问ObserveOnDispatcher不调用UI线程中的处理程序

EN

private void button_Click(object sender, RoutedEventArgs e)
{
  var messageBus = new MessageBus();
  messageBus.GetMessages<Message>().ObserveOnDispatcher().Subscribe(x => TestHanlder(x));

  Trace.WriteLine("Main Thread Id:" + Thread.CurrentThread.ManagedThreadId);
  var deviceManager = new DeviceManager(messageBus);
  deviceManager.Start();
}

private void TestHanlder(Message message)
{
  Trace.WriteLine("UI Handler ThreadId:" + Thread.CurrentThread.ManagedThreadId);
}

public class DeviceManager
{
  private readonly MessageBus _messageBus;

  public DeviceManager(MessageBus messageBus)
  {
    _messageBus = messageBus;
  }

  public void Start()
  {
    for (;;)
    {
      var t = Task.Factory.StartNew(() => BackGroundTask(), TaskCreationOptions.LongRunning);
      t.Wait();
    }
  }

  private void BackGroundTask()
  {
    Thread.Sleep(1000);
    Trace.WriteLine("Push ThreadId:" + Thread.CurrentThread.ManagedThreadId);
    var message = new Message();
    _messageBus.Publish(message);
  }
}

public class MessageBus
{
  readonly ISubject<object> _messages;

  public MessageBus()
  {
    _messages = new Subject<object>();
  }
  public void Publish<TMessage>(TMessage message)
  {
    _messages.OnNext(message);
  }

  public IObservable<TMessage> GetMessages<TMessage>()
  {
    return _messages.OfType<TMessage>();
  }
}

public class Message
{
  public Message()
  {

  }
}

Without ObserveOnDispatcher:
messageBus.GetMessages<Message>().Subscribe(x => TestHanlder(x));
..........................Output.................................
Main Thread Id:8
Push ThreadId:9
UI Handler ThreadId:9
But I need to execute the TestHanlder function in the main thread or UI thread, in my use case above it must be the thread number 8.

When I use ObserveOnDispatcher:
messageBus.GetMessages<Message>().ObserveOnDispatcher().Subscribe(x => TestHanlder(x));
..........................Output.................................

Main Thread Id:9
Push ThreadId:10
 ------------------> UI Handler ThreadId: are missing not there!?
What I'm doing wrong here?!!!!!