Binding a ReactiveCommand prevents a ViewModel from being garbage collected

Question

When I bind a "back button" to a the router in ReactiveUI, my ViewModel is no longer garbage collected (my view too). Is this a bug, or is this me doing something dumb?

Here is my MeetingPageViewModel:

public class MeetingPageViewModel : ReactiveObject, IRoutableViewModel
{
    public MeetingPageViewModel(IScreen hs, IMeetingRef mRef)
    {
        HostScreen = hs;
    }

    public IScreen HostScreen { get; private set; }

    public string UrlPathSegment
    {
        get { return "/meeting"; }
    }
}

Here is my MeetingPage.xaml.cs file:

public sealed partial class MeetingPage : Page, IViewFor<MeetingPageViewModel>
{
    public MeetingPage()
    {
        this.InitializeComponent();

        // ** Comment this out and both the View and VM will get garbage collected.
        this.BindCommand(ViewModel, x => x.HostScreen.Router.NavigateBack, y => y.backButton);

        // Test that goes back right away to make sure the Execute
        // wasn't what was causing the problem.
        this.Loaded += (s, a) => ViewModel.HostScreen.Router.NavigateBack.Execute(null);
    }

    public MeetingPageViewModel ViewModel
    {
        get { return (MeetingPageViewModel)GetValue(ViewModelProperty); }
        set { SetValue(ViewModelProperty, value); }
    }
    public static readonly DependencyProperty ViewModelProperty =
        DependencyProperty.Register("ViewModel", typeof(MeetingPageViewModel), typeof(MeetingPage), new PropertyMetadata(null));

    object IViewFor.ViewModel
    {
        get { return ViewModel; }
        set { ViewModel = (MeetingPageViewModel)value; }
    }
}

I then run, and to see what is up, I use VS 2013 Pro, and turn on the memory analyzer. I also (as a test) put in forced GC collection of all generations and a wait for finalizers. When that line is uncommented above, when all is done, there are three instances of MeetingPage and MeetingPageViewModel. If I remove the BindCommand line, there are no instances.

I was under the impression that these would go away on their own. Is the problem the HostScreen object or the Router that refers to an object that lives longer than this VM? And that pins things down?

If so, what is the recommended away of hooking up the back button? Using Splat and DI? Many thanks!

score 0 · Answer 1 · answered Apr 06 '15 at 22:18

Following up on the idea I had at the end, I can solve this in the following way. In my App.xaml.cs, I make sure to declare the RoutingState to the dependency injector:

        var r = new RoutingState();
        Locator.CurrentMutable.RegisterConstant(r, typeof(RoutingState));

then, in the ctor of each view (the .xaml.cs code) with a back button for my Windows Store app, I no longer use the code above, but replace it with:

        var router = Locator.Current.GetService<RoutingState>();
        backButton.Click += (s, args) => router.NavigateBack.Execute(null);

After doing that I can visit the page as many times as I want and never do I see the instances remaining in the analyzer.

I'll wait to mark this as an answer to give real experts some time to suggest another (better?) approach.

Binding a ReactiveCommand prevents a ViewModel from being garbage collected

1 Answers1