The downloadable sample code contains a number of examples of asynchronous input validation. The simplest is located in the
AsyncValidationView contains two string properties that are required to be non-null or whitespace strings (see Figure 26.12).
When either of the two properties is set to an empty or whitespace string, a data validation error is created for that property. This validation occurs when the user modifies the text or when the Submit button is tapped (see Figure 26.13).