MediaCapture + CaptureElement lifecycle/navigation management

Question

This is in a Windows Phone 8.1 Store app. My MainPage has a CaptureElement to display the preview stream from my MediaCapture object.

Answer 1

You will have to use App.Suspending and App.Resuming (for the cases you have described) with combination of Navigation events (when navigating between Pages). The OnNavigatingFrom event is called when you hit Start, hold Back or use Search (when the App is being suspended), but when you resume the App, OnNavigatedTo is not being called - this event is called only when you are navigating. So in your case, when you hit Start, the preview stops and when you come back it doesn't start again. A refference to MSDN:

Note On Windows Phone, OnNavigatedFrom() is called when the app is suspended. OnNavigatedTo() is not called when the app is resumed.

The other thing is that to debug the App properly you will have to use Lifecycle Events of Debug Location tab in Visual Studio - while you are debbuging the app, it is not being suspended, but when you run your app normally, it gets suspended just after you hit Start.

Note also that the App can be put into Not Running state. More about Lifecycle at MSDN.

Answer 2

The scenarios you described should trigger the Window.Current.VisibilityChanged event where you can use VisibilityChangedEventArgs.Visible passed into the event handler to cleanup preview when not visible and initialize preview when visible. You can subscribe\unsubscribe to Window.Current.VisibilityChanged event in your Loaded\Unloaded handler for your Page\UserControl.

The reason why Suspend/Resume lifecycle events is not sufficient is because the scenarios you mentioned above doesn't deterministically invoke those events at a certain time as the OS will only suspend an app based on an internal policy that can change with OS release updates.

Also as an aside, I would avoid using Navigation handlers and instead rely on Loaded\Unloaded handlers which will allow initialization\cleanup to occur properly if you ever had to move your CaptureElement into its own UserControl as opposed to in a Page and avoids the scenario where WP will call OnNavigatedFrom and not call OnNavigatedTo for suspend\resume (Loaded\Unloaded will always be called in order).