Displays the fields of a ScriptableObject in the inspector

ExtendedScriptableObjectDrawer.cs

// Developed by Tom Kail at Inkle
// Released under the MIT Licence as held at https://opensource.org/licenses/MIT

// Must be placed within a folder named "Editor"
using System;
using System.Reflection;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;
using Object = UnityEngine.Object;

/// <summary>
/// Extends how ScriptableObject object references are displayed in the inspector
/// Shows you all values under the object reference
/// Also provides a button to create a new ScriptableObject if property is null.
/// </summary>
[CustomPropertyDrawer(typeof(ScriptableObject), true)]
public class ExtendedScriptableObjectDrawer : PropertyDrawer {
	
	public override float GetPropertyHeight (SerializedProperty property, GUIContent label) {
		float totalHeight = EditorGUIUtility.singleLineHeight;
		if(property.objectReferenceValue == null || !AreAnySubPropertiesVisible(property)){
			return totalHeight;
		}
		if(property.isExpanded) {
			var data = property.objectReferenceValue as ScriptableObject;
			if( data == null ) return EditorGUIUtility.singleLineHeight;
			SerializedObject serializedObject = new SerializedObject(data);
			SerializedProperty prop = serializedObject.GetIterator();
			if (prop.NextVisible(true)) {
				do {
					if(prop.name == "m_Script") continue;
					var subProp = serializedObject.FindProperty(prop.name);
					float height = EditorGUI.GetPropertyHeight(subProp, null, true) + EditorGUIUtility.standardVerticalSpacing;
					totalHeight += height;
				}
				while (prop.NextVisible(false));
			}
			// Add a tiny bit of height if open for the background
			totalHeight += EditorGUIUtility.standardVerticalSpacing;
			serializedObject.Dispose();
		}
		return totalHeight;
	}

	const int buttonWidth = 66;
	
	static readonly List<string> ignoreClassFullNames = new List<string>{ "TMPro.TMP_FontAsset" };
	
	public override void OnGUI (Rect position, SerializedProperty property, GUIContent label) {
		EditorGUI.BeginProperty (position, label, property);
		var type = GetFieldType();
		
		if(type == null || ignoreClassFullNames.Contains(type.FullName)) {
			EditorGUI.PropertyField(position, property, label);	
			EditorGUI.EndProperty ();
			return;
		}
		
		ScriptableObject propertySO = null;
		if(!property.hasMultipleDifferentValues && property.serializedObject.targetObject != null && property.serializedObject.targetObject is ScriptableObject) {
			propertySO = (ScriptableObject)property.serializedObject.targetObject;
		}
		
		var propertyRect = Rect.zero;
		var guiContent = new GUIContent(property.displayName);
		var foldoutRect = new Rect(position.x, position.y, EditorGUIUtility.labelWidth, EditorGUIUtility.singleLineHeight);
		if(property.objectReferenceValue != null && AreAnySubPropertiesVisible(property)) {
			property.isExpanded = EditorGUI.Foldout(foldoutRect, property.isExpanded, guiContent, true);
		} else {
			// So yeah having a foldout look like a label is a weird hack 
			// but both code paths seem to need to be a foldout or 
			// the object field control goes weird when the codepath changes.
			// I guess because foldout is an interactable control of its own and throws off the controlID?
			foldoutRect.x += 12;
			EditorGUI.Foldout(foldoutRect, property.isExpanded, guiContent, true, EditorStyles.label);
		}
		var indentedPosition = EditorGUI.IndentedRect(position);
		var indentOffset = indentedPosition.x - position.x;
		propertyRect = new Rect(position.x + (EditorGUIUtility.labelWidth - indentOffset), position.y, position.width - (EditorGUIUtility.labelWidth - indentOffset), EditorGUIUtility.singleLineHeight);

		if(propertySO != null || property.objectReferenceValue == null) {
			propertyRect.width -= buttonWidth;
		}
		
		EditorGUI.ObjectField(propertyRect, property, type, GUIContent.none);
		if (GUI.changed) property.serializedObject.ApplyModifiedProperties();

		var buttonRect = new Rect(position.x + position.width - buttonWidth, position.y, buttonWidth, EditorGUIUtility.singleLineHeight);
			
		if(property.propertyType == SerializedPropertyType.ObjectReference && property.objectReferenceValue != null) {
			var data = (ScriptableObject)property.objectReferenceValue;
			
			if(property.isExpanded) {
				// Draw a background that shows us clearly which fields are part of the ScriptableObject
				GUI.Box(new Rect(0, position.y + EditorGUIUtility.singleLineHeight + EditorGUIUtility.standardVerticalSpacing - 1, Screen.width, position.height - EditorGUIUtility.singleLineHeight - EditorGUIUtility.standardVerticalSpacing), "");

