module-inspector/Editor/Drawers/FilePathAttributeDrawer.cs

using Module.Inspector.Editor.Utilities;
using UnityEditor;
using UnityEngine;

namespace Module.Inspector.Editor
{
    [CustomPropertyDrawer(typeof(FilePathAttribute))]
    internal sealed class FilePathAttributeDrawer : DrawerPropertyDrawer
    {
        public override bool Draw(Rect position, DrawerPropertyAttribute attribute, SerializedProperty property, GUIContent label, EditorPropertyUtility.Result result)
        {
            if (property.propertyType != SerializedPropertyType.String)
                return false;
            
            var att = (FilePathAttribute)attribute;
            
            EditorGUI.BeginProperty(position, label, property);
            {
                const float WIDTH = 80;
                    
                var rect0 = new Rect(position.x, position.y, position.width - WIDTH, position.height);
                var rect1 = new Rect(rect0.xMax, position.y, WIDTH, position.height);
                string temp = EditorGUI.TextField(rect0, label, property.stringValue);

                if (!temp.Equals(property.stringValue))
                    property.stringValue = temp;

                if (GUI.Button(rect1, "Find"))
                {
                    string folderPath = GetInitialFolderPath(att.pathType);
                    string path = EditorUtility.OpenFilePanel("File", folderPath, att.extension);

                    if (!string.IsNullOrEmpty(path))
                        path = TransformAbsolutePathTo(path, att.pathType);

                    if (!string.IsNullOrEmpty(path))
                    {
                        property.stringValue = path;
                        property.serializedObject.ApplyModifiedProperties();
                    }
                }
            }
            EditorGUI.EndProperty();
            return true;
        }
        
        private static string GetInitialFolderPath(EFilePathType pathType)
        {
            if (pathType == EFilePathType.RelativeToStreamingFolder)
                return "Assets/StreamingAssets/";

            return "Assets/";
        }

        private string TransformAbsolutePathTo(string absolutePath, EFilePathType pathType)
        {
            absolutePath = absolutePath.Replace('\\', '/');
            var dataPath = Application.dataPath.Replace('\\', '/');
            
            if (pathType == EFilePathType.RelativeToProjectFolder)
            {
                if (absolutePath.StartsWith(dataPath))
                    return absolutePath.Remove(0, dataPath.Length - 6);
                
                Debug.LogWarning("Path didn't include 'Assets'-folder");
                return string.Empty;
            }

            if (pathType == EFilePathType.RelativeToStreamingFolder)
            {
                if (absolutePath.StartsWith($"{dataPath}/StreamingAssets"))
                    return absolutePath.Remove(0, dataPath.Length + 17);
                
                Debug.LogWarning("Path didn't include 'StreamingAssets'-folder");
                return string.Empty;
            }

            return absolutePath;
        }
        
        public override string GetErrorMessage(SerializedProperty property)
        {
            return "Only supports strings";
        }
    }
}