private void setChildDrawableAtIndex(int index, @Nullable Drawable drawable) {
    if (drawable == null) {
      mFadeDrawable.setDrawable(index, null);
      return;
    }
    drawable = WrappingUtils.maybeApplyLeafRounding(drawable, mRoundingParams, mResources);
    getParentDrawableAtIndex(index).setDrawable(drawable);
  }
Sets the drawable at the specified index while keeping the old scale type and rounding. In case
the given drawable is null, scale type gets cleared too.
drawee/src/main/java/com/facebook/drawee/generic/GenericDraweeHierarchy.java