Entry Validation in Xamarin.Forms
You could have a collection of ValueConverters which would validate your TextInputs' strings. For example one could be a PhoneNumberValidator or PhoneToBoolConverter (example names there). This way HasError would bind to HasError="{Binding PhoneNumber, Converter={StaticResource PhoneNumberValidator}}". Converters are handy and you can reuse them in multiple places around your code if they are declared in your App.xaml.cs for example.