EntryLineValidationBehaviour.cs

{
public class EntryLineValidationBehaviour : BehaviorBase<Entry>
    {
        #region StaticFields
        public static readonly BindableProperty IsValidProperty = BindableProperty.Create(nameof(IsValid), typeof(bool), typeof(EntryLineValidationBehaviour), true, BindingMode.Default, null, (bindable, oldValue, newValue) => OnIsValidChanged(bindable, newValue));
        #endregion
        #region Properties
        public bool IsValid
        {
            get
            {
                return (bool)GetValue(IsValidProperty);
            }
            set
            {
                SetValue(IsValidProperty, value);
            }
        }
        #endregion
        #region StaticMethods
        private static void OnIsValidChanged(BindableObject bindable, object newValue)
        {
            if (bindable is EntryLineValidationBehaviour IsValidBehavior &&
                 newValue is bool IsValid)
            {
                IsValidBehavior.AssociatedObject.PlaceholderColor = IsValid ? Color.Default : Color.Red;
            }
        }

        #endregion
    }
    
    public class BehaviorBase<T> : Behavior<T>
    where T : BindableObject
    {
        #region Properties
        public T AssociatedObject
        {
            get;
            private set;
        }
        #endregion
        #region NormalMethods
        private void OnBindingContextChanged(object sender, EventArgs e)
        {
            OnBindingContextChanged();
        }
        #endregion
        #region Overrides
        protected override void OnAttachedTo(T bindable)
        {
            base.OnAttachedTo(bindable);
            AssociatedObject = bindable;
            if (bindable.BindingContext != null)
            {
                BindingContext = bindable.BindingContext;
            }

            bindable.BindingContextChanged += OnBindingContextChanged;
        }
        protected override void OnDetachingFrom(T bindable)
        {
            base.OnDetachingFrom(bindable);
            bindable.BindingContextChanged -= OnBindingContextChanged;
            AssociatedObject = null;
        }
        protected override void OnBindingContextChanged()
        {
            base.OnBindingContextChanged();
            BindingContext = AssociatedObject.BindingContext;
        }
        #endregion
    }
  }