ROS advertisement data publishing tips - latch configuration

         stay ros Of published data advertise The last parameter in the function parameter ＂latch＂, The default is false, Never noticed , Today, when analyzing other problems, we are ros wiki［Publishers and Subscribers］ I accidentally noticed this parameter on the ,

The explanation is as follows ：

latch [optional]

• Enables "latching" on a connection. When a connection is latched, the last message published is saved and automatically sent to any future subscribers that connect. This is useful for slow-changing to static data like a map. Note that if there are multiple publishers on the same topic, instantiated in the same node, then only the last published message from that node will be sent, as opposed to the last published message from each publisher on that single topic.

advertise Functions are defined in the file /opt/ros/melodic/include/ros/node_handle.h, It also explains ．     

    /**
     * \brief Advertise a topic, simple version
     *
     * This call connects to the master to publicize that the node will be
     * publishing messages on the given topic.  This method returns a Publisher that allows you to
     * publish a message on this topic.
     *
     * This version of advertise is a templated convenience function, and can be used like so
     *
     *   ros::Publisher pub = handle.advertise<std_msgs::Empty>("my_topic", 1);
     *
     * \param topic Topic to advertise on
     *
     * \param queue_size Maximum number of outgoing messages to be
     * queued for delivery to subscribers
     *
     * \param latch (optional) If true, the last message published on
     * this topic will be saved and sent to new subscribers when they
     * connect
     *
     * \return On success, a Publisher that, when it goes out of scope,
     * will automatically release a reference on this advertisement.  On
     * failure, an empty Publisher.
     *
     * \throws InvalidNameException If the topic name begins with a
     * tilde, or is an otherwise invalid graph resource name, or is an
     * otherwise invalid graph resource name
     */
    template <class M>
    Publisher advertise(const std::string& topic, uint32_t queue_size, bool latch = false)
    {
      AdvertiseOptions ops;
      ops.template init<M>(topic, queue_size);
      ops.latch = latch;
      return advertise(ops);
    }

         Generally, the receiving node cannot receive data until the publishing node is started , But the starting sequence is sometimes out of control , Some data will be lost , Especially for maps that are released only once . When the publishing data end is set to latch = true, The late node can also receive the last frame message before ． Another particularly useful thing is publishing rviz Visual data debugger , according to namesapce When analyzing some results , You can check repeatedly to show some items , Just in rviz Check this again in the Display Can , No need to replay the data ．

         Here's the picture , Play first bag data , After suspension , stay rviz Check to display