code: @CheckResult
      @NonNull
      public static SpinnerAdapter toSpinnerAdapter(@NonNull PowerAdapter powerAdapter) {
          // SpinnerAdapter adds additional constraints to the ListAdapter contract: getViewTypeCount must return 1.
          // See android.widget.Spinner.setAdapter()
          return new ListAdapterConverterAdapter(checkNotNull(powerAdapter, "powerAdapter"), 1);
      }
docs: Returns the specified {@link PowerAdapter} as a {@link SpinnerAdapter}.
  <p>
  <strong>Note that the supplied adapter must only ever use a single view type</strong>, due to constraints
  imposed by {@link Spinner#setAdapter(SpinnerAdapter)}.
  @param powerAdapter The adapter to be converted.
  @return A spinner adapter that presents the same views as {@code powerAdapter}.
  @see Spinner#setAdapter(SpinnerAdapter)
  @see PowerAdapter#getItemViewType(int)