				EditorGUI.indentLevel++;
				SerializedObject serializedObject = new SerializedObject(data);
				
				// Iterate over all the values and draw them
				SerializedProperty prop = serializedObject.GetIterator();
				float y = position.y + EditorGUIUtility.singleLineHeight + EditorGUIUtility.standardVerticalSpacing;
				if (prop.NextVisible(true)) {
					do {
						// Don't bother drawing the class file
						if(prop.name == "m_Script") continue;
						float height = EditorGUI.GetPropertyHeight(prop, new GUIContent(prop.displayName), true);
						EditorGUI.PropertyField(new Rect(position.x, y, position.width-buttonWidth, height), prop, true);
						y += height + EditorGUIUtility.standardVerticalSpacing;
					}
					while (prop.NextVisible(false));
				}
				if (GUI.changed)
					serializedObject.ApplyModifiedProperties();
				serializedObject.Dispose();
				EditorGUI.indentLevel--;
			}
		} else {
			if(GUI.Button(buttonRect, "Create")) {
				string selectedAssetPath = "Assets";
				if(property.serializedObject.targetObject is MonoBehaviour) {
					MonoScript ms = MonoScript.FromMonoBehaviour((MonoBehaviour)property.serializedObject.targetObject);
					selectedAssetPath = System.IO.Path.GetDirectoryName(AssetDatabase.GetAssetPath( ms ));
				}
				
				property.objectReferenceValue = CreateAssetWithSavePrompt(type, selectedAssetPath);
			}
		}
		property.serializedObject.ApplyModifiedProperties();
		EditorGUI.EndProperty ();
	}

	public static T _GUILayout<T> (string label, T objectReferenceValue, ref bool isExpanded) where T : ScriptableObject {
		return _GUILayout<T>(new GUIContent(label), objectReferenceValue, ref isExpanded);
	}
	
	public static T _GUILayout<T> (GUIContent label, T objectReferenceValue, ref bool isExpanded) where T : ScriptableObject {
		Rect position = EditorGUILayout.BeginVertical();
		
		var propertyRect = Rect.zero;
		var guiContent = label;
		var foldoutRect = new Rect(position.x, position.y, EditorGUIUtility.labelWidth, EditorGUIUtility.singleLineHeight);
		if(objectReferenceValue != null) {
			isExpanded = EditorGUI.Foldout(foldoutRect, isExpanded, guiContent, true);

			var indentedPosition = EditorGUI.IndentedRect(position);
			var indentOffset = indentedPosition.x - position.x;
			propertyRect = new Rect(position.x + EditorGUIUtility.labelWidth - indentOffset, position.y, position.width - EditorGUIUtility.labelWidth - indentOffset, EditorGUIUtility.singleLineHeight);
		} else {
			// So yeah having a foldout look like a label is a weird hack 
			// but both code paths seem to need to be a foldout or 
			// the object field control goes weird when the codepath changes.
			// I guess because foldout is an interactable control of its own and throws off the controlID?
			foldoutRect.x += 12;
			EditorGUI.Foldout(foldoutRect, isExpanded, guiContent, true, EditorStyles.label);

			var indentedPosition = EditorGUI.IndentedRect(position);
			var indentOffset = indentedPosition.x - position.x;
			propertyRect = new Rect(position.x + EditorGUIUtility.labelWidth - indentOffset, position.y, position.width - EditorGUIUtility.labelWidth - indentOffset-60, EditorGUIUtility.singleLineHeight);
		}

		EditorGUILayout.BeginHorizontal();
		objectReferenceValue = EditorGUILayout.ObjectField(new GUIContent(" "), objectReferenceValue, typeof(T), false) as T;

		if(objectReferenceValue != null) {
			
			EditorGUILayout.EndHorizontal();
			if(isExpanded) {
				DrawScriptableObjectChildFields(objectReferenceValue);
			}
		} else {
			if(GUILayout.Button("Create", GUILayout.Width(buttonWidth))) {
				string selectedAssetPath = "Assets";
				var newAsset = CreateAssetWithSavePrompt(typeof(T), selectedAssetPath);
				if(newAsset != null) {
					objectReferenceValue = (T)newAsset;
				}
			}
			EditorGUILayout.EndHorizontal();
		}
		EditorGUILayout.EndVertical();
		return objectReferenceValue;
	}

	static void DrawScriptableObjectChildFields<T> (T objectReferenceValue) where T : ScriptableObject {
		// Draw a background that shows us clearly which fields are part of the ScriptableObject
		EditorGUI.indentLevel++;
		EditorGUILayout.BeginVertical(GUI.skin.box);

