Setting parent in instantiated class

Question

I am trying to get bullets to fire from a parented position. In the Weapon.cs Awake() function I create 2 gunports, pass the parent transform and a Vector3 to offset the position from the parent. When I run the code I get the following error from GunPort.cs:

NullReferenceException GunPort..ctor (UnityEngine.Transform _owner, Vector3 _offsetPosition) (at Assets/GunPort.cs:37) Starfighter.Awake () (at Assets/Starfighter.cs:63)

This error points to where I set the transform.parent to the passed _owner.transform. I have done something similar with parenting a camera to physical object with success and I dont know why there is a problem with the way I have done it here. The _owner is not null as it suggests which I verified from using the statement Debug.Log(_owner.ToString());. Please excuse the Itallic text it is not intentional. The only difference is that my GunPort does not have a physical object, its invisible, as intended.

Weapon.cs:

 void Awake () {
     gunPort = GetComponent<GunPort>();
     gunPorts = new GunPort[2];
     gunPorts[0] = new GunPort(transform, new Vector3(-10, 0, 0));
     gunPorts[1] = new GunPort(transform, new Vector3(10, 0, 0));
 }

 void CheckFiringPrimaryWeapon()
 {    
     if(shipControls.IsFiringPrimary)
     {
         Bullet clone;
         clone = Instantiate(bullet, gunPorts[0].Position, transform.rotation) as Bullet;
         clone = Instantiate(bullet, gunPorts[1].Position, transform.rotation) as Bullet;
     }
 }

GunPort.cs:

 public class GunPort : MonoBehaviour {
 private Transform owner;

 public Vector3 Position
 {
     get { return transform.position; }
 }
 
 public GunPort(Transform _owner, Vector3 _offsetPosition)
 {
     transform.parent = _owner.transform;
     transform.localPosition = 
         (Vector3.right * _offsetPosition.x) +
         (Vector3.up * _offsetPosition.y) +
         (Vector3.forward * _offsetPosition.z);
 }
 }

Answer 1

Best Answer

Answer by syclamoth · Sep 27, 2011 at 06:26 AM

You shouldn't be using the Constructor on objects which derive from MonoBehaviour! The problem is that the object gets 'created' in a different way from how you would expect it to. Use Instantiate, and then pass in variables with an Init(Transform owner, Vector3 offset) method. Also, you don't need to use _owner.transform in your parenting line. _owner.transform will always return _owner, since it is already a transform! You may as well use

 transform.parent = _owner;

It is more efficient, since it does not use component lookups like Component.transform does.

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centaurianmudpig · Sep 27, 2011 at 11:18 AM 0

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Ah, I see, thanks. Is there any way to instantiate the object without it needing to be attached to a GameObject, RigidBody, etc? As the GunPort will be invisible.

syclamoth · Sep 27, 2011 at 11:23 AM 1

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There is no need to attach a renderer to a gameObject, if you need it to be invisible! Just go GameObject -> Create Empty, and put your behaviours on that. If you don't need any of that GameObject stuff at all, then there's no need to inherit from MonoBehaviour at all!

Answer 2

I think I get what you are saying though I am having some trouble trying to implement it. I'm quite new at unity so I hope you will bear with me. I created an empty GameObject, renamed it to GunPort, and attached my GunPort script to it.

In my Main.cs file I added:

 public GameObject port;

Then attached the GunPort GameObject to it within the Unity Editor Inspector.

In the Awake() function I Instantiated the GunPorts as follows (where Init replaces the constructor from the OP in GunPort.cs):

        gunPorts = new GunPort[2];
     gunPorts[0] = Instantiate(port) as GunPort;
     gunPorts[0].Init(transform, 1, new Vector3(10, 0, 0));
     gunPorts[1] = Instantiate(port) as GunPort;
     gunPorts[1].Init(transform, 1, new Vector3(-10, 0, 0));

The problem I have is that the Instantiate() does not return anything, as I get the error:

NullReferenceException: Object reference not set to an instance of an object Starfighter.Awake () (at Assets/Starfighter.cs:66)

The error happens on the Init() call, debugging shows that gunPorts[0] is null. I'm not sure what I have done wrong.

Answer 3

Answer by centaurianmudpig · Sep 27, 2011 at 02:58 PM

OK, I figured out where I went wrong. I should have created port as GunPort and not a GameObject. Thanks for your help in this.

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Bunny83 · Sep 27, 2011 at 03:58 PM 1

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Just fyi: Instantiate clones whatever object you provide as source as long it's derived from UnityEngine.Object. Instantiate will always return the same type that you provide. When you clone a Component like a script it automatically clones the owning GameObject and all attached components / subobjects. When cloning a GameObject it does the same but it will return a reference to the new GameObject.

However, if you want to create an empty GameObject with your script attached you can simply do:

 GunPort newGunPort = (new GameObject()).AddComponent<GunPort>();

The GameObject constructor also allows to automatically add components by their type-object:

 new GameObject("GO-name",typeof(GunPort));

But if you want a reference to the attached script you would have to call GetComponent so it's even more complicated:

 GunPort newGunPort = (new GameObject("GO-name",typeof(GunPort))).GetComponent<GunPort>();

Setting parent in instantiated class

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