Extend Unity's built-in inspectors

DecoratorEditor.cs

using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using UnityEditor;
using UnityEngine;

/// <summary>
/// A base class for creating editors that decorate Unity's built-in editor types.
/// </summary>
public abstract class DecoratorEditor : Editor
{
	// empty array for invoking methods using reflection
	private static readonly object[] EMPTY_ARRAY = new object[0];
	
	#region Editor Fields
	
	/// <summary>
	/// Type object for the internally used (decorated) editor.
	/// </summary>
	private System.Type decoratedEditorType;
	
	/// <summary>
	/// Type object for the object that is edited by this editor.
	/// </summary>
	private System.Type editedObjectType;
	
	private Editor editorInstance;
	
	#endregion

	private static Dictionary<string, MethodInfo> decoratedMethods = new Dictionary<string, MethodInfo>();
	
	private static Assembly editorAssembly = Assembly.GetAssembly(typeof(Editor));
	
	protected Editor EditorInstance
	{
		get
		{
			if (editorInstance == null && targets != null && targets.Length > 0)
			{
				editorInstance = Editor.CreateEditor(targets, decoratedEditorType);
			}
			
			if (editorInstance == null)
			{
				Debug.LogError("Could not create editor !");
			}
			
			return editorInstance;
		}
	}
	
	public DecoratorEditor (string editorTypeName)
	{
		this.decoratedEditorType = editorAssembly.GetTypes().Where(t => t.Name == editorTypeName).FirstOrDefault();
		
		Init ();
		
		// Check CustomEditor types.
		var originalEditedType = GetCustomEditorType(decoratedEditorType);
		
		if (originalEditedType != editedObjectType)
		{
			throw new System.ArgumentException(
				string.Format("Type {0} does not match the editor {1} type {2}", 
			              editedObjectType, editorTypeName, originalEditedType));
		}
	}
	
	private System.Type GetCustomEditorType(System.Type type)
	{
		var flags = BindingFlags.NonPublic	| BindingFlags.Instance;
		
		var attributes = type.GetCustomAttributes(typeof(CustomEditor), true) as CustomEditor[];
		var field = attributes.Select(editor => editor.GetType().GetField("m_InspectedType", flags)).First();
		
		return field.GetValue(attributes[0]) as System.Type;
	}
	
	private void Init()
	{		
		var flags = BindingFlags.NonPublic	| BindingFlags.Instance;
		
		var attributes = this.GetType().GetCustomAttributes(typeof(CustomEditor), true) as CustomEditor[];
		var field = attributes.Select(editor => editor.GetType().GetField("m_InspectedType", flags)).First();
		
		editedObjectType = field.GetValue(attributes[0]) as System.Type;
	}

	void OnDisable()
	{
		if (editorInstance != null)
		{
			DestroyImmediate(editorInstance);
		}
	}
	
	/// <summary>
	/// Delegates a method call with the given name to the decorated editor instance.
	/// </summary>
	protected void CallInspectorMethod(string methodName)
	{
		MethodInfo method = null;
		
		// Add MethodInfo to cache
		if (!decoratedMethods.ContainsKey(methodName))
		{
			var flags = BindingFlags.Instance | BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public;
			
			method = decoratedEditorType.GetMethod(methodName, flags);
			
			if (method != null)
			{
				decoratedMethods[methodName] = method;
			}
			else
			{
				Debug.LogError(string.Format("Could not find method {0}", method));
			}
		}
		else
		{
			method = decoratedMethods[methodName];
		}
		
		if (method != null)
		{
			method.Invoke(EditorInstance, EMPTY_ARRAY);
		}
	}

	public void OnSceneGUI()
	{
		CallInspectorMethod("OnSceneGUI");
	}

	protected override void OnHeaderGUI ()
	{
		CallInspectorMethod("OnHeaderGUI");
	}
	
	public override void OnInspectorGUI ()
	{
		EditorInstance.OnInspectorGUI();
	}
	
	public override void DrawPreview (Rect previewArea)
	{
		EditorInstance.DrawPreview (previewArea);
	}
	
	public override string GetInfoString ()
	{
		return EditorInstance.GetInfoString ();
	}
	
	public override GUIContent GetPreviewTitle ()
	{
		return EditorInstance.GetPreviewTitle();
	}
	
	public override bool HasPreviewGUI ()
	{
		return EditorInstance.HasPreviewGUI ();
	}
	
	public override void OnInteractivePreviewGUI (Rect r, GUIStyle background)
	{
		EditorInstance.OnInteractivePreviewGUI (r, background);
	}
	
	public override void OnPreviewGUI (Rect r, GUIStyle background)
	{
		EditorInstance.OnPreviewGUI (r, background);
	}
	
	public override void OnPreviewSettings ()
	{
		EditorInstance.OnPreviewSettings ();
	}
	
	public override void ReloadPreviewInstances ()
	{
		EditorInstance.ReloadPreviewInstances ();
	}
	
	public override Texture2D RenderStaticPreview (string assetPath, Object[] subAssets, int width, int height)
	{
		return EditorInstance.RenderStaticPreview (assetPath, subAssets, width, height);
	}
	
	public override bool RequiresConstantRepaint ()
	{
		return EditorInstance.RequiresConstantRepaint ();
	}
	
	public override bool UseDefaultMargins ()
	{
		return EditorInstance.UseDefaultMargins ();
	}
}

On line 118, probably Debug.LogError(string.Format("Could not find method {0}", methodName)); is more appropriated.

Man, I'm facing a problem, it looks like that sometimes something is leaking and I start to receive some errors like this:

NullReferenceException: (null) UnityEditor.SerializedObject..ctor (UnityEngine.Object[] objs) (at C:/buildslave/unity/build/artifacts/generated/common/editor/SerializedPropertyBindings.gen.cs:74) UnityEditor.Editor.GetSerializedObjectInternal () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:154) UnityEditor.Editor.get_serializedObject () (at C:/buildslave/unity/build/artifacts/generated/common/editor/EditorBindings.gen.cs:147) UnityEditor.GameObjectInspector.OnEnable () (at C:/buildslave/unity/build/Editor/Mono/Inspector/GameObjectInspector.cs:89)

Have you faced this before, or have any ideas how to prevent this happen?

Error Exception:TargetException: Object does not match target type. will occurred when use this cool script.
When use reflection we pass the wrong instance to Invoke.

I think We should cache the inspector method with decoratedEditorType

private static Dictionary<System.Type, Dictionary<string, MethodInfo>> newDecoratedMethods = new Dictionary<System.Type, Dictionary<string, MethodInfo>>();

Same problem as Dennin, 3 years later, and no fix ? ;) on unity 2019