NavigationService throws NullReferenceException - windows-phone-7

Question

Using MVVM Light, I'm trying to develop a rather simple WP7 application. I've run into a problem using the navigation service. I can navigate to a page, but after pressing the back button I can't navigate to the same page again. NavigationService throws a NullReferenceException.
I have implemented my navigation using Messaging from the GalaSoft.MvvmLight.Messaging namespace. All my views inherits from a customized PhoneApplicationPage base class that registrers a listener on "NavigationRequest":
public class PhoneApplicationPage : Microsoft.Phone.Controls.PhoneApplicationPage
{
public PhoneApplicationPage() : base()
{
Messenger.Default.Register<Uri>(this, "NavigationRequest", (uri) => NavigationService.Navigate(uri));
}
}
From my view models I post Uri's to this listener:
SendNavigationRequestMessage(new Uri("/View/AppSettingsView.xaml", UriKind.Relative));
Like i said, this works except when navigating after pressing the Back button.
Why is this and how can I solve it?
Is there a better way to implement navigation using MVVM Light?

Answer 1

I'm using MVVM Light as well. I have a class called PageConductor, which is based on what John Papa (Silverlight MVP) from Microsoft uses. Here's the PageConductor Service I use
public class PageConductor : IPageConductor
{
protected Frame RootFrame { get; set; }
public PageConductor()
{
Messenger.Default.Register<Messages.FrameMessage>(this, OnReceiveFrameMessage);
}
public void DisplayError(string origin, Exception e, string details)
{
string description = string.Format("Error occured in {0}. {1} {2}", origin, details, e.Message);
var error = new Model.Error() { Description = description, Title = "Error Occurred" };
Messenger.Default.Send(new Messages.ErrorMessage() { Error = error });
}
public void DisplayError(string origin, Exception e)
{
DisplayError(origin, e, string.Empty);
}
private void OnReceiveFrameMessage(Messages.FrameMessage msg)
{
RootFrame = msg.RootFrame;
}
private void Go(string path, string sender)
{
RootFrame.Navigate(new Uri(path, UriKind.Relative));
}
public void GoBack()
{
RootFrame.GoBack();
}
}
In my MainPage.xaml.cs constructor, I have this, which creates an instance of my ContentFrame in my PageConductor service.:
Messenger.Default.Send(new Messages.FrameMessage() { RootFrame = ContentFrame });
I then use dependency injection to instantiate an instance of my PageConductor Service into my MainPage ViewModel. Here is my MainViewModel class:
protected Services.IPageConductor PageConductor { get; set; }
public RelayCommand<string> NavigateCommand { get; set; }
public MainViewModel(Services.IPageConductor pageConductor)
{
PageConductor = pageConductor;
RegisterCommands();
}
private void RegisterCommands()
{
NavigateCommand = new RelayCommand<string>(
(source) => OnNavigate(source));
}
private void OnNavigate(string sender)
{
PageConductor.GoToView(sender, "main");
}
Notice the instance of my PageConductorService as a parameter in my MainViewModel constructor method. I pass this in via my ViewModelLocator:
private readonly TSMVVM.Services.ServiceProviderBase _sp;
public ViewModelLocator()
{
_sp = Services.ServiceProviderBase.Instance;
CreateMain(_sp);
}
#region MainPageViewModel
public static MainViewModel MainStatic
{
get
{
Services.ServiceProviderBase SP = Services.ServiceProviderBase.Instance;
if (_main == null)
{
CreateMain(SP);
}
return _main;
}
}
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
"CA1822:MarkMembersAsStatic",
Justification = "This non-static member is needed for data binding purposes.")]
public MainViewModel Main
{
get
{
return MainStatic;
}
}
public static void ClearMain()
{
_main.Cleanup();
_main = null;
}
public static void CreateMain(Services.ServiceProviderBase SP)
{
if (_main == null)
{
_main = new MainViewModel(SP.PageConductor);
}
}
#endregion
For further reference, my Messages.FrameMessage class is simply:
internal class FrameMessage
{
public Frame RootFrame { get; set; }
}
I've had no issues with forward/back buttons.