Ros node Startup and shutdown
1. ros Operating unit ：
Ros The unit of program operation is ros node.
2. ros Start of node ：
（1） initialization ros node ： By calling ros::init() Interface implementation ; The default node name can be specified by a parameter , The reason is default , It's because when you execute a program on the command line , The node name can be modified .
（2） start-up ros node ： By calling ros::NodeHnadle establish NodeHnadle example nh realization , When you first create an instance , Will call ros::start(), Start this node at this time .
3. ros Node shutdown ：
（1） By calling ros::shutdown() Interface , close ros node , This interface turns off the subscription topic , Release topic , Request service and provide service .
（2） In the last one ros::NodeHnadle When the object is destroyed , Will be called automatically ros::shutdown(), To close ros node .
4. testing ros Is the node down ：
（1）ros::ok() Interface ： return false, Indicates that the node is down .
（2）ros::isShuttingDown() Interface ： return true, explain ros::shutdown() Called , But the node ratio must end .
notes ： So it is generally used ros::ok() judge , The node that ends at but doesn't end at , for example “ prolonged service
callback” in , You can only use ros::isShuttingDown(), Because this node will not be shut down , I wonder what node this is ?
5. interrupt ：
（1） stay ros::init() Options , There is an option ros::init_options::NoSigintHandler, Decide whether to install the default SIGINT Processing function , If this parameter is not used , Then install , The program received SIGINT signal （ctrl+c） Time ,ros::ok() return false, Nodes can go through the closing process . If this parameter is used , You need to install it yourself SIGINT Processing function , To ensure that the node can end .
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