		var serializedObject = new SerializedObject(objectReferenceValue);
		// Iterate over all the values and draw them
		SerializedProperty prop = serializedObject.GetIterator();
		if (prop.NextVisible(true)) {
			do {
				// Don't bother drawing the class file
				if(prop.name == "m_Script") continue;
				EditorGUILayout.PropertyField(prop, true);
			}
			while (prop.NextVisible(false));
		}
		if (GUI.changed)
			serializedObject.ApplyModifiedProperties();
		serializedObject.Dispose();
		EditorGUILayout.EndVertical();
		EditorGUI.indentLevel--;
	}

	public static T DrawScriptableObjectField<T> (GUIContent label, T objectReferenceValue, ref bool isExpanded) where T : ScriptableObject {
		Rect position = EditorGUILayout.BeginVertical();
		
		var propertyRect = Rect.zero;
		var guiContent = label;
		var foldoutRect = new Rect(position.x, position.y, EditorGUIUtility.labelWidth, EditorGUIUtility.singleLineHeight);
		if(objectReferenceValue != null) {
			isExpanded = EditorGUI.Foldout(foldoutRect, isExpanded, guiContent, true);

			var indentedPosition = EditorGUI.IndentedRect(position);
			var indentOffset = indentedPosition.x - position.x;
			propertyRect = new Rect(position.x + EditorGUIUtility.labelWidth - indentOffset, position.y, position.width - EditorGUIUtility.labelWidth - indentOffset, EditorGUIUtility.singleLineHeight);
		} else {
			// So yeah having a foldout look like a label is a weird hack 
			// but both code paths seem to need to be a foldout or 
			// the object field control goes weird when the codepath changes.
			// I guess because foldout is an interactable control of its own and throws off the controlID?
			foldoutRect.x += 12;
			EditorGUI.Foldout(foldoutRect, isExpanded, guiContent, true, EditorStyles.label);

			var indentedPosition = EditorGUI.IndentedRect(position);
			var indentOffset = indentedPosition.x - position.x;
			propertyRect = new Rect(position.x + EditorGUIUtility.labelWidth - indentOffset, position.y, position.width - EditorGUIUtility.labelWidth - indentOffset-60, EditorGUIUtility.singleLineHeight);
		}

		EditorGUILayout.BeginHorizontal();
		objectReferenceValue = EditorGUILayout.ObjectField(new GUIContent(" "), objectReferenceValue, typeof(T), false) as T;

		if(objectReferenceValue != null) {
			EditorGUILayout.EndHorizontal();
			if(isExpanded) {
				
			}
		} else {
			if(GUILayout.Button("Create", GUILayout.Width(buttonWidth))) {
				string selectedAssetPath = "Assets";
				var newAsset = CreateAssetWithSavePrompt(typeof(T), selectedAssetPath);
				if(newAsset != null) {
					objectReferenceValue = (T)newAsset;
				}
			}
			EditorGUILayout.EndHorizontal();
		}
		EditorGUILayout.EndVertical();
		return objectReferenceValue;
	}

	// Creates a new ScriptableObject via the default Save File panel
	static ScriptableObject CreateAssetWithSavePrompt (Type type, string path) {
		path = EditorUtility.SaveFilePanelInProject("Save ScriptableObject", type.Name+".asset", "asset", "Enter a file name for the ScriptableObject.", path);
		if (path == "") return null;
		ScriptableObject asset = ScriptableObject.CreateInstance(type);
		AssetDatabase.CreateAsset (asset, path);
		AssetDatabase.SaveAssets ();
		AssetDatabase.Refresh();
		AssetDatabase.ImportAsset(path, ImportAssetOptions.ForceUpdate);
		EditorGUIUtility.PingObject(asset);
		return asset;
	}
	
	Type GetFieldType () {
		Type type = fieldInfo.FieldType;
		if(type.IsArray) type = type.GetElementType();
		else if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(List<>)) type = type.GetGenericArguments()[0];
		return type;
	}

	static bool AreAnySubPropertiesVisible(SerializedProperty property) {
		var data = (ScriptableObject)property.objectReferenceValue;
		SerializedObject serializedObject = new SerializedObject(data);
		SerializedProperty prop = serializedObject.GetIterator();
		while (prop.NextVisible(true)) {
			if (prop.name == "m_Script") continue;
			return true; //if theres any visible property other than m_script
		}
		serializedObject.Dispose();
		return false;
	}
}

If you end up with a Type Mismatch in your inspector, this script will spam invalid cast warnings at this line in AreAnySubPropertiesVisible:
var data = (ScriptableObject)property.objectReferenceValue;

Does anyone know of a simple way to guard against this?