static ScaleTypeDrawable wrapChildWithScaleType(
      DrawableParent parent, ScalingUtils.ScaleType scaleType) {
    Drawable child = parent.setDrawable(sEmptyDrawable);
    child = maybeWrapWithScaleType(child, scaleType);
    parent.setDrawable(child);
    Preconditions.checkNotNull(child, "Parent has no child drawable!");
    return (ScaleTypeDrawable) child;
  }
Wraps the parent's child with a ScaleTypeDrawable.
drawee/src/main/java/com/facebook/drawee/generic/WrappingUtils.java