Follow up to #1378: NullRef exception in Target Binding when using a value type + fix

[softlion]

🐛 Bug Report

When creating a target binding to a value type using the generic class, SetValue will fail in the conversion from null to the value type, as the value type can not be null.

Expected behavior

No exception.

Reproduction steps

I tracked the problem to be in class MvxTargetBinding<TTarget, TValue>.

The problem is the conversion from null to a value type (here a "bool") which can not be null.

I don't know why the binding would send null in first place.
The caller is C:\projects\mvvmcross\MvvmCross\Binding\Bindings\MvxFullBinding.cs:181,25

This can be reproduced easily by creating a binding for "Selected" on Android, then binding a bool value to the Selected property of any view:

    public class SelectedTargetBinding : MvxAndroidTargetBinding<View, bool>
    {
        public const string Name = "Selected";

        public override MvxBindingMode DefaultMode => MvxBindingMode.OneWay;

        public static void Register(IMvxTargetBindingFactoryRegistry registry)
        {
            registry.RegisterCustomBindingFactory<View>(Name, view => new SelectedTargetBinding(view));
        }

        [Android.Runtime.Preserve(Conditional = true)]
        public SelectedTargetBinding(View target) : base(target)
        {
        }

        protected override void SetValueImpl(View target, bool value)
        {
            if (target != null && value != target.Selected)
                target.Selected = value;
        }
    }

A solution would be to update MvxTargetBinding<TTarget, TValue>:

        public void SetValue(object value)
        {
            SetValue(value == null ? default(TValue) : (TValue) value);
        }

Configuration

Version: 6.2.3

Platform:

• 📱 iOS
• 🤖 Android
• 🏁 WPF
• 🌎 UWP
• 🍎 MacOS
• 📺 tvOS
• 🐒 Xamarin.Forms