title: Delay in google vision callback
body: <p>I am using <code>Google Vision Api</code> for face detection. I want to enable capture button when the face is detected in the camera otherwise disable. Its working fine, only the issue is when there is a face button is enabled, but on face not available, button disables after 1/1.5 seconds because <code>onDone</code> callback of <code>Tracker</code> is called after 1, or 1.5 seconds. </p>
  
  <pre><code>private class GraphicFaceTracker extends Tracker&lt;Face&gt; {
          private GraphicOverlay mOverlay;
          private FaceGraphic mFaceGraphic;
  
          GraphicFaceTracker(GraphicOverlay overlay) {
              mOverlay = overlay;
              mFaceGraphic = new FaceGraphic(overlay);
          }
  
          /**
           * Start tracking the detected face instance within the face overlay.
           */
          @Override
          public void onNewItem(int faceId, Face item) {
              mFaceGraphic.setId(faceId);
          }
  
          /**
           * Update the position/characteristics of the face within the overlay.
           */
          @Override
          public void onUpdate(FaceDetector.Detections&lt;Face&gt; detectionResults, Face face) {
              mOverlay.add(mFaceGraphic);
              mFaceGraphic.updateFace(face);
              iv.post(new Runnable() {
                  @Override
                  public void run() {
                      iv.setEnabled(true);
                  }
              });
          }
  
          /**
           * Hide the graphic when the corresponding face was not detected.  This can happen for
           * intermediate frames temporarily (e.g., if the face was momentarily blocked from
           * view).
           */
          @Override
          public void onMissing(FaceDetector.Detections&lt;Face&gt; detectionResults) {
              mOverlay.remove(mFaceGraphic);
          }
  
          /**
           * Called when the face is assumed to be gone for good. Remove the graphic annotation from
           * the overlay.
           */
          @Override
          public void onDone() {
              iv.post(new Runnable() {
                  @Override
                  public void run() {
                      iv.setEnabled(false);
                  }
              });
              mOverlay.remove(mFaceGraphic);
          }
      }
  </code></pre>
  
  <blockquote>
    <p>How Can I quickly get a callback that face is not in the camera so
    disable the button. How to remove the delay?</p>
  </blockquote>