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我有一个奇怪的例外。我在一个页面中使用了PhoneCamera API，用户可以制作一些照片，拍摄后直接上传。

我的代码：

protected override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    _mediaLibrary = new MediaLibrary();

    // Initialize the camera object
    _phoneCamera = new PhotoCamera(CameraType.Primary);
    _phoneCamera.Initialized += CamInitialized;
    _phoneCamera.AutoFocusCompleted += PhoneCamera_AutoFocusCompleted;
    _phoneCamera.CaptureImageAvailable += PhoneCamera_CaptureImageAvailable;

    // Display the camera feed in the UI
    ViewfinderBrush.SetSource(_phoneCamera);

    // Rotation for transform
    ViewfinderTransform.Rotation = _phoneCamera.Orientation;
}

protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
{
    if (_phoneCamera != null)
    {
        // Cleanup
        _phoneCamera.Dispose();
        _phoneCamera.Initialized -= CamInitialized;
        _phoneCamera.AutoFocusCompleted -= PhoneCamera_AutoFocusCompleted;
        _phoneCamera.CaptureImageAvailable -= PhoneCamera_CaptureImageAvailable;

        if (_cameraInizialized)
        {
            CameraButtons.ShutterKeyHalfPressed -= CameraButtons_ShutterKeyHalfPressed;
            CameraButtons.ShutterKeyPressed -= CameraButtons_ShutterKeyPressed;
        }
    }

    base.OnNavigatingFrom(e);
}

private void CamInitialized(object sender, CameraOperationCompletedEventArgs e)
{
    if (e.Succeeded)
    {
        // Set flashmode
        Dispatcher.BeginInvoke(() => {  _phoneCamera.FlashMode = FlashMode.Auto; });

        // Subscribe to hardeware-button-events
        CameraButtons.ShutterKeyHalfPressed += CameraButtons_ShutterKeyHalfPressed;
        CameraButtons.ShutterKeyPressed += CameraButtons_ShutterKeyPressed;

        _cameraInizialized = true;
    }
    else
    {
        // Show error message
    }
}

当初始化事件被触发时，一切正常。但是当用户在相机初始化时点击手机的主页按钮时出现异常。（初始化事件未触发）。

当用户使用返回按钮返回时，发生以下异常：

[Type]:[AggregateException]
[ExceptionMessage]:[One or more errors occurred.]
[StackTrace]:[
    at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
    at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
    at Microsoft.Devices.Camera.OpenCaptureDevice()
    at Microsoft.Devices.Camera.<>c__DisplayClass2.<InitializeVideoSession>b__0()]
[InnerExceptions]:[[[Type]:[InvalidOperationException]
    [Message]:[The text associated with this error code could not be found.
    Unable to acquire the camera. You can only use this class while in the foreground.]

我尝试了许多其他选择，但没有任何效果。我现在不知道为什么会发生此错误。虽然我知道它发生在哪里，在什么情况下。

欢迎任何帮助。谢谢你。

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