Question

I am trying to write a basic camera app using the auto-focus feature to process a barcode. If I press back just as the camera has focused, it goes to a blank screen and just hangs there – no exception is thrown. I have traced the problem to the CaptureImage() method, I can see it being called, but the OnCaptureImageAvailable method is never invoked. This happens even when I do not unhook the OnCaptureImageAvailable in the OnNavigatedFrom method.

   protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
    {
        base.OnNavigatedFrom(e);
        _camera.AutoFocusCompleted -= OnCameraAutoFocusCompleted;
        _camera.CaptureImageAvailable -= OnCaptureImageAvailable;
        _camera.Initialized -= OnCameraInitialized;
        _camera.Dispose();
        _camera = null;
    }

    private void InitializeCamera()
    {
        _camera = new PhotoCamera();
        _camera.Initialized += OnCameraInitialized;
        viewfinderBrush.SetSource(_camera);
    }

    void OnCameraInitialized(object sender, EventArgs e)
    {
        _camera.Initialized -= OnCameraInitialized;
        _camera.AutoFocusCompleted += OnCameraAutoFocusCompleted;
        _camera.CaptureImageAvailable += OnCaptureImageAvailable;

        _camera.FlashMode = FlashMode.Off;
        _camera.Focus(); 
    }

    private void OnCameraAutoFocusCompleted(object sender, EventArgs e)
    {
        _camera.CaptureImage();
    }

    private void OnCaptureImageAvailable(object sender, ContentReadyEventArgs e)
    {
        if (_camera != null && e.ImageStream != null)
            ScanBarcode(e);
    }

Answer 1

The issue is you are going to the previous page and disposing the camera while the camera is busy capturing the image in “_camera.CaptureImage();”.

How about keeping two boolean flags: “doBack” and “captureCompleted”.

Then, do not go back when the OnCameraAutoFocusCompleted event triggers:

private void OnCameraAutoFocusCompleted(object sender, EventArgs e)
    {
        _camera.CaptureImage();
    }

When the back key is pressed set the doBack flag and ignore, unless captureCompleted is set:

protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
    {
        if (!captureCompleted)
        {
            this.doBack = true;
            e.Cancel = true;
        }

        base.OnBackKeyPress(e);
    }

Then, when the OnCameraCaptureCompleted event triggers, set the captureCompleted flag en go to the previous page (and dispose the camera) if the doBack flag is set:

private void OnCameraCaptureCompleted(object sender, EventArgs e)
    {
        this.captureCompleted = true;

        if (doBack)
        { 
            this.Dispatcher.BeginInvoke(delegate()
            {
                NavigationService.GoBack();
            });
        }
    }