EmptyEntryValidatorBehavior.cs

public class EmptyEntryValidatorBehavior : Behavior<ExtendedEntry>
	{
        ExtendedEntry control;
        string _placeHolder;
        Xamarin.Forms.Color _placeHolderColor;
       
		protected override void OnAttachedTo(ExtendedEntry bindable)
		{
			bindable.TextChanged += HandleTextChanged;
            bindable.PropertyChanged += OnPropertyChanged;
            control = bindable;
            _placeHolder = bindable.Placeholder;
            _placeHolderColor = bindable.PlaceholderColor;
		}

		void HandleTextChanged(object sender, TextChangedEventArgs e)
		{
			if (!string.IsNullOrEmpty(e.NewTextValue))
			{
				((ExtendedEntry)sender).IsBorderErrorVisible = false;
			}
		}

		protected override void OnDetachingFrom(ExtendedEntry bindable)
		{
			bindable.TextChanged -= HandleTextChanged;
            bindable.PropertyChanged -= OnPropertyChanged;
		}

        void OnPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
		{
            if (e.PropertyName == ExtendedEntry.IsBorderErrorVisibleProperty.PropertyName && control != null)
            {
                if (control.IsBorderErrorVisible)
                {
                    control.Placeholder = control.ErrorText;
                    control.PlaceholderColor = control.BorderErrorColor;
                    control.Text = string.Empty;
                }

                else{
                    control.Placeholder = _placeHolder;
                    control.PlaceholderColor = _placeHolderColor;
                }
                    
			}
		}
	}