MVVMCross: How to bind Xamarin.Android events to ViewModel commands

Question

I am trying to go from an activity to another. I am still learning about MVVMCross so this whole pattern is still very new to me. I am applying it with Xamarin.Android only at the moment.

The setup:

1. MainDashboardActivity has an Android Design Support library's NavigationView.

2. The ViewModel MainDashboardViewModel has an IMvxCommand GoToSecondDashboard which is just a simple ShowViewModel to another activity.

The NavigationView has a NavigationItemSelected event. Normally, I would just do this:

navigationView.NavigationItemSelected += (o, e) =>
{
    if(e.MenuItem.ItemId == Resource.Id.SecondDashboardMenu)
    {
        // make new intent to target activity
    }
};

Now I have tucked the navigation logic into the ViewModel's IMvxCommand, and I want to bind it to the NavigationView's event, no longer creating intents and whatnot. How would I achieve this?

I want to use the fluent binding logic in the code file and not in the layout, like how this answer does:

protected override void OnViewModelSet()
{
    SetContentView(Resource.Layout.View_Tip);

    var edit = this.FindViewById<EditText>(Resource.Id.FluentEdit);

    var set = this.CreateBindingSet<TipView, TipViewModel>();
    set.Bind(edit).To(vm => vm.SubTotal);
    set.Apply();

    // for non-default properties use 'For':
    // set.Bind(edit).For(ed => ed.Text).To(vm => vm.SubTotal);

    // you can also use:
    //   .WithConversion("converter", "optional parameter")
    //   .OneTime(), .OneWay() or .TwoWay()
}

But NavigationItemSelected is an event. I have not been able to find a way to bind events to commands. There is also the logic of filtering ItemId before that can happen, so it's not going to even be a straightforward event-to-command binding.

I am not sure if this is the correct approach to this. All I want is to bind menu taps to commands in the code file instead of the layout file.

Answer 1

Since there are no Binding Targets defined for NavigationView, you won't be able to bind as Cyriac describes in his post.

What a target binding does internally is simply subscribe to an event and react to it and exposing that data as a property.

So since there is no way to take an ItemsSource and bind to a NavigationView currently, you have to do something like you are doing already, hooking an EventHandler up to the event, and call directly into your ViewModel, i.e. invoking a Command. This looks something like this:

navigationView.NavigationItemSelected += ItemSelected;

private void ItemSelected(object sender, NavigationItemSelectedEventArgs args)
{
    ViewModel.NavigateCommand.Execute(args.MenuItem.TitleFormatted.ToString());
}

Then in your ViewModel in your Command:

private void DoNavigateCommand(string title)
{
    if (title == "Derp")
        ShowViewModel<DerpViewModel>();
}

Alternatively you could wrap this code in a Target Binding. You can see how these are implemented in the official MvvmCross github repository.

Answer 2

I found an answer by someone else on http://crosscuttingconcerns.com/MvvmCross-Fluent-Databinding , which you should try out. I think you just cant reference directly the Event, rather have to use the string.

protected override void OnViewModelSet ()
{
        SetContentView (Resource.Layout.TermsPage);

        var set = this.CreateBindingSet<TermsView, TermsViewModel>();
        set.Bind(FindViewById<Button>(Resource.Id.acceptTermsButton))
            .For("Click")
            .To(vm => vm.AcceptTermsCommand);
        set.Apply();
}

Well of course you have do adjust it depending on your event.