WPF Form Designer Prototype (MVVM)

using System;
using System.IO;
using System.Windows.Input;
using Microsoft.Win32;
using SimpleMvvmToolkit;

namespace WPFFormDesigner.ViewModels
{
    public class MainWindowViewModel : ViewModelBase<MainWindowViewModel>
    {
        private object _SelectedObject;

        #region Constructor

        public MainWindowViewModel()
        {
            SubscribeToMessageBus();
            InitCommands();
            InitViewModels();
        }

        #endregion

        #region Public Properties

        #region Commands

        public ICommand SaveFormCommand { get; set; }
        public ICommand NewFormCommand { get; set; }
        public ICommand OpenFormFromFileCommand { get; set; }

        #endregion

        private static MainWindowViewModel _Instance;
        private FormDesignerSurfaceViewModel _FormDesignerSurfaceVM;

        private PropertiesViewModel _PropertiesVM;

        public FormDesignerSurfaceViewModel FormDesignerSurfaceVM
        {
            get { return _FormDesignerSurfaceVM; }
            set
            {
                _FormDesignerSurfaceVM = value;
                NotifyPropertyChanged(m => m.FormDesignerSurfaceVM);
            }
        }

        public PropertiesViewModel PropertiesVM
        {
            get { return _PropertiesVM; }
            set
            {
                _PropertiesVM = value;
                NotifyPropertyChanged(m => m.PropertiesVM);
            }
        }

        /// <summary>
        /// Property to hold the currently selected object to be displayed in properties
        /// </summary>
        public object SelectedObject
        {
            get { return _SelectedObject; }
            set
            {
                _SelectedObject = value;
                NotifyPropertyChanged(m => SelectedObject);
                //Notify to subscribers that Selected Object has changed
                Notify(SelectedObjectChangedNotice, new NotificationEventArgs<bool>("SelectedObject changed", true));
            }
        }

        public static MainWindowViewModel Instance
        {
            get
            {
                if (_Instance == null)
                {
                    _Instance = new MainWindowViewModel();
                }
                return _Instance;
            }
            set { _Instance = value; }
        }

        #endregion

        #region Methods

        /// <summary>
        /// Initialize the ViewModels
        /// </summary>
        private void InitViewModels()
        {
            PropertiesVM = new PropertiesViewModel();
            FormDesignerSurfaceVM = new FormDesignerSurfaceViewModel();
        }

        /// <summary>
        /// Subscribes different type of messages to Message bus so ViewModels can send and 
        /// receive information upon request.
        /// </summary>
        private void SubscribeToMessageBus()
        {
            RegisterToReceiveMessages<object>(MessageTokens.PropertiesObjectSelected,
                                              OnPropertiesObjectSelectedRequested);
        }

        private void OnPropertiesObjectSelectedRequested(object sender, NotificationEventArgs<object> e)
        {
            if (e.Data != null)
                SetSelectedObject(e.Data);
        }

        private void SetSelectedObject(object obj)
        {
            SelectedObject = obj;
        }

        private void InitCommands()
        {
            SaveFormCommand = new RelayCommand(OnSaveForm, CanSaveForm);
            OpenFormFromFileCommand = new RelayCommand(OpenFormFromFile);
            NewFormCommand = new RelayCommand(OnNewFormCommand);
        }

        private void OnNewFormCommand()
        {
            var fdsvm = new FormDesignerSurfaceViewModel();
            fdsvm.IsNew = true;
            FormDesignerSurfaceVM = fdsvm;
        }

        private bool CanSaveForm()
        {
            return true;
        }

        private void OnSaveForm()
        {
            if (FormDesignerSurfaceVM != null)
            {
                FormDesignerSurfaceVM.SaveAsCommand.Execute(null);
            }
        }

        public void OpenFormFromFile()
        {
            var dlg = new OpenFileDialog();
            dlg.Filter = "XML File|*.xml";
            dlg.Title = "Open XML File";
            if (dlg.ShowDialog().GetValueOrDefault())
            {
                string xml = File.ReadAllText(dlg.FileName);
                if (!string.IsNullOrEmpty(xml))
                {
                    var fdsvm = new FormDesignerSurfaceViewModel();
                    fdsvm.XamlForm = xml;
                    FormDesignerSurfaceVM = fdsvm;
                }
            }
        }

        #endregion

        #region Events

        public event EventHandler<NotificationEventArgs<bool>> SelectedObjectChangedNotice;

        #endregion
    }
}