Radical 2.0.0 to Radical 2.1.0

In version 2.1.0 the IRequireValidationCallback<TViewModel> has been obsoleted. It'll be removed in version 3.0.0. The IRequireValidationCallback<> behavior can be replaced by using a custom validation rule, like in the following snippet:

class SampleViewModel : AbstractViewModel, IRequireValidation
{
    public SampleViewModel()
    {
        ValidationService = new DataAnnotationValidationService<SampleViewModel>( this )
            .AddRule
            (
                property: () => this.Text,
                rule: ctx => ctx.Failed("This is the error message.")
            );
    }
}