Don't show Update Yarn Commands when source generators are available

Editor/Analysis/ActionSourceCodeGenerator.cs

@@ -1,3 +1,8 @@
+#if UNITY_2021_2_OR_NEWER
+    // Source generators are only available on Unity 2021.2 and later.
+    #define SOURCE_GENERATOR_AVAILABLE
+#endif
+
 using UnityEngine;
 using Yarn.Unity;
 using UnityEditor;
@@ -8,14 +13,31 @@ namespace Yarn.Unity.Editor
     public static class ActionSourceCodeGenerator
     {
 
+        /// <summary>
+        /// Get a path in the current project that can be used for storing
+        /// manually-generated Yarn Action registration code.
+        /// </summary>
+        /// <remarks>
+        /// This property checks to see if a file exists in the Assets folder
+        /// that is both named "YarnActionRegistration.cs", and contains a
+        /// marker indicating that it was generated by Yarn Spinner's code
+        /// generation systems. If this is found, the path to the file is
+        /// returned. Otherwise, the path
+        /// <c>Assets/YarnActionRegistration.cs</c> is returned.
+        /// </remarks>
         public static string GeneratedSourcePath
         {
             get
             {
                 const string YarnRegistrationFileName = "YarnActionRegistration.cs";
                 const string DefaultOutputFilePath = "Assets/" + YarnRegistrationFileName;
+
+                // Note the lack of a closing parenthesis in this string - we
+                // only want to check to see if it was generated by
+                // "YarnActionAnalyzer", not any specific version of that
+                // analyzer
                 const string YarnGeneratedCodeSignature = "GeneratedCode(\"YarnActionAnalyzer\"";
-                
+
                 var existingFile = System.IO.Directory.EnumerateFiles(System.Environment.CurrentDirectory, YarnRegistrationFileName, System.IO.SearchOption.AllDirectories).FirstOrDefault();
 
                 if (existingFile == null)
@@ -27,10 +49,14 @@ public static string GeneratedSourcePath
                     try
                     {
                         var text = System.IO.File.ReadAllText(existingFile);
-                        return text.Contains(YarnGeneratedCodeSignature) ? existingFile : DefaultOutputFilePath;
+                        return text.Contains(YarnGeneratedCodeSignature) 
+                            ? existingFile
+                            : DefaultOutputFilePath;
                     }
                     catch (System.Exception e)
                     {
+                        // Something happened while checking the file. Return
+                        // our default, and log that we encountered a problem.
                         Debug.LogWarning($"Can't check to see if {existingFile} is a valid action registration script, using {DefaultOutputFilePath} instead: {e}");
                         return DefaultOutputFilePath;
                     }
@@ -39,7 +65,32 @@ public static string GeneratedSourcePath
             }
         }
 
+#if SOURCE_GENERATOR_AVAILABLE
+#pragma warning disable IDE0051 // private member is unused
+
+        // On versions of Unity where source generators are not available, we
+        // provide a menu item that manually generates the appropriate code that
+        // registers actions when the domain reloads.
+        //
+        // We don't need to do this when source generators ARE available, so we
+        // don't offer this item in this case.
         [MenuItem("Window/Yarn Spinner/Update Yarn Commands")]
+        private static void OnUpdateYarnCommands()
+        {
+            GenerateYarnActionSourceCode();
+        }
+#pragma warning restore IDE0051
+#endif
+
+        /// <summary>
+        /// Generates and imports a C# source code file in the project
+        /// containing Yarn Action registration code at the path indicated by
+        /// <see cref="GeneratedSourcePath"/>.
+        /// </summary>
+        /// <remarks>
+        /// This method should not be called in projects where Unity has support
+        /// for source generators (i.e. Unity 2021.2 and later).
+        /// </remarks>
         public static void GenerateYarnActionSourceCode()
         {
             var analysis = new Yarn.Unity.ActionAnalyser.Analyser("Assets");