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Resolved Making a trigger for an icicle to drop

Discussion in 'Scripting' started by BrownBrew, Mar 19, 2023.

  1. BrownBrew


    Jan 6, 2023
    Hello. I'm having a problem with seemingly a very simple task.

    I have an icicle that contains a trigger object and a damagebox object.
    Trigger has a static rigidbody, a boxcollider2d and a script that is supposed to run a method of a IBasicTriggerable object (in this instance - make the icicle drop on the player)
    Code (CSharp):
    1. public class BasicTrigger : MonoBehaviour
    2. {
    3.     [SerializeField] private Object _object;
    5.     private void OnTriggerEnter2D(Collider2D collision)
    6.     {
    7.         (_object as IBasicTriggerable).BasicTriggerEvent();
    8.     }
    9. }
    I drag n drop the Icicle object in the inspector
    (Icicle itself has the IBasicTriggerable interface and the code to trigger the drop)
    But it gives me the nullreference error on collission proc:
    I also tried to use UnityEvents and dragndrop the Icicle to call it's Drop method (the UnityEvent was on the child trigger object), but it gives me the same NullReferenceException.
    Seems like there's something I fundamentally don't understand here.
    I also want to mention that I understand this is a really weird way to implement a trigger when you can easilly do this by just adding a boxcollider2d set as trigger on the icicle itself. But I want to understand how to do it this way exactly - by referencing an object on the trigger.
    Please help! Thank you in advance!
  2. spiney199


    Feb 11, 2021
    You've probably referenced
    via it's game object, which mean it will never be
    . When you cast with
    , if the cast is invalid, you will be given

    You'll want to make sure you reference it via the component, though I believe that will be cumbersome with Unity's default inspectors. Probably easier to use
    for the interface type instead.
    BrownBrew and Bunny83 like this.
  3. Bunny83


    Oct 18, 2010
    Right. When you just drag an object from the hierarchy into a serialized slot in the inspector, the Unity editor will actually drag the GameObject instance around. The editor is clever enough to detect component types when the field you try to assign a value to is a component. So dragging a GameObject onto a component field means the Unity editor searches for this component type on the dragged gameobject. So it tries to assign the dragged object and if it's not compatible it tries to use GetComponent of the field type. Since the field here is a UnityEngine.Object, any class that is derived from UntiyEngine.Object can be assigned to that field. So when you just drag an object, you would assign a GameObject.

    As it was mentioned, you can actually drag specific component instances as well. However it would require you to open two inspectors, lock one of them so you can actually grab the component header bar and drag that into the slot. In this case the object you're dragging is the actual component and not just the GameObject.

    I once made this SerializableInterface which may be useful in such cases. It also just stores a UnityEngine.Object (which can be a component or ScriptableObject instance) and does lazily convert the serialized UnityEngine.Object into the actual interface type. The property drawer actually filters out an appropriate instance. If more than one component exist on the dragged gameobject, it displays a drop down menu which lets you select one of the matching instances.


    So you could implement your BasicTrigger class like this:

    Code (CSharp):
    1.     public class BasicTrigger : MonoBehaviour
    2.     {
    3.         [SerializeField] private SerializableInterface<IBasicTriggerable> _target;
    5.         private void OnTriggerEnter2D(Collider2D collision)
    6.         {
    7.             _target.Instance.BasicTriggerEvent();
    8.         }
    9.     }
    BrownBrew and spiney199 like this.
  4. BrownBrew


    Jan 6, 2023
    Thank you guys. I'm also a complete idiot and the nullreference was caused by the method in the icicle, not the trigger, so it works perfectly with a UnityEvent as well.