GC Loop

Behaviors.cs

namespace MySolution.ValueConverters
{
    public class NullableIntConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
            int result;
            int.TryParse(value as string, out result);
            return result <= 0 ? null : result as int?;
        }
    }
    
    public class RegexValidator : EventToCommand<View>
    {
      public static readonly BindableProperty RegularExpressionProperty = 
        BindableProperty.Create("RegularExpression", typeof(string), typeof(RegexValidator));
        
      public string RegularExpression {
        get { return (string)GetValue(RegularExpressionProperty); }
        set { SetValue(RegularExpressionProperty, value); }
      }
      
      /* Scenario: new value is 'null', old value is '0'
       * null is not a match in the regex so it would set the text to 0 triggering the regex validation again.
       * Because the ValueConverter is changing 0 to null the regex is triggered again.
       * Infinity...
       * */
      
      protected override void OnEvent(object sender, object eventArgs) {
        string text = 
          (eventArgs as TextChangedEventArgs)?.NewTextValue
          ?? (sender as Entry)?.Text
          ?? (sender as Editor)?.Text;
          
        /* Solution */
        if (Equals(text, null))
          return;
        /* End Solution */
        
        string oldValue = (eventArgs as TextChangedEventArgs)?.OldTextValue;
        if (!Equals(text, oldValue) && !this.IsMatch(text)) {
          base.OnEvent(sender, eventArgs);
          this.SetText(oldValue);
        }
      }
      
      bool IsMatch(string value) {
        return (!Equals(input, null) && Regex.IsMatch(input, this.RegularExpression, RegexOptions.None));
      }
      
      void SetText(object sender, string value) {
        if (sender is Entry) {
          ((Entry)sender).Text = value;
        } else if (sender is Editor) {
          ((Editor)sender).Text = value;
        } else {
          throw new ArgumentException("The RegexValidator can only be applied to an Entry or Editor control");
        }
      }
    }
}

Page.xaml

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:behaviors="clr-namespace:MySolution.Behaviors"
             xmlns:converters="clr-namespace:MySolution.ValueConverters"
             x:Class="MySolution.MyClass"
             Title="MyTitle">
  <ContentPage.Resources>
    <ResourceDictionary>
      <converters:NullableIntConverter x:Key="NullableIntConverter" />
    </ResourceDictionary>
  </ContentPage.Resources>
  <ContentPage.Content>
    <Entry 
      Text="{Binding MyNullableProperty, Converter={StaticResource NullableIntConverter}}">
      <Entry.Behaviors>
        <behaviors:NumericValidator />
        <behaviors:NumberRangeValidator MinValue="1" MaxValue="999" />
        <behaviors:MaxLengthValidator MaxLength="3" />
        <behaviors:CursorToEndBehavior />
        <behaviors:RegexValidator 
          RegularExpression="^([1-9]*|[1-9]+[0-9]*)$"
          EventName="TextChanged" />
          <behaviors:EventToCommand 
            EventName="Unfocused"
            Command="{Binding SaveOnEventCommand}"/>
      </Entry.Behaviors>
    </Entry>
  </ContentPage.Content>
</ContentPage>