how to use Twist with Timer()

edit

I'd like to make the robot moves a certain time toward x-direction using Twist with Timer

full code:

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
class GoForward():
    def __init__(self):
        rospy.init_node('GoForward', anonymous=False)
        rospy.on_shutdown(self.shutdown)
        self.cmd_vel = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
        rospy.Timer(rospy.Duration(2), self.my_callback)
    def my_callback(self, event):
        move_cmd = Twist()
        move_cmd.linear.x = 0.2
        move_cmd.angular.z = 0
        self.cmd_vel.publish(move_cmd)
    def shutdown(self):
        rospy.loginfo("Stop TurtleBot")
        self.cmd_vel.publish(Twist())
        rospy.sleep(1)
if __name__ == '__main__':
    try:
        GoForward()
except:
    rospy.loginfo("GoForward node terminated.")

What is wrong, the code stops without continue.

any help or suggestions, it would be appreciated.

UPDATE

I modified the code to:

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
class GoForward():
    def __init__(self):
        rospy.init_node('GoForward', anonymous=False)
        rospy.on_shutdown(self.shutdown)
        self.cmd_vel = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
        event_period = rospy.get_param('~event_period', 0.02)
        rospy.Timer(rospy.Duration(event_period), self.my_callback)
    def my_callback(self, event):
        move_cmd = Twist()
        move_cmd.linear.x = 0.2
        move_cmd.angular.z = 0
        self.cmd_vel.publish(move_cmd)
    def shutdown(self):
        rospy.loginfo("Stop TurtleBot")
        self.cmd_vel.publish(Twist())
        rospy.sleep(1)
if __name__ == '__main__':
    try:
        GoForward()
        rospy.spin()
    except:
        rospy.loginfo("GoForward node terminated.")

the output:

the robot moved without stopping.