title: Update WPF with externalevent and MVVM schema
body: <p>I'm currently developping a new soft which analyze a lot of data on Revit.
  I have my WPF View, my ViewModel and a long methode.</p>
  
  <p>init.cs:</p>
  
  <pre><code>IExternalEventHandler handlerEvent = new FamilyAnalysis();
  var familyAnalysisEvent = ExternalEvent.Create(handlerEvent);
  Gui = new AnalysisView(familyAnalysisEvent);
  Gui.Show();
  </code></pre>
  
  <p>AnalysisView.xaml.cs:</p>
  
  <pre><code>public AnalysisView(ExternalEvent externalEvent)
  {
      InitializeComponent();
      Windows.SetLocation(this);
      Language = System.Windows.Markup.XmlLanguage.GetLanguage(System.Threading.Thread.CurrentThread.CurrentCulture.Name);
      DataContext = new AnalysisViewModel(externalEvent, this);
  }
  </code></pre>
  
  <p>AnalysisViewModel.cs:</p>
  
  <pre><code>public ICommand StartAnalysisCommand
  {
      get
      {
          return _startAnalysisCommand ?? (_startAnalysisCommand = new RelayCommand(() =&gt;
          {
              //Some check before then..
              _externalEvent.Raise();
          }));
      }
  }
  </code></pre>
  
  <p>during the _externalEvent.Raise() I would like to have a loading form updating the state of the methode.</p>
  
  <p>I tried to search about dispatcher but I'm not sure how to do that and all my try didn't worked. The loading screen pop but the value are not getting updated.</p>
  
  <p>Any advice ?</p>