Turn on torch with MediaCapture on Windows Phone S

问题:

My problem is quite simple, I cannot turn on the flash light with the MediaCapture API from windows phone 8.1. (I succedded with the 8.0 API)

I built a very simple project with 2 buttons, one to toggle the FlashControl and the other one to toggle TorchControl.

There is no crash, no exception. My phones support FlashControl and TorchControl. I also debug step-by-step and everything looks good, values are changed when buttons are clicked.

Here is my code:

MediaCapture m_captureManager;

public MainPage()
{
  InitializeComponent();
}

private static async Task<DeviceInformation> GetCameraID(Windows.Devices.Enumeration.Panel desiredCamera)
{
  DeviceInformation deviceID = (await DeviceInformation.FindAllAsync(DeviceClass.VideoCapture))
      .FirstOrDefault(x => x.EnclosureLocation != null && x.EnclosureLocation.Panel == desiredCamera);
  if (deviceID != null) return deviceID;
  else throw new Exception(string.Format("Camera {0} doesn't exist", desiredCamera));
}

protected async override void OnNavigatedTo(NavigationEventArgs e)
{
  var cameraID = await GetCameraID(Windows.Devices.Enumeration.Panel.Back);
  m_captureManager = new MediaCapture();

  await m_captureManager.InitializeAsync(new MediaCaptureInitializationSettings
  {
    StreamingCaptureMode = StreamingCaptureMode.Video,
    PhotoCaptureSource = PhotoCaptureSource.VideoPreview,
    AudioDeviceId = string.Empty,
    VideoDeviceId = cameraID.Id
  });

}

private void button_ClickTorch(object sender, RoutedEventArgs e)
{
  var torch = m_captureManager.VideoDeviceController.TorchControl;
  if (torch.Supported)
  {
    if (torch.Enabled)
      torch.Enabled = false;
    else
      torch.Enabled = true;
  }
}

private void button_ClickFlash(object sender, RoutedEventArgs e)
{
  if (captureManager.VideoDeviceController.FlashControl.Supported)
  {
    if (captureManager.VideoDeviceController.FlashControl.Enabled)
      captureManager.VideoDeviceController.FlashControl.Enabled = false;
    else
      captureManager.VideoDeviceController.FlashControl.Enabled = true;
  }
}

It's a simple piece of code and I cannot make it works... I was quite desperate so I tried to toggle by using an intermediate object and without, as you can see, but it did not change the result (which is in conformity).

回答1:

I finally found out what was wrong. To be able to use camera services, we have to start a preview. Since with Silverlight we can’t use a CaptureElement, we have to use a CustomPreviewSink with a VideoBrush

This is how to do it ( from microsoft doc)

private async void StartPreview()
{
previewSink = new Windows.Phone.Media.Capture.MediaCapturePreviewSink();

// List of supported video preview formats to be used by the default preview format selector.
var supportedVideoFormats = new List<string> { "nv12", "rgb32" };

// Find the supported preview format
var availableMediaStreamProperties = mediaCaptureManager.VideoDeviceController.GetAvailableMediaStreamProperties(
    Windows.Media.Capture.MediaStreamType.VideoPreview)
        .OfType<Windows.Media.MediaProperties.VideoEncodingProperties>()
        .Where(p => p != null 
            && !String.IsNullOrEmpty(p.Subtype) 
            && supportedVideoFormats.Contains(p.Subtype.ToLower()))
        .ToList();
var previewFormat = availableMediaStreamProperties.FirstOrDefault();

// Start Preview stream
await mediaCaptureManager.VideoDeviceController.SetMediaStreamPropertiesAsync(
    Windows.Media.Capture.MediaStreamType.VideoPreview, previewFormat);
await mediaCaptureManager.StartPreviewToCustomSinkAsync(
    new Windows.Media.MediaProperties.MediaEncodingProfile { Video = previewFormat }, previewSink);


// Set the source of the VideoBrush used for your preview
Microsoft.Devices.CameraVideoBrushExtensions.SetSource(viewfinderBrush, previewSink);
}

Add this piece of code to previous code and it will work. The important point is to start the preview before changing any parameters