Dependent key

gistfile1.cs

public partial class MyController : UIViewController
{
	private bool deviceAuthorized {
		[Export("isDeviceAuthorized")]
		get;
		[Export("setDeviceAuthorized:")]
		set;
	}

	private bool sessionRunningAndDeviceAuthorized {
		[Export("isSessionRunningAndDeviceAuthorized")]
		get {
			return deviceAuthorized;
  		}
  	}
  
	[Export("keyPathsForValuesAffectingSessionRunningAndDeviceAuthorized")]
	private static NSSet keyPathsForValuesAffectingSessionRunningAndDeviceAuthorized ()
	{
		return NSSet.MakeNSObjectSet (new NSString[] {
			(NSString)"deviceAuthorized",
		});
	}
	
	public override void ViewWillAppear (bool animated)
	{
		base.ViewWillAppear (animated);

		NSKeyValueObservingOptions oldNew = NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New;
		NSKeyValueObservingOptions initOldNew = NSKeyValueObservingOptions.Initial | NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New;
		AddObserver (this, (NSString)"sessionRunningAndDeviceAuthorized", oldNew, SessionRunningAndDeviceAuthorizedContext);

      		// after this call I expect ObserveValue method invokation.
		deviceAuthorized = !deviceAuthorized;
	}
	
	public override void ObserveValue (NSString keyPath, NSObject ofObject, NSDictionary change, IntPtr context)
	{
	}
}

I found solution, but I think it is not great.
I can surround deviceAuthorized = !deviceAuthorized; with WillChangeValue / DidChangeValue:

WillChangeValue ("deviceAuthorized");
deviceAuthorized = !deviceAuthorized;
DidChangeValue ("deviceAuthorized");

But why [Export()] doesn't do all job?