参数回调 OnSetParametersCallbackHandle

OnSetParametersCallbackHandle 在于foxy版本添加，dashing中还尚未支持。

在foxy版本中，也添加了返回此类型的回调函数，

/// Add a callback for when parameters are being set.
/**
 * The callback signature is designed to allow handling of any of the above
 * `set_parameter*` or `declare_parameter*` methods, and so it takes a const
 * reference to a vector of parameters to be set, and returns an instance of
 * rcl_interfaces::msg::SetParametersResult to indicate whether or not the
 * parameter should be set or not, and if not why.
 *
 * For an example callback:
 *
 * ```cpp
 * rcl_interfaces::msg::SetParametersResult
 * my_callback(const std::vector<rclcpp::Parameter> & parameters)
 * {
 *   rcl_interfaces::msg::SetParametersResult result;
 *   result.successful = true;
 *   for (const auto & parameter : parameters) {
 *     if (!some_condition) {
 *       result.successful = false;
 *       result.reason = "the reason it could not be allowed";
 *     }
 *   }
 *   return result;
 * }
 * ```
 *
 * You can see that the SetParametersResult is a boolean flag for success
 * and an optional reason that can be used in error reporting when it fails.
 *
 * This allows the node developer to control which parameters may be changed.
 *
 * Note that the callback is called when declare_parameter() and its variants
 * are called, and so you cannot assume the parameter has been set before
 * this callback, so when checking a new value against the existing one, you
 * must account for the case where the parameter is not yet set.
 *
 * Some constraints like read_only are enforced before the callback is called.
 *
 * The callback may introspect other already set parameters (by calling any
 * of the {get,list,describe}_parameter() methods), but may *not* modify
 * other parameters (by calling any of the {set,declare}_parameter() methods)
 * or modify the registered callback itself (by calling the
 * add_on_set_parameters_callback() method).  If a callback tries to do any
 * of the latter things,
 * rclcpp::exceptions::ParameterModifiedInCallbackException will be thrown.
 *
 * The callback functions must remain valid as long as the
 * returned smart pointer is valid.
 * The returned smart pointer can be promoted to a shared version.
 *
 * Resetting or letting the smart pointer go out of scope unregisters the callback.
 * `remove_on_set_parameters_callback` can also be used.
 *
 * The registered callbacks are called when a parameter is set.
 * When a callback returns a not successful result, the remaining callbacks aren't called.
 * The order of the callback is the reverse from the registration order.
 *
 * \param callback The callback to register.
 * \returns A shared pointer. The callback is valid as long as the smart pointer is alive.
 * \throws std::bad_alloc if the allocation of the OnSetParametersCallbackHandle fails.
 */
RCLCPP_PUBLIC
RCUTILS_WARN_UNUSED
OnSetParametersCallbackHandle::SharedPtr
add_on_set_parameters_callback(OnParametersSetCallbackType callback);

这是用于替代dashing版本中的set_on_parameters_set_callback

/// Register a callback to be called anytime a parameter is about to be changed.
/**
 * \deprecated Use add_on_set_parameters_callback instead.
 * With this method, only one callback can be set at a time. The callback that was previously
 * set by this method is returned or `nullptr` if no callback was previously set.
 *
 * The callbacks added with `add_on_set_parameters_callback` are stored in a different place.
 * `remove_on_set_parameters_callback` can't be used with the callbacks registered with this
 * method. For removing it, use `set_on_parameters_set_callback(nullptr)`.
 *
 * \param[in] callback The callback to be called when the value for a
 *   parameter is about to be set.
 * \return The previous callback that was registered, if there was one,
 *   otherwise nullptr.
 */
[[deprecated("use add_on_set_parameters_callback(OnParametersSetCallbackType callback) instead")]]
RCLCPP_PUBLIC
OnParametersSetCallbackType
set_on_parameters_set_callback(rclcpp::Node::OnParametersSetCallbackType callback);