ValidationBehavior.cs

...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationGroupBehavior: Behavior<View>
    {
        ...
        public void Update()
        {
            bool isValid = true;

            foreach (ValidationBehavior validationItem in _validationBehaviors)
            {
                isValid = isValid && validationItem.Validate();
            }

            IsValid = isValid;
        }

        public bool IsValid
        {
            get
            {
                return (bool)GetValue(IsValidProperty);
            }
            set
            {
                SetValue(IsValidProperty, value);
            }
        }
    }
}