Create a button in a custom ribbon in Revit and add the functionality to enable/disable the revit addin button

App.cs

 class App : IExternalApplication
    {
        public static PushButton Pushbutton { get; set; }
    
        public Result OnStartup(UIControlledApplication application)
        {
            OnButtonCreate(application);

            return Result.Succeeded;
        }

        public Result OnShutdown(UIControlledApplication a)
        {
            return Result.Succeeded;
        }



        /// <summary>
        /// Create a custom button on the ribbon
        /// </summary>
        /// <param name="application"></param>
        private void OnButtonCreate(UIControlledApplication application)
        {

            // Create a custom ribbon tab
            String tabName = "NewTab";

            //Create Ribbon panel if it is not created already
            try { application.CreateRibbonTab(tabName); } catch { }


            string executableLocation = Path.GetDirectoryName(
                Assembly.GetExecutingAssembly().Location);
            string dllLocation = Path.Combine(executableLocation, "ProjectName.dll");

            PushButtonData buttondata = new PushButtonData("ProjectNameBtn", "Project \nName",
                dllLocation, "ProjectName.Command");
            buttondata.ToolTip = "This addin will do something";

            BitmapImage pb1Image = new BitmapImage(new Uri("pack://application:,,,/ProjectName;component/Resources/Project.ico"));
            buttondata.LargeImage = pb1Image;

            var ribbonPanel = application.CreateRibbonPanel(tabName, "Productivity");
            
            // Store the PushButton reference. Later we can use 
            // Pushbutton.Enable = false/true
            Pushbutton = ribbonPanel.AddItem(buttondata) as PushButton;
        }
    }

EventControllerWPF.cs

   class EventControllerWPF : IExternalApplication
    {
        // class instance
        public static EventControllerWPF Instance;
        // ModelessForm instance
        private UserControl defaultControl;
        public System.Windows.Window Window = new System.Windows.Window();

        public Result OnStartup(UIControlledApplication application)
        {

            defaultControl = null;
            return Result.Succeeded;
        }

        public Result OnShutdown(UIControlledApplication application)
        {
            if (Window != null)
            {
                Window.Close();
            }

            return Result.Succeeded;

        }


        public void ShowForm(UIApplication uiapp)
        {
            if (defaultControl == null)
            {
                if(Instance == null)
                {
                    Instance = this;
                }

                // A new handler to handle request posting by the dialog
                ExternalRevitFileSelecter handler = new ExternalRevitFileSelecter();

                // External Event for the dialog to use (to post requests)
                ExternalEvent exEvent = ExternalEvent.Create(handler);


                // We give the objects to the new dialog;
                // The dialog becomes the owner responsible for disposing them, eventually.
                defaultControl = new DefaultControl(exEvent, handler);
                this.Window.Content = defaultControl;
                BitmapImage pb1Image = new BitmapImage(new Uri("pack://application:,,,/ProjectName;component/Resources/ProjectName.ico"));
                this.Window.Icon = pb1Image;
                this.Window.Title = "Project Name";
                this.Window.Show();
                
                // When application runs, make push button disable
                App.Pushbutton.Enabled = false;
                
                // Add an event to to trigger a method
                // when user close the application
                // so that we could enable the pushbutton again
                this.Window.Closed += OnProjectNameClosed;
                


            }
        }

        // When application closes, enable the Pushbutton again
        private void OnProjectNameClosed(object sender, EventArgs e)
        {
        
            App.Pushbutton.Enabled = true;
        }
    }