How to bind Toolbar MenuItem.IsEnabled with MvvmCross and Xamarin.Android - xamarin.android

Question

Using MvvmCross and Xamarin.Android is it possible to bind the IsEnabled property of a Toolbar MenuItem to a boolean value in a viewmodel? If so, how is it done?

Answer 1

I believe it's impossible at this time to bind Android IMenuItem.IsEnabled to a boolean value on a viewmodel due to the fact that IsEnabled is readonly and changed the enabled state of a menu item requires a call to SetIsEnabled(bool).
I worked around this limitation by adding an event handler for MvxViewModel.PropertyChanged in the Activity/Fragment. All my viewmodels inherit from MvxViewModel so I'll share a way this can be accomplished. Most of my app is implemented with Fragments, so my example reflects that, (in my actual code I've put most of this in a base Fragment class, but I wanted to keep it simple):
public class MyViewModel : MvxViewModel
{
public bool MenuItemIsEnabled {
get{return _menuItemIsEnabled;}
set{SetProperty(ref _menuItemIsEnabled, value);
}
}
[MvxFragment(typeof(MainViewModel), Resource.Id.content_frame, true)]
[Register(nameof(MyFragment))]
public class MyFragment : MvxFragment
{
private Toolbar Toolbar;
//menuitem whose enabled state should change with viewmodel property
private IMenuItem MyMenuItem;
public new MyViewModel ViewModel
{
get { return (MyViewModel)base.ViewModel; }
set { base.ViewModel = value; }
}
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
var view = base.OnCreateView(inflater, container, savedInstanceState);
//we'll need the Toolbar later when we set up the IsEnabled "binding"
Toolbar = view.FindViewById<Toolbar>(Resource.Id.toolbar);
return view;
}
public override void OnViewModelSet()
{
base.OnViewModelSet();
ViewModel.PropertyChanged += ViewModel_PropertyChanged;
}
public override void OnCreateOptionsMenu(IMenu menu, MenuInflater inflater)
{
base.OnCreateOptionsMenu(menu, inflater);
//create the menu based on a menu resource
inflater.Inflate(Resource.Menu.my_menu, menu);
//save a reference to the menu item so we can update it when the viewmodel changes
MyMenuItem = menu.FindItem(Resource.Id.my_menu_item);
//set the initial enabled state based on the viewmodel
MyMenuItem.SetEnabled(ViewModel.MenuItemIsEnabled);
}
private void ViewModel_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
//when ViewModel.MenuItemIsEnabled is updated, update the menuitem enabled state as well
if (e.PropertyName == nameof(ViewModel.MenuItemIsEnabled ))
BinContentsMenuItem?.SetEnabled(ViewModel.MenuItemIsEnabled);
}
}
This should be considered pseudocode, since it's trying to demonstrate the general concepts I used to work around my original problem.