MQTT Plotter (PyQt5)

This utility GUI application is a tool for visualizing the content of multiple data streams passed as short numeric messages back and forth across the network via a MQTT server. It supports opening an authenticated connection to the server, subscribing to a class of messages in order to receive them, viewing message traffic, and plotting the received messages as points in a dynamically updated graphic.

The expected use is that each message will be generated by an Arduino sketch and forwarded to the MQTT network using the Arduino-MQTT Bridge (PyQt5) application. The following section documents the message formatting options.

Data Format

Each message received is processed as plain text integer numbers separated by spaces. Message lengths of either two or five value: with two values, they are interpreted as the X and Y location (“X Y”), with five values as a position and an RGB color (“X Y R G B”). All values may range from 0 to 100 inclusive. The topic name is used to identify the point, so multiple messages on the same topic will dynamically move a plot point. Some sample messages follow.

message text	graphical result on a plotted point
0 0	move the point to the lower left corner
50 50	move the point to the center
100 50 100 0 0	move to right edge and paint it red
75 75 0 0 100	move near upper right and paint it blue

Installation Requirements

The code requires a working installation of Python 3 with PyQt5 and paho-mqtt. For suggestions on setting up your system please see Python 3 Installation.

User Guide

The connection GUI is derived from MQTT Monitor (PyQt5). The procedure for use generally follows this sequence:

1. Launch the qt_mqtt_plotter.py program using Python 3.

2. Select the port corresponding to your course number.

3. Enter the username and password provided by your instructor.

4. Verify that the server address is mqtt.ideate.cmu.edu, then click Connect.

5. Enter a subscription pattern according to your needs. This tool is designed to display all messages, so the default wildcard # is typical for capturing all messages. However, a pattern such as username/# will limit displayed traffic to a particular sender.

6. Verify that data messages are appearing in the text area.

7. The plot tab will then show an animated graphic in which each topic stream is represented by a single moving point with 2D location and 3D color.

Source Code Documentation

If you’re curious about the inner workings of the app, the rest of this page provides detailed documentation. This section is not necessary for using the system.

MainApp

class mqtt.qt_mqtt_plotter.MainApp

Main application object holding any non-GUI related state.

MainGUI

class mqtt.qt_mqtt_plotter.MainGUI(main, *args, **kwargs)

A custom main window which provides all GUI controls. Requires a delegate main application object to handle user requests.

closeEvent(self, QCloseEvent)

write(string)

Write output to the console text area in a thread-safe way. Qt only allows calls from the main thread, but the service routines run on separate threads.

QtParticles

class mqtt.qt_mqtt_plotter.QtParticles

Custom widget to draw a 2D plot of a set of particles. Each particle has 5-D state: [x, y, r, g, b]. Each axis is defined to have unit scaling and is valid on [0,1].

paintEvent(e)

Subclass implementation of parent QWidget class callback to repaint the graphics.

Full Code

#!/usr/bin/env python3
"""A PyQt5 GUI utility to monitor and plot MQTT server messages."""

################################################################
# Written in 2018-2020 by Garth Zeglin <garthz@cmu.edu>

# To the extent possible under law, the author has dedicated all copyright
# and related and neighboring rights to this software to the public domain
# worldwide. This software is distributed without any warranty.

# You should have received a copy of the CC0 Public Domain Dedication along with this software.
# If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.

################################################################
# standard Python libraries
from __future__ import print_function
import os, sys, struct, time, logging, functools, queue, signal, getpass, math

# documentation: https://doc.qt.io/qt-5/index.html
# documentation: https://www.riverbankcomputing.com/static/Docs/PyQt5/index.html
from PyQt5 import QtCore, QtGui, QtWidgets, QtNetwork

# documentation: https://www.eclipse.org/paho/clients/python/docs/
import paho.mqtt.client as mqtt

# default logging output
log = logging.getLogger('main')

# logger to pass to the MQTT library
mqtt_log = logging.getLogger('mqtt')
mqtt_log.setLevel(logging.WARNING)

# IDeATE server instances, as per https://mqtt.ideate.cmu.edu/#ports

ideate_ports = { 8884 : '16-223',
                 8885 : '16-375',
                 8886 : '60-223',
                 8887 : '62-362',
}

mqtt_rc_codes = ['Success', 'Incorrect protocol version', 'Invalid client identifier', 'Server unavailable', 'Bad username or password', 'Not authorized']

