ValidationBehavior.cs

...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationBehavior : Behavior<View>
    {
        ...
        public bool Validate()
        {
            ...
        }

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

            _view = bindable as View;
            _view.PropertyChanged += OnPropertyChanged;
            _view.Unfocused += OnUnFocused;
        }

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

            _view.PropertyChanged -= OnPropertyChanged;
            _view.Unfocused -= OnUnFocused;
        }
        void OnUnFocused(object sender, FocusEventArgs e)
        {
            Validate();
        }

        void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == PropertyName)
            {
                Validate();
            }
        }
    }
}