Navigating to a new page from the View Model in Wi

Question

I am working on a windows phone 8.1 universal app and want to find the best way of handling page navigations without having large amounts of logic in the code behind. I want to keep the code behind in my View as uncluttered as possible. What is the accepted MVVM way of navigating to a new page in response to a button click?

I currently have to send a RelayComnmand message from the ViewModel to the view with the details of the page to navigate to. This means that the code behind has to be wired up as follows:

    public MainPage()
    {
      InitializeComponent();
      Messenger.Default.Register<OpenArticleMessage>(this, (article) => ReceiveOpenArticleMessage(article));
...
}

    private object ReceiveOpenArticleMessage(OpenArticleMessage article)
    {
     Frame.Navigate(typeof(ArticleView));
   }

This just doesn't seem the best way although it does work. How can I do the page navigations directly from the ViewModel? I am using MVVM-Light in my project.

Answer 1

There is a new and simpler implementation here: https://marcominerva.wordpress.com/2014/10/10/navigationservice-in-mvvm-light-v5/
First we create the NavigationService and DialogService (for the page navigation params):

public ViewModelLocator() {
    ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);

    var navigationService = this.CreateNavigationService();
    SimpleIoc.Default.Register<INavigationService>(() => navigationService);

    SimpleIoc.Default.Register<IDialogService, DialogService>();

    SimpleIoc.Default.Register<MainViewModel>();
    SimpleIoc.Default.Register<DetailsViewModel>();
}

private INavigationService CreateNavigationService() {
    var navigationService = new NavigationService();
    navigationService.Configure("Details", typeof(DetailsPage));
    // navigationService.Configure("key1", typeof(OtherPage1));
    // navigationService.Configure("key2", typeof(OtherPage2));

    return navigationService;
}

Then we create a RelayCommand and NavigationService in your ViewModel, like so:

public class MainViewModel : ViewModelBase {
    private INavigationService _navigationService;
    public RelayCommand<Tuple<string, string>> DetailsCommand { get; set; }

    public MainViewModel(INavigationService navigationService) {
        this._navigationService = navigationService;
        DetailsCommand = new RelayCommand<Tuple<string, string>>((args) => NavigateTo(args));
    }

    public void NavigateTo(Tuple<string, string> args) {
        this._navigationService.NavigateTo(args.Item1, args.Item1);
    }

    public void ClickAndNavigate() {
        NavigateTo(new Tuple<string, string>("AdminPivotPage", "Test Params"));
    }
}

And finally, we can get the page navigation params like so:

public sealed partial class DetailsPage : Page {
    // ... 
    protected override void OnNavigatedTo(NavigationEventArgs e) {
        var parameter = e.Parameter as string;  // "My data"
        base.OnNavigatedTo(e);
    }
}

But to read the arguments passed in page navigation in MVVM pattern, you can take a look here.

Answer 2

Ok, I have found an answer to this question. Took a bit of investigation but I eventually found the preferred MVVM-Light way of doing this. I don't take credit for this answer in anyway but just posting it here in case people are looking for an answer to this question.

Create an INavigationService interface as follows:

public interface INavigationService
{
    void Navigate(Type sourcePageType);
    void Navigate(Type sourcePageType, object parameter);
    void GoBack();
}

Create a NavigationService class as follows:

public class NavigationService : INavigationService
{
    public void Navigate(Type sourcePageType)
    {
        ((Frame)Window.Current.Content).Navigate(sourcePageType);
    }

    public void Navigate(Type sourcePageType, object parameter)
    {
        ((Frame)Window.Current.Content).Navigate(sourcePageType, parameter);
    }

    public void GoBack()
    {
        ((Frame)Window.Current.Content).GoBack();
    }
}

Now in the ViewModelLocator, set it up like this:

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance",
        "CA1822:MarkMembersAsStatic",
        Justification = "This non-static member is needed for data binding purposes.")]
    public MainViewModel Main
    {
        get
        {
            return ServiceLocator.Current.GetInstance<MainViewModel>();
        }
    }

    static ViewModelLocator()
    {
        ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);

        if (ViewModelBase.IsInDesignModeStatic)
        {
            SimpleIoc.Default.Register<INavigationService, Design.DesignNavigationService>();
        }
        else
        {
            SimpleIoc.Default.Register<INavigationService>(() => new NavigationService());
        }

        SimpleIoc.Default.Register<MainViewModel>();
    }

Next setup a navigation service for design time as follows:

public class DesignNavigationService : INavigationService
{
    // This class doesn't perform navigation, in order
    // to avoid issues in the designer at design time.

    public void Navigate(Type sourcePageType)
    {
    }

    public void Navigate(Type sourcePageType, object parameter)
    {
    }

    public void GoBack()
    {
    }
}

My MainViewModel constructor is as follows:

   public MainViewModel(INavigationService navigationService)
    {
        _navigationService = navigationService;

        ...

Now you can simply use this to navigate in your viewmodel:

_navigationService.Navigate(typeof(WelcomeView));

For more details on the original author Laurent Bugnion see this article and associated code. http://msdn.microsoft.com/en-us/magazine/jj651572.aspx

Answer 3

I agree with ricochete above, its simpler, though my direct implimentation messed up with my Design Data Binding in Blend.

I decided to create an a class that inherited from the NavigationService

public class NavigationServiceHelper : NavigationService
{
    public NavigationServiceHelper()
    {
        this.Configure("Page1", typeof(View.Page1));
        this.Configure("Page2", typeof(View.Page2));
    }
}

Then in the ViewModelLocator I registerred it this way

 SimpleIoc.Default.Register<INavigationService, NavigationServiceHelper>();

My Design View Data Bindings worked again. If someone could explain why the design data wun't work in ricochete above, please do. Thank you!