Find a Target

Lesson 8 of 25 in Coddy's Unity Game: Tower Defense course.

A tower shoots the enemy nearest to it, as long as that enemy is within the tower's range. To choose, the tower needs every enemy on the board. FindObjectsByType<Enemy> would search the whole scene every frame for every tower, so instead the enemies keep a list of themselves: a static list on the Enemy class, which each enemy joins in OnEnable and leaves in OnDisable. Unity calls OnDisable on an object that is being destroyed, so a dead enemy leaves the list too. Pickups in a game can do the same:

public class Pickup : MonoBehaviour
{
    public static readonly List<Pickup> All = new List<Pickup>();

    void OnEnable()
    {
        All.Add(this);
    }

    void OnDisable()
    {
        All.Remove(this);
    }
}

Finding the nearest is a loop that remembers the best so far. A magnet that pulls the nearest pickup within pullRange starts with no pickup and with the range as the best distance, so a pickup further away can never win:

Pickup best = null;
float bestDistance = pullRange;
foreach (Pickup pickup in Pickup.All)
{
    // closer than the best so far? keep it, and its distance
}

Vector3.Distance(a, b) gives the distance between two points. When nothing is within range, the loop keeps nothing, and the answer is null.

Challenge

Medium

Each tower picks the nearest enemy in range.

In Enemy, add public static readonly List<Enemy> All = new List<Enemy>();; add this in OnEnable, remove it in OnDisable.
In Tower, add public float range = 4f; and private Enemy target;.
Every frame, find the nearest enemy in Enemy.All within range (or null), log "Target: " + found.name when it is new, and keep it in target.

Press Play and build by the path: the Console names each new target. Then Check.

Try it yourself

using UnityEngine;

// Comes later: a tower finds an enemy in range, turns to it and fires.
public class Tower : MonoBehaviour
{
}

All lessons in Unity Game: Tower Defense

1The Path

4Waves

2Towers

Find a Target Project lesson

3Strategy

Practice on your own: Unity playground