Clean viewmodel from memory when their associated pages are removed from backstack

Question

In my WP8.1 universal application there are a few pages say - A, B, C which are associated with their respective ViewModels - VMA, VMB, VMC. When the user reaches page C, I want to remove page B from backstack as well as remove its ViewModel (VMB) from
memory .
The result should b A->C
I am able to remove the page from backstack by setting the navigation state of the frame. How to remove the viewmodel reference to prevent memory leak? The associated viewmodel is subscribed to global events like HardwareButtons.BackPressed, how to
unsubscribe to such events?
I am not using any framework for MVVM.

Answer 1

1. Why should the back button be restricted to app.xaml.cs only? It may be that some functions in the view-model depend on back button. example closing of popup or switching between semantic zooms (jumplist view to full list view). Given that, I have handled
it in App.xaml.cs
private void HardwareButtons_BackPressed(object sender, Windows.Phone.UI.Input.BackPressedEventArgs e)
if (!BackHandled && this.GoBackCommand.CanExecute(null))
e.Handled = true;
this.GoBackCommand.Execute(null);
else
BackHandled = false;
and in my viewmodel I do this:
private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
if (IsCountryListVisible)
App.BackHandled = true;
IsCountryListVisible = false;
e.Handled = true;
return;
2. I took the back button event as an example. My viewmodels may be subscribed to a pubsub/eventbus event which I want to unsubscribe. Another example is disposing or clearing undisposed members/cache.