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Visual State Triggers in Windows 10

Developers writing UWP apps in XAML have the capability to create VisualState triggers that can automatically trigger a VisualState change. One example is the AdaptiveTrigger that can be triggered by changing the size of the window

  <!--VisualState to be triggered when window width is >=720 effective pixels.-->
      <AdaptiveTrigger MinWindowWidth="720" />

      <Setter Target="myPanel.Orientation" Value="Horizontal" />


PeekABoo is an app that uses the StateTriggerBase class to create a PeekTrigger for detecting when a face is present in the view of the camera

    <local:PeekTrigger Mode="FaceDetected"></local:PeekTrigger>

By creating a property Mode on the PeekTrigger class, we can allow the developer to choose if the trigger is active when a face is detected (FaceDetected) or when a face is not detected (FaceNotDetected).

FaceDetection is done by using the new FaceDetectionEffect class in Windows 10 to detect faces in a video stream, in this case the camera.

var definition = new FaceDetectionEffectDefinition();
definition.SynchronousDetectionEnabled = false;
definition.DetectionMode = FaceDetectionMode.HighPerformance;

_faceDetectionEffect = (FaceDetectionEffect) await _mediaCapture.AddVideoEffectAsync(definition, MediaStreamType.VideoPreview);

// detect 30 frames/second
_faceDetectionEffect.DesiredDetectionInterval = TimeSpan.FromMilliseconds(33);
_faceDetectionEffect.Enabled = true;
_faceDetectionEffect.FaceDetected += FaceDetectionEffect_FaceDetected;

We set the trigger to active by using the SetActive(true) method of the StateTriggerBase when the condition is satisfied

private async void FaceDetectionEffect_FaceDetected(FaceDetectionEffect sender, FaceDetectedEventArgs args)
    if (args.ResultFrame.DetectedFaces.Count > 0 && mode.ToLower() == "facedetected" ||
        args.ResultFrame.DetectedFaces.Count == 0 && mode.ToLower() == "facenotdetected")
        await this.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>

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