How do I make game objects move towards the player, when the player is within a set range?

Question

I am trying to make game objects, like a coin or ammo clip, gravitate towards the player when they are within a set radius. I have seen this implemented in quite a few old school 90's games; when the player walks near anything, I want it to move to the player for collection. I can already handle the collection.

This is what I have, so far:

using UnityEngine;
using System.Collections;

public class MoveCoinsTowardsPlayer : MonoBehaviour 
{
    public Transform target;
    public float speed;

    void Update() 
    {
        float step = speed * Time.deltaTime;
        transform.position = Vector3.MoveTowards(transform.position, target.position, step);
    }
}

How do I make game objects move towards the player, when the player is within a set range?

Answer 1

You could simply check the distance of the coin to the player, however, you would need to store a record of each coin and iterate through each instance during each Update() method, in order to check.

We will assume you have a static number of coins, and have pre-loaded them into an array. This is the most cost effective, in comparison to using a List. If you dynamically add coins to the level, you will have to use a List<MoveCounTowardsPlayer>, and add the coins to the list as they are created.

class MoveCoinTowardsPlayer : MonoBehaviour
{
    public Transform target;
    public float speed;
    public bool isMoving = false;

    void Update()
    {
        if(isMoving)
        {
            transform.position = Vector3
                .MoveTowards(transform.position, target.position, speed * Time.deltaTime;
        }
    }

    public void MoveToPlayer(Transform player)
    {
        target = player;
        isMoving = true;
    }
}

class Player : MonoBehaviour
{
    ///<summary>The array of coins available in the level.</summary>
    public MoveCoinTowardsPlayer[] coin;
    ///<summary>The range within which the player will pick up coins.</summary>
    public float pickupRange;

    void Update()
    {
        for(int i = 0; i < coin.Length; i++)
        {
            if(coin[i] != null && Vector3
                .Distance(transform.position, coin[i].transform.position) <= pickupRange)
            {
                // If the coin still references a valid object, and its distance to 
                // the player transform is within pickup range, 
                // start moving it towards the player.
                coin[i].MoveToPlayer(transform);
            }
        }
    }        
}

---

All in all, the above solution seems like a waste of resources, given that it iterates through all coins during a single update. I would personally add a kinematic sphere collider (circle, if your working in 2D). You can make this collider the size of your pickup radius, and use collision to determine collection.

You would also need a collider on the coins, as well as a tag to identify them as coins.

class Player : MonoBehaviour
{
    public string coinTag;

    void OnTriggerEnter(Collider other)
    {
        if(other.tag == coinTag)
        {
            other.gameObject.GetComponent<MoveCoinTowardsPlayer>()
                .MoveToPlayer(transform);
        }
    }
}