App Resuming event not firing when the app is resumed in WP 8.1 store app

Question

My WP 8.1 store app behaves very strangely. App Resuming event fires as expected when I quickly navigate away and then come back. But If I keep the app in background for some time and when comes back, the app fires the Constructor and OnNavigatedTo events instead of the Resuming event and has the black "Resuming..." screen for a couple of seconds(about 4 seconds).This is an app with a BackgroundAudio task. Even the sample BackgroundAudio app from MS behaves like this. Anyone knows what's wrong here?

Answer 1

I'm in the process of building a WP 8.1 app which uses a background audio task as well. Everything you explained happens to me also.

If you see "Resuming..." for a few seconds, then it probably means your app was terminated by the OS after being suspended first. In this case, the Resuming event won't fire because your app was completely killed and must start again. "Resuming" usually means when a suspended (not terminated) app resumes execution.

When your app is terminated and then "resumed" from the app switcher, your Application.OnLaunched() method override will be invoked with e.PreviousExecutionState == ApplicationExecutionState.Terminated. In this method, you should check if the previous execution state was Terminated, and if so, restore the app to the state it was in prior to suspension. This gives the illusion to the user that the app was never terminated and they can resume what they were doing at the time.

If you create a new Pivot App Windows Phone 8.1 project (for example), you'll see that the application lifecycle events are taken care of correctly in App.xaml.cs.

App Resuming event fires as expected when I quickly navigate away and then come back.

This is correct behavior. It takes a few seconds once the app has been backgrounded before it is suspended by the OS, so if you resume the app before the OS has suspended it, then it will simply resume from memory.

I'm not sure why background audio apps are more susceptible to termination. I even find that the Xbox Music app behaves similarly. Hopefully in the next version of Windows Phone, this issue will be addressed.

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FYI here's a diagram of the application lifecycle from MSDN (I recommend you read this page for more information about the application lifecycle):

"Resuming" only occurs from the suspended to running states.