ROS Theoretical Question - How does work properly a node

Question

I'm new at ROS and I'm trying to figure out how does the node works with a pyton/C++ code. I have two codes for the same situation:

Use a Xbox Controller to move the turtle in turtlesim

FIRST - Using class python and a while as a main:

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
from sensor_msgs.msg import Joy

# DEFINING CLASS
class pubSub:
    def __init__(self):
        # DEFINE VARIABLE TO KEEP LAST VALUE
        rospy.init_node('c2t')
        self.twt = Twist()
        self.hold = Twist()

    def callback_function(self,joyData):
        self.twt.linear.x = 4*joyData.axes[1]
        self.twt.angular.z = 4*joyData.axes[0]
        self.hold = self.twt

    def assuming_cont(self):
        pub = rospy.Publisher('turtle1/cmd_vel', Twist, queue_size=10)
        rospy.Subscriber("joy", Joy, self.callback_function, queue_size=10)
        pub.publish(self.twt)

ps = pubSub()
while not rospy.is_shutdown():
    ps.assuming_cont()

SECOND - Using global variables and a if __name__ == '__main__' with rospy.spin()

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist 
from sensor_msgs.msg import Joy 

# FUNCTION - GET JOY VALUES TRANSFORM TO TURTLE MOVE
def joy_to_twist_transform(jData):
    twt = Twist()
    twt.linear.x = 5*jData.axes[1]
    twt.angular.z = 4*jData.axes[0]
    pub.publish(twt)

# FUNCTION - START PROCESS
def process():
    global pub
    pub = rospy.Publisher('turtle1/cmd_vel', Twist, queue_size=10)
    sub = rospy.Subscriber("joy", Joy, joy_to_twist_transform, queue_size=1)
    rospy.init_node('control_2turtle')
    rospy.spin()

# MAIN
if __name__  == '__main__':
    try:
        process()
    except rospy.ROSInterruptException:
        pass

My questions are more theoretical:

1 - What I understand about how a node works is: he run the code in a loop until is shutdown. But this does not answer Why can I still save the last value published in a topic (in my head, if I run my code in a loop I start from the beginning everytime)

2 - Why is necessary to keep creating a rospy.Publisher and rospy.Subscriber in every loop? it is possible to create those guy one time only?(for example using them into init function in the seconde example)

3 - What is the difference betweenwhile not rospy.is_shutdown(): and if __name__ == '__main__' with rospy.spin()?

4 - What is rospy.spin()? in any documentation says that keeps your node running, but the way I see he stuck the code in that line and keep there only.

If any could give any recommendation to read about those problems I would thanks a lot.

Answer 1

if __name__ == '__main__': doesn't matter at here (in the second code), also you could run your code without it like here:

Second code:

.
.
.
# MAIN
try:
    process()
except rospy.ROSInterruptException:
    pass

i.e. when you don't want your code to run when it imported in another python code, you should use if __name__ == '__main__':

More information.

---

The main difference between First code and Second code is that the Second code used the rospy.spin() method which is a built-in ROS loop that triggers the publisher and subscriber in another thread, so you wouldn't need to an inner while in your code like the First example.

Relevant Question.

Answer 2

Well, I made this question in ROS Answers and I made the answer (I got some experience by now): answer