Watchdog Listener Script

The watchdog listener monitors changes in the barcodes.txt file and sends those changes back to the robot and shuffleboard. The watchdog listener is also used to read the networktables and see if a new barcode must be read.

Imports

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from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler
from networktables import NetworkTables
from networktables.util import ntproperty
import threading
import os

There are a few more imports for this script over the barcode script.

  • Line 4 is the observer import and is used to create the listener for a file.

  • Line 5 is the FileSystemEventHandler which creates functions that check for changes to files

  • Line 6 is the main NetworkTables import, used to send and receive info from the robot.

  • Line 7 is the ntproperty import and is used to create tables and properties.

  • Line 8 is the thread import to create threads.

  • Line 9 is the os import and used for sending commands to the terminal.

Create Barcodes File

The watchdog script will be run as a startup script, which makes it a good idea to create the barcodes.txt file if it does not exist to avoid errors when reading the file.

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f = open('/home/pi/barcodes.txt', 'w')
f.close()

  • Line 12 will create the barcodes.txt if it does not exist

  • Line 13 closes the file to prevent issues of the file being open when it should be closed

Connect to NetworkTables

Before anything can happen, a connection to NetworkTables needs to be established. NetworkTables does not run right away and needs some time for the server and client to start. The below code handles this.

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#Create thread to make sure networktables is connected
cond = threading.Condition()
notified = [False]

#Create a listener
def connectionListener(connected, info):
    with cond:
        notified[0] = True
        cond.notify()

#Instantiate NetworkTables
NetworkTables.initialize(server="10.12.34.2")
NetworkTables.addConnectionListener(connectionListener, immediateNotify=True)

#Wait until connected
with cond:
    if not notified[0]:
        cond.wait()

The above may look complicated, but it is pretty simple.

  • Line 16 creates a conditional thread

  • Line 17 creates a boolean array

  • Line 20 defines a new function call connectionListener; this function listens for a connection and changes the condition when connected.

  • Lines 21 - 23 is a statement that when connected to update conditions

  • Line 26 will initialize a connection

  • Line 27 adds a listener to check if connected

  • Lines 30 - 32 will hang until a connection is made

Important

Make sure the IP address used in line 26 matches the IP address of the VMX WiFi AP.

Create the Vision Tables

To successfully send data between the watchdog and the robot code, it is good to create a couple of properties to hold this data. The properties can be read on the watchdog side and the robot side.

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ntBarcodeData = ntproperty('/Vision/barcodeData', "null")
ntBarcodeType = ntproperty('/Vision/barcodeType', "null")
ntReadBarcode = ntproperty('/Vision/readBarcode', False)

#Get Table
table = NetworkTables.getTable('Vision')

  • Lines 35 & 36 create the barcode properties.

  • Line 37 creates the command property used for executing the barcode script for a new barcode.

  • Line 40 assigns the Vision table to a variable for later use.

The first value of property is the key and the second is the default value. The default value also creates the property type. In the above cases ntBarcodeData & ntBarcodeType will be strings, whereas ntReadBarcode is a boolean.

Note

When creating a table, the key of the table must always start with a /.

File System Handler

To optimize the code that it does not open, read, close the code continuously. A FileSystemEventHandler can be used. In this case, the use of watchdog is perfect. This way, nothing will happen unless the file is modified. If there is no modification to the file, i.e., a new barcode is read, it will do nothing.

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class MyHandler(FileSystemEventHandler):
    def on_modified(self, event):
        try:
            file = open('./home/pi/barcodes.txt', 'r')
            table.putString('barcodeData', file.readline())
            table.putString('barcodeType', file.readline())
            file.close()
        except:
            pass #when file is not created yet

event_handler = MyHandler()
observer = Observer()
observer.schedule(event_handler, path='./home/pi/barcodes.txt', recursive=False)
observer.start()

  • Line 43 creates a class that uses the FileSystemEventHandler

  • Line 44 creates the function on_modified which is an extension of the FileSystemEventHandler. This function will be called when the event handler detects a modification to the file.

  • Line 45 & 50 is used to catch errors.

  • Line 46 opens the barcodes.txt in read mode.

  • Line 47 will read the first line of the file and add it as the barcodeData data.

  • Line 48 will read the second line of the file and add it as the barcodeType data.

  • Line 49 closes the file to ensure no issues.

  • Line 53 creates the event handler

  • Line 54 creates the observer

  • Line 55 configures the observer with the event handler and file to watch

  • Line 56 starts the observer thread

Forever Loop

This script needs to run forever and handle a flag sent from the robot to take a new barcode reading.

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while(True):
    if table.getBoolean('readBarcode', False) == True:
        table.putBoolean('readBarcode', False)
        os.system('python3 /home/pi/readBarcode.py')
    try:
        pass
    except KeyboardInterrupt:
        observer.stop()

  • Line 59 is the while loop that never ends

  • Line 60 checks to see if the robot code is requesting a new barcode scan.

  • Line 61 flips the readBarcode flag to False to prevent a double read.

  • Line 62 runs the readBarcode.py script

  • Lines 63 - 66 are used if the script ever wants to end. In this case, a KeyboardInterrupt is required to end. As this script will run as a startup script, the observer should never end.

Setting the Script to run as at Startup

Setting the watchdog script to run at startup is very simple.

In terminal open rc.local

sudo nano /etc/rc.local

Scroll to the bottom with the arrow keys.

Above exit 0 put:

python3 /home/pi/watchdogListener.py &

To save, hit CTRL + X then Y and hit Enter.

Important

Including the & at the end will fork the process to the background and prevent other processes from hanging.

It is now possible to reboot, and the script will be running.

Note

There will be no indication that the script is running.

Full Script

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#!/usr/bin/python3

#imports
from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler
from networktables import NetworkTables
from networktables.util import ntproperty
import threading
import os

#Create the barcodes file
f = open('/home/pi/barcodes.txt', 'w')
f.close()

#Create thread to make sure networktables is connected
cond = threading.Condition()
notified = [False]

#Create a listener
def connectionListener(connected, info):
    with cond:
        notified[0] = True
        cond.notify()

#Instantiate NetworkTables
NetworkTables.initialize(server="10.12.34.2")
NetworkTables.addConnectionListener(connectionListener, immediateNotify=True)

#Wait until connected
with cond:
    if not notified[0]:
        cond.wait()

#Create the vision Table
ntBarcodeData = ntproperty('/Vision/barcodeData', "null")
ntBarcodeType = ntproperty('/Vision/barcodeType', "null")
ntReadBarcode = ntproperty('/Vision/readBarcode', False)

#Get Table
table = NetworkTables.getTable('Vision')

#Create the system handler
class MyHandler(FileSystemEventHandler):
    def on_modified(self, event):
        try:
            file = open('./home/pi/barcodes.txt', 'r')
            table.putString('barcodeData', file.readline())
            table.putString('barcodeType', file.readline())
            file.close()
        except:
            pass #when file is not created yet

event_handler = MyHandler()
observer = Observer()
observer.schedule(event_handler, path='./home/pi/barcodes.txt', recursive=False)
observer.start()

#The forever loop
while(True):
    if table.getBoolean('readBarcode', False) == True:
        table.putBoolean('readBarcode', False)
        os.system('python3 /home/pi/readBarcode.py')
    try:
        pass
    except KeyboardInterrupt:
        observer.stop()