private ScaleTypeDrawable getScaleTypeDrawableAtIndex(int index) {
    DrawableParent parent = getParentDrawableAtIndex(index);
    if (parent instanceof ScaleTypeDrawable) {
      return (ScaleTypeDrawable) parent;
    } else {
      return WrappingUtils.wrapChildWithScaleType(parent, ScalingUtils.ScaleType.FIT_XY);
    }
  }
Gets the ScaleTypeDrawable at the specified index. In case there is no child at the specified
index, a NullPointerException is thrown. In case there is a child, but the ScaleTypeDrawable
does not exist, the child will be wrapped with a new ScaleTypeDrawable.
drawee/src/main/java/com/facebook/drawee/generic/GenericDraweeHierarchy.java