UnityGame/Library/PackageCache/com.unity.timeline/Samples~/Customization/Annotation/Editor/CreateAnnotationAction.cs

using System.Collections.Generic;
using System.Linq;
using UnityEditor;
using UnityEditor.Timeline;
using UnityEditor.Timeline.Actions;
using UnityEngine.Timeline;

namespace Timeline.Samples
{
    // Adds an additional item in context menus that will create a new annotation
    // and sets its description field with the clipboard's contents.
    [MenuEntry("Create Annotation from clipboard contents")]
    public class CreateAnnotationAction : TimelineAction
    {
        // Specifies the action's prerequisites:
        // - Invalid (grayed out in the menu) if no text content is in the clipboard;
        // - NotApplicable (not shown in the menu) if no track is selected;
        // - Valid (shown in the menu) otherwise.
        public override ActionValidity Validate(ActionContext context)
        {
            // get the current text content of the clipboard
            string clipboardTextContent = EditorGUIUtility.systemCopyBuffer;
            if (clipboardTextContent.Length == 0)
            {
                return ActionValidity.Invalid;
            }

            // Timeline's current selected items can be fetched with `context`
            IEnumerable<TrackAsset> selectedTracks = context.tracks;
            if (!selectedTracks.Any() || selectedTracks.All(track => track is GroupTrack))
            {
                return ActionValidity.NotApplicable;
            }

            return ActionValidity.Valid;
        }

        // Creates a new annotation and add it to the selected track.
        public override bool Execute(ActionContext context)
        {
            // to find at which time to create a new marker, we need to consider how this action was invoked.
            // If the action was invoked by a context menu item, then we can use the context's invocation time.
            // If the action was invoked through a keyboard shortcut, we can use Timeline's playhead time instead.
            double time;
            if (context.invocationTime.HasValue)
            {
                time = context.invocationTime.Value;
            }
            else
            {
                time = TimelineEditor.inspectedDirector.time;
            }

            string clipboardTextContent = EditorGUIUtility.systemCopyBuffer;

            IEnumerable<TrackAsset> selectedTracks = context.tracks;
            foreach (TrackAsset track in selectedTracks)
            {
                if (track is GroupTrack)
                    continue;

                AnnotationMarker annotation = track.CreateMarker<AnnotationMarker>(time);
                annotation.description = clipboardTextContent;
                annotation.title = "Annotation";
            }

            return true;
        }
    }
}