VisualElementLostFocusBehavior.cs

public class VisualElementLostFocusBehavior : Behavior<VisualElement>
    {
        public static readonly BindableProperty CommandProperty =
            BindableProperty.Create("Command", typeof(ICommand), typeof(VisualElementLostFocusBehavior));

        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        private VisualElement _bindable;

        protected override void OnAttachedTo(VisualElement bindable)
        {
            base.OnAttachedTo(bindable);

            _bindable = bindable;

            bindable.BindingContextChanged += BindableOnBindingContextChanged;
            bindable.Unfocused += BindableOnUnfocused;
        }

        private void BindableOnBindingContextChanged(object sender, EventArgs eventArgs)
        {
            this.BindingContext = _bindable.BindingContext;
        }

        private void BindableOnUnfocused(object sender, FocusEventArgs focusEventArgs)
        {
            Command?.Execute(null);
        }

        protected override void OnDetachingFrom(VisualElement bindable)
        {
            base.OnDetachingFrom(bindable);

            bindable.BindingContextChanged -= BindableOnBindingContextChanged;
            bindable.Unfocused -= BindableOnUnfocused;

            _bindable = null;
        }
    }