveDistance = 0.1f;   // how close is "there"

    private Transform target;                               // which way it is walking
    private Rigidbody2D body;

    void Start()
    {
        // Looked up once, never inside Update() — the rule from 2.2.
        body = GetComponent<Rigidbody2D>();

        // Seed the memory. Either end works; this one gives a full first beat.
        target = waypointB;
    }

    void Update()
    {
        // Arrive first, walk second. The order only matters on the one frame
        // where the enemy reaches a waypoint.
        if (Vector3.Distance(transform.position, target.position) < arriveDistance)
        {
            TurnAround();
        }

        // No Time.deltaTime: velocity is per second, not per frame (2.2).
        Vector2 direction = (target.position - transform.position).normalized;
        body.linearVelocity = direction * moveSpeed;
    }

    // The only thing that ends a beat. The player lets go of the keys; the
    // enemy has no keys, so the turn has to be written here.
    private void TurnAround()
    {
        if (target == waypointA)
        {
            target = waypointB;
        }
        else
        {
            target = waypointA;
        }
    }
}

// Inspector, on Assets/Prefabs/MobileEnemy (this lesson's editor half):
//   Collider 2D                       =  on the ROOT, resized by hand until the
//                                        outline covers the art on the Sprite child
//   Rigidbody 2D ▸ Body Type          =  Dynamic   <- only Dynamic is stopped by walls
//   Rigidbody 2D ▸ Gravity Scale      =  0         <- the Overworld has no down
//   Rigidbody 2D ▸ Freeze Rotation Z  =  ON        <- solid hits impart spin
//   Waypoint A / Waypoint B           =  two empty GameObjects placed in the
//                                        Overworld, wired per enemy INSTANCE