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MonitorState (ROS)
Description: Monitor a state!Tutorial Level: INTERMEDIATE
NB - This is just what I coded up while trying to understand the MonitorState. Please don't take it for best practices or anything, until JonBohren or somebody with more experience takes a look at it. -LauraLindzey
Contents
Setup
Here's an example of a simple state machine using MonitorState. In order to transition from state Foo to state Bar, send a message to the /sm_reset topic:
rostopic pub -1 /sm_reset std_msgs/Empty
The Code
1 #!/usr/bin/env python
2 import roslib; roslib.load_manifest('smach_MonitorState_example')
3 import rospy
4 import smach
5 import smach_ros
6
7 from std_msgs.msg import Empty
8
9 class bar(smach.State):
10 def __init__(self):
11 smach.State.__init__(self, outcomes=['bar_succeeded'])
12 def execute(self, userdata):
13 rospy.sleep(3.0)
14 return 'bar_succeeded'
15
16 def monitor_cb(ud, msg):
17 return False
18
19 def main():
20 rospy.init_node("monitor_example")
21
22 sm = smach.StateMachine(outcomes=['DONE'])
23 with sm:
24 smach.StateMachine.add('FOO', smach_ros.MonitorState("/sm_reset", Empty, monitor_cb), transitions={'invalid':'BAR', 'valid':'FOO', 'preempted':'FOO'})
25 smach.StateMachine.add('BAR',bar(), transitions={'bar_succeeded':'FOO'})
26
27 sis = smach_ros.IntrospectionServer('smach_server', sm, '/SM_ROOT')
28 sis.start()
29 sm.execute()
30 rospy.spin()
31 sis.stop()
32
33 if __name__=="__main__":
34 main()
Code with Notes
Arguments are the userdata and the message. This needs to return False when we want the monitor state to terminate. In this case, the monitor state will return 'invalid'
Note that your MonitorState needs to define transitions for all three possible outcomes ('valid', 'invalid', 'preempted').
From the source code for MonitorState:
def __init__(self, topic, msg_type, cond_cb, max_checks=-1)
The max_checks argument is a limit on how many times the monitor_cb can be called before the MonitorState will return 'valid'.
