How to receive gameobject in property drawer? (interface in inspector workaround)

Questions & Answers
1

Hi! I found this workaround for assigning interfaces in inspector (https://forum.unity.com/threads/serialized-interface-fields.1238785/), but I’m trying to find a way to drop gameobjects in the target field and get the component with the interface (as one would do with “gameobject.GetComponent()”). I tried assigning gameobjects with the same workaround but using custom classes (with monobehaviour) instead of interfaces and it works.

I’ll be leaving here the code of the property Drawer.

using UnityEngine;
using UnityEditor;

namespace Universal.Attributes
{
    /// <summary>
    /// Drawer for the RestrictType attribute.
    /// </summary>
    [CustomPropertyDrawer(typeof(RestrictTypeAttribute))]
    public class RestrictTypeDrawer : PropertyDrawer
    {
        /// <summary>
        /// Overrides GUI drawing for the attribute.
        /// </summary>
        /// <param name="position">Position.</param>
        /// <param name="property">Property.</param>
        /// <param name="label">Label.</param>
        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            // Check if this is reference type property.
            if (property.propertyType == SerializedPropertyType.ObjectReference)
            {
                // Get attribute parameters.
                var requiredAttribute = this.attribute as RestrictTypeAttribute;
                
                // Begin drawing property field.
                EditorGUI.BeginProperty(position, label, property);
                
                // Draw property field.
                property.objectReferenceValue = EditorGUI.ObjectField(position, label, property.objectReferenceValue, requiredAttribute.requiredType, true);

                // Finish drawing property field.
                EditorGUI.EndProperty();
            }
            else
            {
                // If field is not reference, show error message.
                // Save previous color and change GUI to red.
                var previousColor = GUI.color;
                GUI.color = Color.red;

                // Display label with error message.
                EditorGUI.LabelField(position, label, new GUIContent("Property is not a reference type"));

                // Revert color change.
                GUI.color = previousColor;
            }
        }
    }
}
2

I found a solution! A video about interfaces and property drawers that tackled the same problem. I’ll leave the code here just in case.

using UnityEngine;
using UnityEditor;

namespace Universal.Attributes
{
    /// <summary>
    /// Drawer for the RestrictType attribute.
    /// https://www.patrykgalach.com/2020/01/27/assigning-interface-in-unity-inspector/
    /// https://youtu.be/zkr6LU3kHE4 (from OtterKnight)
    /// </summary>
    [CustomPropertyDrawer(typeof(RestrictTypeAttribute))]
    public class RestrictTypeDrawer : PropertyDrawer
    {
        /// <summary>
        /// Overrides GUI drawing for the attribute.
        /// </summary>
        /// <param name="position">Position.</param>
        /// <param name="property">Property.</param>
        /// <param name="label">Label.</param>
        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            // Check if this is reference type property.
            if (property.propertyType == SerializedPropertyType.ObjectReference)
            {
                // Get attribute parameters.
                var requiredAttribute = this.attribute as RestrictTypeAttribute;
                System.Type reqType = requiredAttribute.requiredType;
                
                // Begin drawing property field.
                EditorGUI.BeginProperty(position, label, property);
                
                // Draw property field.
                Object obj = EditorGUI.ObjectField(position, label, property.objectReferenceValue, typeof(Object), !EditorUtility.IsPersistent(property.serializedObject.targetObject));

                if (EditorGUI.EndChangeCheck())
                {
                    if (obj == null)
                        property.objectReferenceValue = null;
                    else if (reqType.IsInstanceOfType(obj))
                        property.objectReferenceValue = obj;
                    else if (obj is GameObject)
                    {
                        //This would be risky in another situation, but we know it's a monobehaviour
                        MonoBehaviour c = (MonoBehaviour)((GameObject)obj).GetComponent(reqType);
                        if (c != null)
                            property.objectReferenceValue = c;
                    }
                }

                // Finish drawing property field.
                EditorGUI.EndProperty();
            }
            else
            {
                // If field is not reference, show error message.
                // Save previous color and change GUI to red.
                var previousColor = GUI.color;
                GUI.color = Color.red;

                // Display label with error message.
                EditorGUI.LabelField(position, label, new GUIContent("Property is not a reference type"));

                // Revert color change.
                GUI.color = previousColor;
            }
        }
    }
}