Product::get_inventory_level( $variant_id )

	public function get_inventory_level( $variant_id = 0 ) {
		$data = $this->get_source_data();
		if ( $data->inventory_tracking == 'none' ) {
			return - 1;
		}
		if ( $data->inventory_tracking == 'variant' && ! empty( $variant_id ) ) {
			foreach ( $data->variants as $variant ) {
				if ( $variant_id == $variant->id ) {
					return (int) $variant->inventory_level;
				}
			}
		}

		return (int) $data->inventory_level;
	}