How to store references to scene objects in prefabs?

Question

i am instantiating a few game objects in my scene using prefabs and i would like to attach a script to some of them to do something at a certain time but for that i need to reference to scene objects and its not allowing me. anyone know why? and whats the workaround?

i can pull those prefabs into my scene and use them as game objects but that would destroy the idea of prefabs isn't?

Answer 1

In the editor, prefabs can only reference other prefabs. Prefabs are supposed to be shared between scenes. That means they can't rely on the object being present in every scene.

If you need them to reference something in the scene, you need to get that reference at runtime. There are several ways to do that.

• Attach the reference to the object which instantiates the prefabs and have the spawning script set it on every object it instantiates:

  GameObject bullet = Instantiate(bulletPrefab, transform.position, transform.rotation); bullet.GetComponent<BulletScript>().shooter = gameObject;

• Search for the game object in the Start method of your prefab.

  • With Find (warning, slow!)
  • Its variants like FindWithTag or FindObjectOfType (warning, not as slow as Find but still slow)
  • By traversing your object hierarchy with transform.parent, transform.GetChild, transform.Find etc. (warning, breaks very easily)
• Store the reference to the object in a global variable (a public static variable of an appropriate class) so all the instantiated prefabs can access it.

Answer 2

Unity does not allow you to reference scene objects to prefabs. It allows any other reference. It's because prefabs have a template-like nature and thus are unaware of the scene.

There are many workarounds for this case, most of them involve a static variable.

1. Assign a Tag to your scene object and then find it by searching through objects

   var all = FindObjectsOfType<GameObject>();
   foreach ( var item in all ) { 
       if (item.Tag.CompareTo("MyTag") == 0) 
           return item;
   }
   //or
   var item = GameObject.FindWithTag("MyTag");
   

2. Add a script to your scene object and store the reference there

   internal GameObject MyRef;
   void Awake() { MyRef = this; }
   

3. Add an empty game object (reference manager) in your scene with a script to store all references

   //assign these in inspector
   public GameObject MyRef1;
   public GameObject MyRef2;
   

Note that

in case 2,3 you may use static variables or singleton pattern.

e.g.

case 2:

public class SceneObject1 : MonoBehaviour
{
    public static GameObject MyRef { get; private set; }
    void Awake() { MyRef = this; }
}

case 3:

public class RefManager : MonoBehaviour
{
    static RefManager instance;
    void Awake() { instance = this; }

    public GameObject MyRef1;

    //use this in prefab script
    public static GameObject MyRef1 { get { return instance.MyRef1; } }
}

//in prefab
var inPrefabVariable = RefManager.MyRef1;

Answer 3

You should definitively read the Unity manual about prefabs

The prefab acts as a template from which you can create new object instances in the scene.

The purpose of prefabs is to be instantiated into the scene.

When in the Project tab, it acts as a "regular file" and must be independant from any object in any scene (but it can have references from other prefabs also in the Project tab) so as to be reusable in any scene.

The advantage of prefabs is, when you edit the values of one of the and hit "Apply", all the instances in the other scenes will have the new values. (Except if you have already overriden the values of the instance)

You have to instantiate the prefab into your scene so as to be able to link objects of the scene into your prefab. The instantiation and creation of the links to the prefab instance can be done using the inspector, or, at runtime, using code.

Answer 4

I think there is a way around this. If I am getting your question right, you want your prefab's public variables to have objects reference from the scene. What you can do is make prefab instances particularly for that scene. Which means

1. you can drag and drop you prefab in hierarchy.
2. Unpack the Prefab object in hierarchy
3. Now attach the references of other object to the script of Unpacked Prefab.
4. Disable the Unpacked Prefab in hierarchy.
5. Now where ever you have code to instantiate the prefab originally, replace it with this Unpacked Prefab object from hierarchy.
6. Set Active(true) the instantiated object.

What basically we are doing here is instead of using prefab we are using object as a reference for instantiating objects. And since the object are is disabled in hierarchy it is not working but holding all the property of its prefab, just like the prefab itself.

Note while instantiating the object, don't forget to SetActive true. Here is the example how to do it :

GameObject cloneObject = Instantiate(Object);
cloneObject.SetActive(true);

Answer 5

One workaround is to note the instanceID of any scene object references, they are merely ints, and will save to the prefab just fine. In order to restore missing object references in new instances of the prefab, write a script that searches for GameObjects whose instanceID match with the ones we stored in the prefab. To get a full list of all available GameObjects in all open scenes, you can use eg GameObject.FindObjectsOfType(typeof(GameObject));

I'm not suggesting this is a robust solution, or a good idea, just that it is possible to deal with this kind of scenario.