SearchBar does not work with DataBinding (MVVM)

Hello,
At first I'd like to say this is my first issue I create here. Also I'm really impressed by all the work you did to provide developers with all this amazing controls.

My problem is that I cannot use SearchBar with MVVM, in particular IsError and ErrorStr properties.
When I bind to above mentioned properties there is no effect on the control. This is because internal code in TextBox.cs in VerifyData() overrides these properties based on default validation code, when no VerifyFunction has been provided.

If I were to provide VerifyFunction this has to return OperationResult<bool> which is technology-specific and therefore should not be used within ViewModel. What's worse I cannot event bind any function to VerifyFunction because this is not a DependencyProperty !

The only thing I can do now is to provide a VerifyFunction implementation in code-behind of my view object and from there I need to call a ViewModel by explicit casting it from DataContex to fetch logic for validation.

Please feel free to ask if anything is unclear or you need more details on this problem.
I think the SearchBar control is great, it just need a small improvement to fully support MVVM architecutre.

Start in TextBox.cs at VerifyData() and trace how SearchBar handles the IsError and ErrorStr properties when no VerifyFunction is supplied. Compare that behavior with MVVM binding expectations; done means those bound properties can control validation without requiring a code-behind VerifyFunction or explicit ViewModel casting.