MVVMCross binding to badgevalue - xamarin

Question

I am trying to bind to the badgevalue of a TabBarItem like this:
var set = this.CreateBindingSet<MyView, MyViewModel>();
set.Bind(ViewControllers[0].TabBarItem.BadgeValue).To(vm => vm.MyNumber);
set.Apply();
but I get the following error:
MvxBind: Error: 6.30 Empty binding target passed to MvxTargetBindingFactoryRegistry
However if I set the value directly like this the badge appears:
ViewControllers[0].TabBarItem.BadgeValue = ((MyViewModel)ViewModel).MyNumber;
Why is the binding not working?
Thanks!

Answer 1

This doesn't work because you need to set up your own binding extensions if what you are trying to bind to doesn't exist as a valid binding target already.
Look here where Stuart answered a similar question for Android.

Answer 2

Create a custom binding:
public class TabBadgeBinding : MvxTargetBinding
{
public const string TargetPropertyName = nameof(TabBadgeBinding);
private readonly UIViewController _viewController;
public TabBadgeBinding(UIViewController viewController) : base(viewController)
{
_viewController = viewController;
}
public override Type TargetType => typeof(UIViewController);
public override MvxBindingMode DefaultMode => MvxBindingMode.OneWay;
public override void SetValue(object value)
{
if (value is string badgeValue)
{
_viewController.TabBarItem.BadgeValue = badgeValue;
}
}
}
Register Binding in Setup.cs:
protected override void FillTargetFactories(IMvxTargetBindingFactoryRegistry registry)
{
base.FillTargetFactories(registry);
registry.RegisterFactory(new MvxCustomBindingFactory<UIViewController>(TabBadgeBinding.TargetPropertyName, (controller) => new TabBadgeBinding(controller)));
}
Use Binding in your VC:
set.Bind(ParentViewController).For(TabBadgeBinding.TargetPropertyName).To(vm => vm.UnreadNotificationsCount);