Making file picker asynchronous - Windows Phone 8.1

Question

I tried to make File open picker asynchronous using TaskComplectionSource however sometimes I get my application closed with -1 return value, sometimes I get exception like:

[System.Runtime.InteropServices.COMException] = {System.Runtime.InteropServices.COMException (0x80004005): Unspecified error

Unspecified error

   at Windows.Storage.Pickers.FileOpenPicker.PickSingleFileAndContinue()
   at PhotosGraphos.Mobile.Common.StorageFileExtensions.<PickSingleFileAsyncMobile..

Code:

public static class StorageFileExtensions
{
    private static TaskCompletionSource<StorageFile> PickFileTaskCompletionSource;

    private static bool isPickingFileInProgress;
    public static async Task<StorageFile> PickSingleFileAsyncMobile(this FileOpenPicker openPicker)
    {
        if (isPickingFileInProgress)
            return null;

        isPickingFileInProgress = true;
        PickFileTaskCompletionSource = new TaskCompletionSource<StorageFile>();

        var currentView = CoreApplication.GetCurrentView();
        currentView.Activated += OnActivated;
        openPicker.PickSingleFileAndContinue();

        StorageFile pickedFile;
        try
        {
            pickedFile = await PickFileTaskCompletionSource.Task;
        }
        catch (TaskCanceledException)
        {
            pickedFile = null;
        }
        finally
        {
            PickFileTaskCompletionSource = null;
            isPickingFileInProgress = false;
        }

        return pickedFile;
    }

    private static void OnActivated(CoreApplicationView sender, IActivatedEventArgs args)
    {
        var continuationArgs = args as FileOpenPickerContinuationEventArgs;
        sender.Activated -= OnActivated;

        if (continuationArgs != null && continuationArgs.Files.Any())
        {
            StorageFile pickedFile = continuationArgs.Files.First();
            PickFileTaskCompletionSource.SetResult(pickedFile);
        }
        else
        {
            PickFileTaskCompletionSource.SetCanceled();
        }
    }
}

What's weird - this bug is hardly reproduced while debugging. Does anyone have any idea what could be reason of that?

Answer 1

I've changed file picker to standard way provided by @Romasz - it still was crashing. I've been debugging it for hours and I get same COMException but sometimes with information provided:

"GetNavigationState doesn't support serialization of a parameter type which was passed to Frame.Navigate"

It seems that code with TaskCompletionSource works and there is nothing wrong with that. I found out in msdn documentation for Frame

Note: The serialization format used by these methods is for internal use only. Your app should not form any dependencies on it. Additionally, this format supports serialization only for basic types like string, char, numeric and GUID types.

And I was passing my model-class object in navigation parameter - so it was kept in navigation stack therefore it couldn't be serialized. The lesson is: do not use non-primitive types for navigation parameter - Frame.Navigate should disallow such navigation and throw exception - but it doesn't..

EDIT: Another bug - if you bind tapped (let say button tapped) or event like that to command which launch FileOpenPicker you need to check if picker.PickFile.. was called before - otherwise when you tap fast on that button you'll get few calls to picker.PickFile.. and UnauthorizedAccessException will be thrown.

Answer 2

Don't do that (don't try to turn Continuation behaviour into async). Why?

Normally when your app is put into the background (for example when you call file picker), it's being suspended, and here is one small pitfall - when you have a debugger attached, your app will work without being suspended. Surely that can cause some troubles.

Note also that when you normally run your app and you fire a picker, then in some cases your app can be terminated (low resources, user closes it ...). So you need here two things which are added by VS as a template: ContinuationManager and SuspensionManager. More you will find at MSDN. At the same link you will find a good procedure to debug your app:

Follow these steps to test the case in which your app is terminated after calling the AndContinue method. These steps ensure that the debugger reattaches to your app after completing the operation and continuing.

1. In Visual Studio, right-click on your project and select Properties.

2. In Project Designer, on the Debug tab under Start action, enable Do not launch, but debug my code when it starts.

3. Run your app with debugging. This deploys the app, but does not run it.

4. Start your app manually. The debugger attaches to the app. If you have breakpoints in your code, the debugger stops at the breakpoints. When your app calls the AndContinue method, the debugger continues to run.

5. If your app calls a file picker, wait until you have opened the file provider (for example, Phone, Photos, or OneDrive). If your app calls an online identity provider, wait until the authentication page opens.

6. On the Debug Location toolbar, in the Process dropdown list, select the process for your app. In the Lifecycle Events dropdown list, select Suspend and Shutdown to terminate your app but leave the emulator running.

7. After the AndContinue operation completes, the debugger reattaches to your app automatically when the app continues.