################################################################
class QtParticles(QtWidgets.QWidget):
    """Custom widget to draw a 2D plot of a set of particles.  Each particle has 5-D
       state: [x, y, r, g, b].  Each axis is defined to have unit scaling and is
       valid on [0,1].
    """

    def __init__(self):
        super().__init__()
        self.setMinimumSize(QtCore.QSize(100, 100))
        self.setAutoFillBackground(True)

        # Graphical state variables. Define a placeholder point while testing.
        # self.particles  = {'placeholder' : [0.5, 0.5, 0.5, 0.5, 0.5] }
        self.particles  = {}

        # finish initialization
        self.show()
        return

    # === particle update methods ============================================================
    def update_particle_position(self, name, location):
        # limit the coordinate range to slightly larger than the plotting bounds
        x = min(max(location[0], -0.1), 1.1)
        y = min(max(location[1], -0.1), 1.1)

        particle = self.particles.get(name)
        if particle is None:
            self.particles[name] = [x, y, 0.5, 0.5, 0.5]
        else:
            particle[0:2] = x, y
        self.repaint()

    def update_particle_color(self, name, rgb):
        particle = self.particles.get(name)
        if particle is None:
            self.particles[name] = [0.0, 0.0, rgb[0], rgb[1], rgb[2]]
        else:
            particle[2:5] = rgb
        self.repaint()

    # === Qt API methods ============================================================
    def paintEvent(self, e):
        """Subclass implementation of parent QWidget class callback to repaint the graphics."""
        geometry = self.geometry()
        view_width = geometry.width()
        view_height = geometry.height()

        # Clear the background.
        qp = QtGui.QPainter()
        qp.begin(self)
        qp.fillRect(QtCore.QRectF(0, 0, view_width, view_height), QtCore.Qt.white)
        # qp.setRenderHint(QtGui.QPainter.Antialiasing)

        # Set up a coordinate system scaled to unit dimension that keeps the
        # minimum visible area in view.
        scene_width   = 1.3            # minimum visible width
        scene_height  = scene_width    # minimum visible height
        scene_aspect  = scene_width / scene_height
        view_aspect = view_width / view_height
        if scene_aspect > view_aspect:
            scaling = view_width / scene_width
        else:
            scaling = view_height/scene_height

        # Capture the default graphics transformation.
        qp.save()

        # Move the origin to the center (in pixel coordinates).
        qp.translate(QtCore.QPointF(view_width/2, view_height/2))

        # Apply scaling to draw in unit coordinates.
        qp.scale(scaling, scaling)

        # Translate in the new scaled coordinates to place the origin near the
        # upper left corner; the default coordinates using +Y pointing down.
        qp.translate(QtCore.QPointF(-0.5, -0.5))

        # Draw the bounds of the unit square.
        pen = QtGui.QPen(QtCore.Qt.black)
        pen.setWidthF(0.005)
        qp.setPen(pen)
        qp.drawRect(QtCore.QRectF(0.0, 0.0, 1.0, 1.0))

        # Draw the particles.  The Y dimension is inverted to a normal
        # mathematical plot with +Y up.
        color = QtGui.QColor()
        for particle in self.particles.values():
            color.setRgbF(min(1.0, abs(particle[2])), min(1.0, abs(particle[3])), min(1.0, abs(particle[4])), 1.0)
            brush = QtGui.QBrush(color)
            qp.setBrush(brush)
            qp.drawEllipse(QtCore.QPointF(particle[0], 1.0 - particle[1]), 0.02, 0.02)

        # Restore the initial unscaled coordinates.
        qp.restore()
        qp.end()


################################################################
class MainGUI(QtWidgets.QMainWindow):
    """A custom main window which provides all GUI controls.  Requires a delegate main application object to handle user requests."""

    def __init__(self, main, *args, **kwargs):
        super(MainGUI,self).__init__()

