ValidationBehavior.cs

...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationBehavior : Behavior<View>
    {
        ...
        public bool Validate()
        {
            bool isValid = true;
            string errorMessage = "";

            foreach (IValidator validator in Validators)
            {
                bool result = validator.Check(_view.GetType()
                                       .GetProperty(PropertyName)
                                       .GetValue(_view) as string);
                isValid = isValid && result;

                if (!result)
                {
                    errorMessage = validator.Message;
                    break;
                }
            }

            if (!isValid)
            {
                _style.ShowError(_view, errorMessage);

                return false;
            }
            else
            {
                _style.RemoveError(_view);

                return true;
            }

        }
        ...
    }
}