ROS中的message_filter中的时间同步

#ifndef MESSAGE_FILTERS_TIME_SYNCHRONIZER_H
#define MESSAGE_FILTERS_TIME_SYNCHRONIZER_H

#include <memory>

#include "message_filters/message_event.h"
#include "message_filters/synchronizer.h"
#include "message_filters/sync_policies/exact_time.h"

namespace message_filters
{

/**
 * \brief Synchronizes up to 9 messages by their timestamps.
 *
 * TimeSynchronizer synchronizes up to 9 incoming channels by the timestamps contained in their messages' headers.
 * TimeSynchronizer takes anywhere from 2 to 9 message types as template parameters, and passes them through to a
 * callback which takes a shared pointer of each.
 *
 * The required queue size parameter when constructing the TimeSynchronizer tells it how many sets of messages it should
 * store (by timestamp) while waiting for messages to arrive and complete their "set"
 *
 * \section connections CONNECTIONS
 *
 * The input connections for the TimeSynchronizer object is the same signature as for rclcpp subscription callbacks, ie.
\verbatim
void callback(const std::shared_ptr<M const>&);
\endverbatim
 * The output connection for the TimeSynchronizer object is dependent on the number of messages being synchronized.  For
 * a 3-message synchronizer for example, it would be:
\verbatim
void callback(const std::shared_ptr<M0 const>&, const std::shared_ptr<M1 const>&, const std::shared_ptr<M2 const>&);
\endverbatim
 * \section usage USAGE
 * Example usage would be:
\verbatim
TimeSynchronizer<sensor_msgs::CameraInfo, sensor_msgs::Image, sensor_msgs::Image> sync_policies(caminfo_sub, limage_sub, rimage_sub, 3);
sync_policies.registerCallback(callback);
\endverbatim
 * The callback is then of the form:
\verbatim
void callback(const sensor_msgs::CameraInfo::ConstPtr&, const sensor_msgs::Image::ConstPtr&, const sensor_msgs::Image::ConstPtr&);
\endverb