        # save the main object for delegating GUI events
        self.main = main

        # create the GUI elements
        self.console_queue = queue.Queue()
        self.setupUi()

        self._handler = None
        self.enable_console_logging()

        # finish initialization
        self.show()

        # manage the console output across threads
        self.console_timer = QtCore.QTimer()
        self.console_timer.timeout.connect(self._poll_console_queue)
        self.console_timer.start(50)  # units are milliseconds

        return

    # ------------------------------------------------------------------------------------------------
    def setupUi(self):
        self.setWindowTitle("IDeATe MQTT Plotter")
        self.resize(600, 600)

        # set up tabbed page structure
        self.tabs = QtWidgets.QTabWidget()
        self.setCentralWidget(self.tabs)

        # set up a main tab with the connection controls
        self.mainTab = QtWidgets.QWidget(self)
        self.tabs.addTab(self.mainTab, 'Main')
        self.verticalLayout = QtWidgets.QVBoxLayout(self.mainTab)
        self.verticalLayout.setContentsMargins(-1, -1, -1, 9) # left, top, right, bottom

        # generate GUI for configuring the MQTT connection

        # server name entry and port selection
        hbox = QtWidgets.QHBoxLayout()
        self.verticalLayout.addLayout(hbox)
        hbox.addWidget(QtWidgets.QLabel("MQTT server address:"))
        self.mqtt_server_name = QtWidgets.QLineEdit()
        self.mqtt_server_name.setText(str(self.main.hostname))
        self.mqtt_server_name.editingFinished.connect(self.mqtt_server_name_entered)
        hbox.addWidget(self.mqtt_server_name)

        hbox.addWidget(QtWidgets.QLabel("port:"))
        self.port_selector = QtWidgets.QComboBox()
        hbox.addWidget(self.port_selector)

        self.port_selector.addItem("")
        for pairs in ideate_ports.items():
            self.port_selector.addItem("%d (%s)" % pairs)
        self.port_selector.activated['QString'].connect(self.mqtt_port_selected)

        # attempt to pre-select the stored port number
        try:
            idx = list(ideate_ports.keys()).index(self.main.portnum)
            self.port_selector.setCurrentIndex(idx+1)
        except ValueError:
            pass

        # instructions
        explanation = QtWidgets.QLabel("""Username and password provided by instructor.  Please see help tab for details.""")
        explanation.setWordWrap(True)
        self.verticalLayout.addWidget(explanation)

        # user and password entry
        hbox = QtWidgets.QHBoxLayout()
        self.verticalLayout.addLayout(hbox)
        hbox.addWidget(QtWidgets.QLabel("MQTT username:"))
        self.mqtt_username = QtWidgets.QLineEdit()
        self.mqtt_username.setText(str(self.main.username))
        self.mqtt_username.editingFinished.connect(self.mqtt_username_entered)
        hbox.addWidget(self.mqtt_username)

        hbox.addWidget(QtWidgets.QLabel("password:"))
        self.mqtt_password = QtWidgets.QLineEdit()
        self.mqtt_password.setText(str(self.main.password))
        self.mqtt_password.editingFinished.connect(self.mqtt_password_entered)
        hbox.addWidget(self.mqtt_password)

        # instructions
        explanation = QtWidgets.QLabel("""A subscription specifies topics to receive.  Please see help tab for details.""")
        explanation.setWordWrap(True)
        self.verticalLayout.addWidget(explanation)

        # subscription topic entry
        hbox = QtWidgets.QHBoxLayout()
        label = QtWidgets.QLabel("MQTT message subscription:")
        self.mqtt_sub = QtWidgets.QLineEdit()
        self.mqtt_sub.setText(self.main.subscription)
        self.mqtt_sub.editingFinished.connect(self.mqtt_sub_entered)
        hbox.addWidget(label)
        hbox.addWidget(self.mqtt_sub)
        self.verticalLayout.addLayout(hbox)

        # connection indicator
        self.connected = QtWidgets.QLabel()
        self.connected.setLineWidth(3)
        self.connected.setFrameStyle(QtWidgets.QFrame.Box)
        self.connected.setAlignment(QtCore.Qt.AlignCenter)
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Fixed, QtWidgets.QSizePolicy.Fixed)
        self.connected.setSizePolicy(sizePolicy)
        self.set_connected_state(False)

