feat: add SubclassSelector generic class to allow "nested" PropertyDrawers

Description

The problem:

I have and interface and I want to select subclasses with SubclassSelector attribute. One of the subclasses has its own PrepertyDrawer. Let's look to an example code:

public interface IInterace {}

[Serializable]
public class SubclassA : IInterface {}

[Serializable]
public class SubclassB : IInterface {}

public class MyObject : MonoBehaviour {
    [SerializeReference, SubclassSelector] private IInterface _interface;
}

// in an Editor script
[CustomPropertyDrawer(typeof(SubclassB))]
public class SubclassBPropertyDrawer : PropertyDrawer
{
    // ... editor code
}

The problem is the property drawer of SubclassB wont be used as it competes with SubclassSelectorDrawer because both drawers are applied to the same field _interface.

MySolution

Instead of an attribute I used a generic class SubclassSelector<T> that have a SerializeReference field. I added this class to your code so users can use either SubclassSelectorAttribute or SubclassSelector<T> depending on their needs. The usage is as follows:

public class MyObject : MonoBehaviour {
    [SerializeField] private SubclassSelector<IInterface> _interface;
}

Advantages:

Subclasses' property drawers are correctly applied.

Disadvantages

To get the value of _interface you have call _interface.Value which is more verbose. Value is a property of SubclassSelector<T> that returns the value of the actual object you want.

Changes made

I created the class SubclassSelector.cs inside MackySoft.SerializeReferenceExtensions.Runtime folder. The code is simple

[Serializable]
public class SubclassSelector<T> where T : class
{
[SerializeReference] private T _value;

public T Value
{
    get => _value as T;
    set => _value = value;
}
}

I created the class SubclassSelectorGenericDrawer.cs inside MackySoft.SerializeReferenceExtensions.Editor folder. This has the drawer code and is almost identical to your SubclassSelectorDrawer.cs.

mackysoft / Unity-SerializeReferenceExtensions