Problem in getting event for allow or don't allow on LOcation GPS enable

Hi Evenryone,

I am facing problem to get the code wait for "Allow" or Don't Allow"
below is my code
bool istrue = false;
CLLocationManager locationManager = new CLLocationManager();
Version version = new Version (UIDevice.CurrentDevice.SystemVersion);
if (version >= new Version (8,0))
{

            locationManager.RequestWhenInUseAuthorization ();
        } 
        else 
        {
            locationManager.StartUpdatingLocation();
        }

        while (!istrue)
        {

        }

I want to wait my code for the action "Allow" or "Don't Allow" by user.
Any suggestion is most welcome.

Many many thanks to you in advance.

Thanks,
Praveen Kumar