Attached properties

Another way to implement behaviors in order to modify default control behavior is to use attached properties to declare a bindable extension to existing controls. This approach is generally used for small behavioral adjustments, such as enabling/disabling other behaviors, as well as adding/removing effects. Let's get started:

1. In order to implement such a behavior, we need to create a bindable property that will be used on other controls:

 public static class Validations
 {
  public static readonly BindableProperty ValidateRequiredProperty =
  BindableProperty.CreateAttached(
  "ValidateRequired",
  typeof(bool),
  typeof(RequiredValidation),
  false, 
  propertyChanged: OnValidateRequiredChanged);
 
  public static bool GetValidateRequired(BindableObject view)
  {
  return (bool)view.GetValue(ValidateRequiredProperty);
  }
 
  public static void SetValidateRequired(BindableObject view, 
 bool value)
  {
  view.SetValue(ValidateRequiredProperty, value);
  }
  
  private static void OnValidateRequiredChanged(
 BindableObject bindable, object oldValue, object
 newValue)
  {
         // TODO:
  }
 } 

1. In the case of attached behaviors, the static class can be directly accessed so that it sets the attached property on the current control (instead of creating and adding a behavior):

 <Entry x:Name="usernameEntry" Placeholder="username" 
 Text="{Binding UserName, Mode=OneWayToSource}" 
 behaviors:Validations.ValidateRequired="true" >

1. By implementing the ValidateRequired property changed handler, we can have the attached property insert and remove the required validation to various Entry views:

 private static void OnValidateRequiredChanged(
 BindableObject bindable, 
 object oldValue, 
 object newValue)
 {
  if(bindable is Entry entry)
  {
  if ((bool)newValue)
  {
  entry.Behaviors.Add(new ValidationBehavior() 
 { 
 ValidationRule = new RequiredValidationRule() 
 });
  }
  else
  {
  var behaviorToRemove = entry.Behaviors
 .OfType<ValidationBehavior>()
 .FirstOrDefault(
 item => item.ValidationRule is 
 RequiredValidationRule);
 
  if (behaviorToRemove != null)
  {
  entry.Behaviors.Remove(behaviorToRemove);
  }
  }
  }
 }