Adding onMissing handling

    Adding handling of the onMissing event allows the graphic trackers to
    hide a temporarily missing item from view, which can happen occasionally
    during tracking.  This can reduce the perception of "lag", since it
    won't show a stale detection on screen when the detection hasn't really
    occurred.

Author: claywilkinson

face/FaceTracker/app/src/main/java/com/google/android/gms/samples/vision/face/facetracker/FaceTrackerActivity.java

@@ -162,17 +162,27 @@ private class GraphicFaceTracker extends Tracker<Face> {
         @Override
         public void onNewItem(int faceId, Face item) {
             mFaceGraphic.setId(faceId);
-            mOverlay.add(mFaceGraphic);
         }
 
         /**
          * Update the position/characteristics of the face within the overlay.
          */
         @Override
         public void onUpdate(FaceDetector.Detections<Face> detectionResults, Face face) {
+            mOverlay.add(mFaceGraphic);
             mFaceGraphic.updateFace(face);
         }
 
+        /**
+         * 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<Face> detectionResults) {
+            mOverlay.remove(mFaceGraphic);
+        }
+
         /**
          * Called when the face is assumed to be gone for good. Remove the graphic annotation from
          * the overlay.

face/multi-tracker/app/src/main/java/com/google/android/gms/samples/vision/face/multitracker/GraphicTracker.java

@@ -40,17 +40,27 @@ class GraphicTracker<T> extends Tracker<T> {
     @Override
     public void onNewItem(int id, T item) {
         mGraphic.setId(id);
-        mOverlay.add(mGraphic);
     }
 
     /**
      * Update the position/characteristics of the item within the overlay.
      */
     @Override
     public void onUpdate(Detector.Detections<T> detectionResults, T item) {
+        mOverlay.add(mGraphic);
         mGraphic.updateItem(item);
     }
 
+    /**
+     * Hide the graphic when the corresponding face was not detected.  This can happen for
+     * intermediate frames temporarily, for example if the face was momentarily blocked from
+     * view.
+     */
+    @Override
+    public void onMissing(Detector.Detections<T> detectionResults) {
+        mOverlay.remove(mGraphic);
+    }
+
     /**
      * Called when the item is assumed to be gone for good. Remove the graphic annotation from
      * the overlay.