A dynamic Rehosted Workflow Designer for WF 4

using System;
using System.Activities.Core.Presentation;
using System.Activities.Presentation;
using System.ComponentModel;
using System.IO;
using System.Windows.Input;
using System.Windows.Threading;
using RehostedDesigner.Designer.ViewModels.Contracts;
using RehostedDesigner.LoadErrorHandling;

namespace RehostedDesigner.Designer.ViewModels
{
    public class StandardDesignerViewModel : INotifyPropertyChanged, IDesignerViewModel, ILoadErrorDesignerViewModel
    {
        private readonly Dispatcher _dispatcher = Dispatcher.CurrentDispatcher;

        public StandardDesignerViewModel()
        {
            this.CurrentSurface = new EmptyDesignerSurface();
        }

        public object CurrentSurface
        {
            get;
            private set;
        }

        public event Action SurfaceChanged;
        public event PropertyChangedEventHandler PropertyChanged;

        public void ReloadError(string xaml)
        {
            this.CurrentSurface = new LoadErrorDesignerSurface(xaml);
            this._dispatcher.Invoke(new Action(this.OnSurfaceChanged));
        }

        public void ReloadDesigner(object root)
        {
            WorkflowDesigner designer = null;

            this._dispatcher.Invoke(new Action(() => designer = ReloadDesignerSTA(root)));
            this.CurrentSurface = new StandardDesignerSurface(designer);
            this._dispatcher.Invoke(new Action(this.OnSurfaceChanged));
        }

        private WorkflowDesigner ReloadDesignerSTA(object root)
        {
            var result = new WorkflowDesigner();
            new DesignerMetadata().Register();
            result.Load(root);

            return result;
        }

        private void OnPropertyChanged(string name)
        {
            if (this.PropertyChanged != null)
            {
                this.PropertyChanged(this, new PropertyChangedEventArgs(name));
            }
        }

        private void OnSurfaceChanged()
        {
            this.OnPropertyChanged("CurrentSurface");

            if (this.SurfaceChanged != null)
            {
                this.SurfaceChanged();
            }
        }
    }
}