How do you pick up an instantiated object?

Question

Hello! So I have the standard code to pick up an object within the scene:

 public class PickUp : MonoBehaviour
 {
 
     public Transform ObjectPos;
 
     void OnMouseDown()
     {
         GetComponent<Rigidbody>().useGravity = false;
         this.transform.position = ObjectPos.position;
         this.transform.parent = GameObject.Find("ObjectPosition").transform;
     }
 
     void OnMouseUp()
     {
         this.transform.parent = null;
         GetComponent<Rigidbody>().useGravity = true;
     }
 }

My issue is that you can't assign a transform to a prefab object because it hasn't been spawned yet. I have a series of objects that spawn during Runtime that I need the player to be able to pick up. This seems like it should be super simple but I can't find any information on it at all.

Answer 1

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Answer by Hellium · Mar 16 at 11:17 AM

Reference the ObjectPos object from the class responsible for spawning the PickUp objects and inject the reference when instantiating the objects

  public class PickUpSpawner : MonoBehaviour
  {     
      public PickUp PickUpPrefab;
      public Transform ObjectPos;
  
      void SpawnPickUp()
      {
          PickUp pickUp = Instantiate(PickUpPrefab);
          pickUp.ObjectPos = ObjectPos;
      }
 }

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moominlittlesocks · Mar 16 at 01:06 PM 0

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That works! Thank you very much. So now Unity is obviously throwing an error because the PickUp script hasn't been given a Transform for ObjectPos, however if I remove that line of code, I get the following error:

CS1061 'PickUp' does not contain a definition for 'ObjectPos' and no accessible extension method 'ObjectPos' accepting a first argument of type 'PickUp' could be found (are you missing a using directive or an assembly reference?)

Surely now that ObjectPos is being referenced within the PickUpSpawner script, the old one is redundant?

Hellium moominlittlesocks · Mar 16 at 01:24 PM 0

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You must keep public Transform ObjectPos; inside your PickUp class.

So now Unity is obviously throwing an error because the PickUp script hasn't been given a Transform for ObjectPos

Can you provide the exact error thrown by Unity? Is it a NullReferenceException? If so, you may have forgotten to call pickUp.ObjectPos = ObjectPos; after instantiating the object. (make sure to inject the value inside the instanciated object, not inside the prefab)

moominlittlesocks Hellium · Mar 16 at 01:46 PM 0

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So right now, I have the following code within my PickUpSpawner script, attached to the spawner:

 public PickUp PickupPrefab;
 public Transform ObjectPos;
 
 public void SpawnBox()
     {
         PickUp pickup = Instantiate(PickupPrefab);
         pickup.ObjectPos = ObjectPos;
     }

Then, in the PickUp script attached to the prefab I have the following:

     public Transform ObjectPos;    
 
     void OnMouseDown()
     {
         GetComponent<Rigidbody>().useGravity = false;
         this.transform.position = ObjectPos.position;
         this.transform.parent = GameObject.Find("ObjectPosition").transform;
     }
 
     void OnMouseUp()
     {
         this.transform.parent = null;
         GetComponent<Rigidbody>().useGravity = true;
     }
 }

The Pickup Prefab and Object Pos are then referenced correctly. But by keeping the line public Transform ObjectPos; within the PickUp script, attached to the prefab, I'm getting the NullReferenceException error because I can't give the prefab the transform it wants. Have I got the PickUp script in the right place? Should it not be attached to the prefab?

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How do you pick up an instantiated object?

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