Unity C# attribute for serializing component and interface references within the scene or prefab during OnValidate, rather than using GetComponent* functions in Awake/Start/OnEnable at runtime.

This project aims to provide:

• a simple set of tags for declaring dependency locations
• resolution, validation and serialisation of references (including interface types!)
• determinate results so you never have to worry about Unity losing your references, they'll always be safely regenerated

Mainly to avoid framerate hiccups when additively loading scenes in Farewell North, but I also find this to be a much cleaner way to declare references.

For more details on how this project came about, check out this Farewell North devlog on YouTube.

Instead of declaring your references, finding them in Awake or assigning them in the editor, and then validating them in OnValidate like so:

[SerializeField] private Player _player; 
[SerializeField] private Collider _collider;
[SerializeField] private Renderer _renderer;
[SerializeField] private Rigidbody _rigidbody;
[SerializeField] private ParticleSystem[] _particles;

private void Awake()
{
    this._collider = this.GetComponent<Collider>();
    this._renderer = this.GetComponentInChildren<Renderer>();
    this._rigidbody = this.GetComponentInParent<Rigidbody>();
    this._particles = this.GetComponentsInChildren<ParticleSystem>();
}

private void OnValidate()
{
    Debug.Assert(this._player != null, "Player must be assigned in the editor");
}

You can declare their location inline using the Self, Child, Parent and Anywhere attributes:

[SerializeField, Anywhere] private Player _player; 
[SerializeField, Self] private Collider _collider;
[SerializeField, Child] private Renderer _renderer;
[SerializeField, Parent] private Rigidbody _rigidbody;
[SerializeField, Child]  private ParticleSystem[] _particles;

private void OnValidate()
{
    this.ValidateRefs();
}

The ValidateRefs function is made available as a MonoBehaviour extension for ease of use, and handles finding, validating and serialisating the references in the editor so they're always available at runtime.

By default Unity doesn't allow you to serialise interfaces, but this project allows it by wrapping them with the InterfaceRef type, like so:

[SerializeField, Self] private InterfaceRef<IDoorOpener> _doorOpener;

From here they'll be serialised like any other component reference, with the interface available using .Value:

IDoorOpener doorOpener = this._doorOpener.Value;
doorOpener.OpenDoor();

• Self looks for the reference on the same game object as the attributed component using GetComponent(s)()
• Parent looks for the reference on the parent hierarchy of the attributed components game object using GetComponent(s)InParent()
• Child looks for the reference on the child hierarchy of the attributed components game object using GetComponent(s)InChildren()
• Anywhere will only validate the reference isn't null, but relies on you to assign the reference yourself.

The attributes all allow for optional flags to customise their behaviour:

[SerializeField, Self(Flag.Optional)]    
private Collider _collider;

[SerializeField, Parent(Flag.IncludeInactive)]
private Rigidbody _rigidbody;

You can also specify multiple flags:

[SerializeField, Child(Flag.Optional | Flag.IncludeInactive)] 
private ParticleSystem[] _particles;

• Optional won't fail validation for empty (or null in the case of non-array types) results.
• IncludeInactive only affects Parent and Child, and determines whether inactive components are included in the results. By default, only active components will be found, same as GetComponent(s)InChildren and GetComponent(s)InParent.

• Supports all MonoBehaviour and Component types (basically anything you can use with GetComponent)
• Determinate results, so there's no worry of Unity losing all your serialised references (which has happened to me a few times on upgrading editor versions). All references will be reassigned automatically.
• Fast, this tool won't slow down your editor or generate excessive garbage.
• Fully serialised at edit time, so all references are already available at runtime
• The ValidateRefs function is made available as a MonoBehaviour extension for ease of use. You can also invoke it outside of a MonoBehaviour like so, if preferred:

MyScript script = ...;
SceneRefAttributeValidator.Validate(script);

• One-click to regenerate all references for every script under Tools > Validate All Refs.
• Regenerate references for any component by right-clicking the component and selecting Validate Refs.

• Supports arrays but not Lists. Feel free to submit a PR if you'd like to see this:
  • Valid: [Child] Renderer[] _renderers;
  • Invalid: [Child] List<Renderer> _renderers;
• Support arrays of InterfaceRef.
  • Valid: [Child] InterfaceRef<IInterface>[] _interfaces;
  • Invalid: [Child] InterfaceRef<IInterface[]> _interfaces;

This project is made available under the MIT License, so you're free to use it for any purpose.