Xamarin.Forms Behavior Example

NumericValidationBehavior.cs

//Add functionality to visual elements without subclassing.
//Xamarin.Form controls can have a list of Behaviors.

public class NumericValidationBehavior : Behavior<Entry>
{
 protected override void OnAttachedTo(Entry entry)
 {
  entry.TextChanged += OnEntryTextChanged;
  base.OnAttachedTo(entry);
 }
 
 protected override void OnDetachingFrom(Entry entry)
 {
  entry.TextChanged -= OnEntryTextChanged;
  base.OnDetachingFrom(entry);
 }
 
 void OnEntryTextChanged(object sender, TextChangedEventArgs args)
 {
  double result;
  bool isValid = Double.TryParse(args.NewTextValue, out result);
  ((Entry)sender).TextColor = isValid ? Color.Default : Color.Red;
 }
}