WP8 app resuming with sensors

Question

In my application on WP8 I work with all of the sensors. Accelerometer, Compass (from Windows.Devices.Sensors and from Microsoft.Devices.Sensors too) and Inclinometer. Everything works fine until I push the Windows button and after that the Back button. I get a full black screen with a "Resuming..." message and nothing happens. Once I got an exception (from 5 fauilure only once):

{System.Runtime.InteropServices.SEHException: External component has thrown an exception.
   at Microsoft.Devices.Sensors.SensorBase`1.PauseSensor()
   at Microsoft.Devices.Sensors.SensorBase`1.<>c__DisplayClass7.<.ctor>b__4(Object sender, NotificationCallbackEventArgs args)
   at Microsoft.Devices.Sensors.SensorCallback.MS.Internal.Interop.INotificationCallback.Pausing(XPauseType pauseType)
   at MS.Internal.Interop.NotificationService.NotificationPausing(XPauseType pauseType)
   at MS.Internal.FrameworkCallbacks.NotificationPausing(UInt32 pauseType)}

Did someone meet with this exception?

Or is there a best practice working correctly with sensors during the wp8 application lifecycle?

Thanks

Answer 1

This problem was more complex than I thought. I’ve worked a lot with this. I share what I found because I think my experiences with it could help later someone else too.

I worked with the sensors through service classes: these classes update my VMs and in the VMs I use my DispatcherHelper to update these on the UI. I knew if I want to update UI bound properties in valuechanged events of the sensors I need to call this update in the eventhandler with a dispatcherhelper but what I did not know is how they behave if they are communicating with VMs instead of Views. When I started the app I had no problems with updating the UI if the sensors had new value. My only problem was in this case that if I wanted to deactivate (pressed windows button) and activate (pressed back button) my app, I had no exception, I did not lost my debug session but only got a black page with a „Resuming…” text and running progressbar and nothing happened.

What I missed was a crossthread/accessing exception. If I do it with a UI bound data it throws an exception but in this case it did not.

One other thing: the accelerometer (I had a version only working with the accelerometer) works with the emulator but on the device it does not. I think the emulator try to simulate the accelerator by giving back values, but not in a right way (without a threading issue).

Solution

If I update within an eventhandler on the UI thread (with my own DispatcherHelper) everything works fine. It is a good lesson learned, always pay attention when interacting with sensors and other layers/threads (VM,V).

Answer 2

It does sound like a lifecycle issue.

Have you tried invoking SensorBase.Stop() on the sensors from Microsoft.Devices.Sensors on the App.Deactivated event and resuming using SensorBase.Start() on the App.Activated event? Depending on how many pages in your app need sensor access you might even be able to override OnNavigatedTo/OnNavigatedFrom methods and use those.

You shouldn't have to do that, but depending on your app it might be a good idea. (does your app support lock screen idle execution? does it have background agents? does it try to take sensor readings while running in the background?)