ROS study notes 7- understand the services (services) and parameters (parameter)

In this article from the official Wiki: http://wiki.ros.org/ROS/Tutorials/UnderstandingServicesParams

ROS service (service)
service is a communication mechanism in ROS, but different from the publish and subscribe theme, the service adopted a "request (request) - response (response)" mechanism for data exchange.
And service-related functions can be carried out using the rosservice command.
```
rosservice list lists the active service information 
rosservice call the given parameter called service 
rosservice type print out the type of service 
rosservice find to find a given type of service 
rosservice uri print services ROSRPC source
```
1. rosservice list
  This command lists the services that are currently active, such as in the example of a small turtle, we run rosservice list
  returns the following:
```
$ rosservice list
/clear
/kill
/reset
/rosout/get_loggers
/rosout/set_logger_level
/rostopic_21128_1568429034667/get_loggers
/rostopic_21128_1568429034667/set_logger_level
/rostopic_25467_1568471203069/get_loggers
/rostopic_25467_1568471203069/set_logger_level
/rqt_gui_py_node_25560/get_loggers
/rqt_gui_py_node_25560/set_logger_level
/rqt_gui_py_node_25926/get_loggers
/rqt_gui_py_node_25926/set_logger_level
/spawn
/teleop_turtle/get_loggers
/teleop_turtle/set_logger_level
/turtle1/set_pen
/turtle1/teleport_absolute
/turtle1/teleport_relative
/turtlesim/get_loggers
/turtlesim/set_logger_level
```
2. rosservice type
  then use rosservice type can view the type of service:
```
$ rosservice type /clear
std_srvs/Empty
```
  Empty indicates that the service is returned without specifying parameters, a feature that is only that action, without data.
3. rosservice call
  using rosservice call can invoke the service.
  Usage is as follows:
```
rosservice call [service] [args]
```
  For example, want to call / clear, then use the following statement:
```
rosservice call /clear
```
  It will be removed leaving a small turtle movement track.
  
  We can look at the parameters of service / spwan with parameters:
```
$ rosservice type /spawn | rossrv show
float32 x
float32 y
float32 theta
string name
---
string name
```
  It can be seen / spawn service parameters for the type of data represented by three float32 position (x, y) and facing and theta represents the name of a string name.
  Call the service with the following command:
```
rosservice call /spawn 2 2 0.2 ""
```
  Another is to produce a small turtle:
ROS parameters (rosparam)
rosparam allows ROS parameter server ( the Parameter Server storage) and operating parameters (like personal understanding global variable programming). Server parameters to take YAML markup language to store the following data types:
integers (integer), floats (float), boolean (Boolean), dictionaries (dictionaries (key type)), and lists (list type). Parameters related operations using rosparam command is used as follows:
```
rosparam set the parameter value 
rosparam get get parameter values 
rosparam load Load parameters from file 
rosparam dump storage parameters to file 
rosparam delete delete parameters 
rosparam list parameter names are listed
```
1. rosparam list
  
  This command can list all parameter names, such as:
```
$ rosparam list
/background_b
/background_g
/background_r
/rosdistro
/roslaunch/uris/host_localhost__35773
/rosversion
/run_id
```
2. rosparam get rosparam set and
  may set the parameter values rosparam set, for example, the following command set the background color in the red channel value of 200:
```
rosparam set /background_r 200
```
  You must then call clear service to refresh the background:
```
$ rosservice call /clear
```
  The background turns pink
  
  Use the following command to get the background of green:
```
$ rosparam get /background_g
86
```
  Use rosparam get / all command parameter values available:
```
$ rosparam get /
background_b: 255
background_g: 86
background_r: 200
rosdistro: 'kinetic

  '
roslaunch:
  uris: {host_localhost__35773: 'http://localhost:35773/'}
rosversion: '1.12.14

  '
run_id: d0977b84-d694-11e9-b3cd-502b73e82f34
```
3. rosparam dump and rosparam load
  can rosparam dump storage parameters, rosparam load loading parameters:
```
rosparam dump [file_name] [namespace]
rosparam load [file_name] [namespace]
```
  You can specify a file name and namespace parameters
  following command to write the file params.yaml all parameters of the current namespace.
```
$ rosparam dump params.yaml
```
  Load all parameters following command to copy the file namespace from params.yaml.
```
$ rosparam load params.yaml copy
```
  Then read the parameter space:
```
$ rosparam get /copy/background_b
255
```

ROS study notes 7- understand the services (services) and parameters (parameter)

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