Unity power-up is not working as intended

Question

First off, I am quite new to scripting so there's probably going to be a few flaws in my script.

So basically, I've made a script for the power up, but once my shot or the player touches the power up coin the fire rate does increase however it won't go back to the normal fire rate after 5 seconds... I have no idea what might be the cause, any advice would be helpful!

using UnityEngine;
using System.Collections;

public class FireRatePowerUp : MonoBehaviour {

    private bool isPowerUp = false;

    private float powerUpTime = 5.0f;

    private PlayerShoot playerShoot;

    private void Start()
    {
        playerShoot = PlayerShoot.FindObjectOfType<PlayerShoot>();
    }

    private void OnTriggerEnter2D(Collider2D collision)
    {
        if (collision.gameObject.tag == "Player" || collision.gameObject.tag == "Projectile")
        {
            StartCoroutine(PowerUpTime());
            isPowerUp = true;

            Destroy(gameObject);

            if (collision.gameObject.tag == "Projectile")
            {
                Destroy(collision.gameObject);
            }
        }
    }

    IEnumerator PowerUpTime()
    {
        playerShoot.fireRate -= 0.13f;

        yield return new WaitForSeconds(powerUpTime);

        playerShoot.fireRate += 0.13f;
    }
}

Answer 1

I think the issue here is that you're destroying the gameobject this script is attached to (the coin) and by so doing, the script itself is destroyed, therefor its code, coroutine or otherwise won't execute.

        StartCoroutine(PowerUpTime());
        isPowerUp = true;
        Destroy(gameObject); //oops, our script has been destroyed :(

You would have to do this very differently, basically moving the bulk of the code to the PlayerShoot class.

Something like this (this being in PlayerShoot.cs)

public void ActivatePowerupFireRate(float time, float amt) {
    StartCoroutine(DoActivatePowerupFireRate(time, amt));
}

public IEnumerator ActivatePowerupFireRate(float time, float amt) {
    fireRate -= amt;
    yield return WaitForSeconds(time);
    fireRate += amt;
}

Answer 2

IEumerator is definately one of the ways you can solve this issue. However I'm not a fan of them here's my solution if you have a timer in game.

public int timePassed = 0;
public int gotPowerUp = 0;

void Start(){
 InvokeRepeating("Timer", 0f, 1.0f);
 //Starting at 0 seconds, every second call Timer function.
}

void Timer(){
  timePassed++; // +1 second.
}

That way when you obtained the powerup you can set gotPowerUp = timePassed. So you have the exact time when powerup is activated.

then you do something like

if( (gotPowerUp + 5) == timePassed ){
 //5 seconds have passed.
 //deactivate powerup here
}