@Override
  public @Nullable Drawable.ConstantState getConstantState() {
    if (mCurrentDelegate == null) {
      return super.getConstantState();
    }

    return mCurrentDelegate.getConstantState();
  }
As {@code setCurrent}, but without invalidating a drawable. Subclasses are responsible to call
{@code invalidateSelf} on their own.

@param newDelegate
@return the previous delegate
drawee/src/main/java/com/facebook/drawee/drawable/ForwardingDrawable.java