How do I get the NavigationService without being a Page?

This seems to be a really naive question, but how on earth does one get the NavigationService from outside of a page, like say perhaps a view model? Everybody says that navigation should occur at the view, but I keep thinking, t开发者_开发问答his is not a web page, its an application. The view model and business logic should control application flow, not the view. Is this in fact naive?

As far as I understand it, operations involving the View, i.e. UI, should be done by the View exclusively. When working with MVVM, the UI should not be controlled by the ViewModel or BusinessLogic directly (since they are not supposed to know anything about the concrete implementation of the View) but work with Messages.

That means, if we want to open an Editor window from the ViewModel we send a Message from the ViewModel that we want to open it and receive it in the View and open the window there. The same is valid for Navigating through different pages, where you would receive the Message in the MainPage (or whatever holds you pages that you want to navigate through) and handle everything there.

An alternative to that would be using a DialogService or something like that, which handles opening windows in a central place. However, since the NavigationService is a property of the Page class, we need to handle the message in the Page.

Example code, using the MVVM Light Toolkit: (not tested, partly taken from Shawn Wildermuth's RiaXBoxGames example):

ViewModel (e.g., put that in a Command for a Button):

Messenger.Default.Send<bool>(true, "GoToNextPage");

View (e.g. put that in a Constructor):

Messenger.Default.Register<bool>(this, "GoToNextPage", ignore =>
{
  // your code to go to next page
});

another option is to create an event on ViewModel, fire this event when Command occurs and subscribe View to this event. Inside EventArgs you can carry which page to navigate to etc. I think simple and testable solution.

Robert

I just pass a reference to the Frame when I create the View-Model.