Cannot get a handle on a GameObject velocity through another GameObject in Unity

Question

I'm trying to make a brick breaker game in Unity, using 2D. I have an object that randomly spawns when a brick is destroyed. One of them is a "slowdown" object to slow down the projectile. However, whenever I "catch" the spawn with my paddle, it seems as if I don't have the same instance of the ball.

For instance, I Debug.Log the velocity of the ball through RandomSpawns.csscript, it will show as (0,0). At the same time, if I show the ball velocity through ball.cs, it will show the right velocity

RandomSpawning.cs:

using UnityEngine;
using System.Collections;

public class RandomSpawns : MonoBehaviour 
{
    public paddle paddle;
    public LoseCollider loseCollider;
    public ball ball;
    public Brick brick;

    public string nameOfSpawn;

void Update()
{

    // Finds object with Spawn tag and if lower than paddle y pos, means
    // that player missed the spawn -->destroy to not reset the game

    if (GameObject.FindGameObjectsWithTag("Spawn") != null)
    {
        if (this.transform.position.y < paddle.transform.position.y)
        {
            Destroy(gameObject);
        }
    }
}

void OnTriggerEnter2D(Collider2D collision)
{
    if (collision.gameObject.tag == "paddle")
    {
        triggeredEffect();
        Debug.Log("Right here");
        Destroy(this.gameObject);
    }
}

void triggeredEffect()
{
    if (string.Equals(nameOfSpawn, "slowdown"))
    { 
        Debug.Log("current velocity " + ball.GetComponent<Rigidbody2D>().velocity);
        Vector2 velocity = ball.GetComponent<Rigidbody2D>().velocity;
        ball.GetComponent<Rigidbody2D>().velocity = velocity * 0.5f;
        Debug.Log("slowdown: new velocity" + ball.GetComponent<Rigidbody2D>().velocity);
    }
}

I also tried using this, to no avail:

 GameObject ball = GameObject.Find("ball");
 ball currentBall = ball.GetComponent<ball>();
 Debug.Log("current velocity " + currentBall.GetComponent<Rigidbody2D>().velocity);
 Vector2 velocity = currentBall.GetComponent<Rigidbody2D>().velocity;
 currentBall.GetComponent<Rigidbody2D>().velocity = velocity * 0.5f;
 Debug.Log("slowdown: new velocity" + currentBall.GetComponent<Rigidbody2D>().velocity);

This just gives me an "Object reference not set to instance of object" error.

ball.cs:

using UnityEngine;
using System.Collections;

public class ball : MonoBehaviour 
{
    private paddle paddle;
    private bool hasStarted = false;
    private Vector3 paddleToBall;

    void Start () 
    {
        paddle = GameObject.FindObjectOfType<paddle>();
        paddleToBall = this.transform.position - paddle.transform.position;
        print(paddleToBall);
    }

    // Update is called once per frame
    void Update () 
    {
        if (!hasStarted)
        {
            this.transform.position = paddle.transform.position + paddleToBall;
            if (Input.GetMouseButtonDown(0))
            {
                this.GetComponent<Rigidbody2D>().velocity = new Vector2(.5f, 10f);
                hasStarted = true;
            }
        }
    }   

    void OnCollisionEnter2D(Collision2D collision)
    {
        Vector2 tweak = new Vector2(Random.Range(0f, .2f),Random.Range(0f, .2f));
        if (hasStarted)
        {
            GetComponent<AudioSource>().Play();
            GetComponent<Rigidbody2D>().velocity += tweak;
        }
    }
}

your Random spawns script does not seem to asing the ball object at all. But the second piece of code you gave should have fixed that problem. 2 questions where exactly did you write that code and is your ball actually named "ball" in the scene? — Uri Popov
– Uri Popov, Commented Sep 8, 2016 at 20:33
@UriPopov, the referencing problems are far more rooted than that. Take a closer look at the users variable declarations for ball and paddle. — Gnemlock
– Gnemlock, Commented Sep 9, 2016 at 0:08

Gnemlock · Accepted Answer · 2016-09-09 00:42:46Z

Your referencing your ball incorrectly. I believe that your second attempt may expose an additional error, but you still use incorrect naming conventions, that potentially lead to completely loosing reference to your ball.cs class.

Attempt One - Naming Conventions and Class References

It is very important that you ensure the use of unique names. This includes between references and class names. You can give the name "ball" to both the class and the instance name.

In both scripts, this is how you create your references:

ball ball;
paddle paddle;

This is an example of how you should be creating your references:

ball _ball;
paddle paddleInstance;

When you use identical names for the instance, as you did for the actual class, you leave the door open for many peculiar behaviors. In this case, I believe you are creating a reference that is automatically interpreted as an 'instantiated default', leading to the wrong reference.

Attempt Two - Finding via Tag and Name

In your second attempt, you hit an error before you even look to the class. This could also be caused by the way you are looking for the object. When you use GameObject.Find(string), you are looking for the actual name you have given the game object in the Unity editor. You can not provide the name of a component or script in this way, as GameObject.Find() only looks at GameObject.name.

You may very well be using it correctly, in your example. Once again, your naming your game object with the identical name you give to your class. This can cause additional problems, including those which may lead to an error in linking the reference to an actual game object, in the first place.

Ultimately, if you want the object with the Ball component on it, FindObjectOfType is probably a better way to go than relying on its name being a specific string. — DMGregory
– DMGregory ♦, Commented Mar 8, 2018 at 18:14
Also, Ball (the class) vs. ball (the variable) would be sufficient and more proper. — Draco18s no longer trusts SE
– Draco18s no longer trusts SE, Commented Mar 8, 2018 at 18:29

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Cannot get a handle on a GameObject velocity through another GameObject in Unity

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Attempt One - Naming Conventions and Class References

Attempt Two - Finding via Tag and Name

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Cannot get a handle on a GameObject velocity through another GameObject in Unity

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