AppDelegate.cs

using AppKit;
using Foundation;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;
using System.IO;
using ScriptingBridge;

namespace NowPlayingTest
{
    [Register("AppDelegate")]
    public class AppDelegate : NSApplicationDelegate
    {
        public AppDelegate()
        {
        }

        public override void DidFinishLaunching(NSNotification notification)
        {
            // Insert code here to initialize your application
            var center = NSDistributedNotificationCenter.DefaultCenter as NSNotificationCenter;
            center.AddObserver(new NSString("com.swinsian.Swinsian-Track-Playing"), async obj =>
            {
                var dict = obj.UserInfo;
                await Task.Run(() =>
                {
                    Song song = new Song();
                    song.Album = dict["album"] as NSString;
                    song.Artist = dict["artist"] as NSString;
                    song.Title = dict["title"] as NSString;
                    var apath = dict["thumbnailPath"] as NSString;
                    song.AlbumArt = new NSImage(Path.GetDirectoryName(apath) + "/cover_512.png");
                    Song.LastPlayedSong = song;
                    dict.ToList().ForEach((item) =>
                    {
                        Debug.WriteLine($"{item.Key} : {item.Value}");
                    });
                });
            });
            center.AddObserver(new NSString("com.apple.iTunes.playerInfo"), async obj =>
            {
                var dict = obj.UserInfo;
                await Task.Run(() =>
                {
                    Song song = new Song();
                    song.Album = dict["Album"] as NSString;
                    song.Artist = dict["Artist"] as NSString;
                    song.Title = dict["Name"] as NSString;
                    dict.ToList().ForEach((item) =>
                    {
                        Debug.WriteLine($"{item.Key} : {item.Value}");
                    });
                    //get album art
                    var itunes = SBApplication.FromBundleIdentifier("com.apple.iTunes");
                    var ctrack = itunes.ValueForKey(new NSString("currentTrack"));
                    var front_artwork = ctrack.ValueForKey(new NSString("artworks")) as SBElementArray;
                    if (front_artwork.Count > 0)
                    {
                        var aitem = front_artwork.Get()[0];
                        var raw_image = aitem.ValueForKey(new NSString("rawData")) as NSData;
                        song.AlbumArt = new NSImage(raw_image);
                    }
                    Song.LastPlayedSong = song;
                    Debug.WriteLine(ctrack.ToString());
                });
            });
        }

        public override void WillTerminate(NSNotification notification)
        {
            // Insert code here to tear down your application
        }
    }
}

Song.cs

using System;
using AppKit;
namespace NowPlayingTest
{
    public class Song
    {
        public static Song LastPlayedSong;
        public String Artist { get; set; }
        public String Title { get; set; }
        public String Album { get; set; }
        public NSImage AlbumArt { get; set; }
    }
}

ViewController.cs

using System;

using AppKit;
using Foundation;
using ObjCRuntime;
using ScriptingBridge;
using System.Collections.Generic;
using System.Linq;
using System.Diagnostics;
using System.Threading.Tasks;
using System.Threading;

namespace NowPlayingTest
{
    public partial class ViewController : NSViewController
    {
        public ViewController(IntPtr handle) : base(handle)
        {
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();
            // Do any additional setup after loading the view
            //change text
            deheheButton.Title = "でへへ〜";
            deheheButton.Activated += (sender, e) =>
            {
                var alert = new NSAlert();
                alert.MessageText = "でへへ〜";
                if (Song.LastPlayedSong != null)
                {
                    var song = Song.LastPlayedSong;
                    alert.Icon = song.AlbumArt;
                    alert.InformativeText = $"{song.Title}\n{song.Artist}\n{song.Album}";
                }
                else
                {
                    alert.InformativeText = "取得できなかった〜でへへ〜";
                }
                alert.RunSheetModal(this.View.Window);
            };
        }

        public override NSObject RepresentedObject
        {
            get
            {
                return base.RepresentedObject;
            }
            set
            {
                base.RepresentedObject = value;
                // Update the view, if already loaded.
            }
        }
    }
}