        # connection control buttons
        connect = QtWidgets.QPushButton('Connect')
        connect.pressed.connect(self.connection_requested)
        disconnect = QtWidgets.QPushButton('Disconnect')
        disconnect.pressed.connect(self.main.disconnect_from_mqtt_server)
        hbox = QtWidgets.QHBoxLayout()
        hbox.addWidget(self.connected)
        hbox.addWidget(connect)
        hbox.addWidget(disconnect)
        self.verticalLayout.addLayout(hbox)

        # text area for displaying both internal and received messages
        self.consoleOutput = QtWidgets.QPlainTextEdit()
        self.consoleOutput.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarAsNeeded)
        self.verticalLayout.addWidget(self.consoleOutput)

        # set up the graphics tab
        self.plot = QtParticles()
        self.tabs.addTab(self.plot, 'Plot')

        # set up the help tab
        self.helpTab = QtWidgets.QWidget(self)
        self.tabs.addTab(self.helpTab, 'Help')
        self._make_help(self.helpTab)

        # set up the status bar which appears at the bottom of the window
        self.statusbar = QtWidgets.QStatusBar(self)
        self.setStatusBar(self.statusbar)

        # set up the main menu
        self.menubar = QtWidgets.QMenuBar(self)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 500, 22))
        self.menubar.setNativeMenuBar(False)
        self.menubar.setObjectName("menubar")
        self.menuTitle = QtWidgets.QMenu(self.menubar)
        self.setMenuBar(self.menubar)
        self.actionQuit = QtWidgets.QAction(self)
        self.menuTitle.addAction(self.actionQuit)
        self.menubar.addAction(self.menuTitle.menuAction())
        self.menuTitle.setTitle("File")
        self.actionQuit.setText("Quit")
        self.actionQuit.setShortcut("Ctrl+Q")
        self.actionQuit.triggered.connect(self.quitSelected)

        return

    # --- verbose function to create the help tab -----------------------------------------
    def _make_help(self, parent):
        vbox = QtWidgets.QVBoxLayout(parent)
        hbox = QtWidgets.QHBoxLayout()
        vbox.addLayout(hbox)
        text = QtWidgets.QTextEdit()
        hbox.addWidget(text)
        text.insertHtml("""
<style type="text/css">
table { margin-left: 20px; }
td { padding-left: 20px; }
</style>
<a href="#top"></a>
<h1>IDeATe MQTT Plotter</h1>
<p>This Python application is a tool intended for visualizing the content of multiple data streams passed as short numeric messages back and forth across the network via a MQTT server.  It supports opening an authenticated connection to the server, subscribing to a class of messages in order to receive them, viewing message traffic, and plotting the received messages as points in a dynamically updated graphic.</p>
<h2>Connecting</h2>
<p>The first set of controls configures server parameters before attempting a connection.  Changes will not take effect until the next connection attempt.</p

<dl>
  <dt>server address</dt><dd>The network name of the MQTT server. (Defaults to mqtt.ideate.cmu.edu.)</dd>
  <dt>server port</dt><dd>The numeric port number for the MQTT server.  IDeATe is using a separate server for each course, so the drop-down menu also identifies the associated course number.</dd>
  <dt>username</dt><dd>Server-specific identity, chosen by your instructor.</dd>
  <dt>password</dt><dd>Server-specific password, chosen by your instructor.</dd>
</dl>

<p>Your username and password is specific to the MQTT server and will be provided by your instructor.  This may be individual or may be a shared login for all students in the course.  Please note, the password will not be your Andrew password.</p>

<h2>Listening</h2>

<p>MQTT works on a publish/subscribe model in which messages are published on <i>topics</i> identified by a topic name.  The name is structured like a path string separated by <tt>/</tt> characters to organize messages into a hierarchy of topics and subtopics.
Our course policy will be to prefix all topics with a student andrew ID, e.g. if your user name is xyzzy, we ask that you publish on the 'xyzzy' topic and sub-topics, as per the following examples.</p>


<p>
<table>
<tr><td><b>xyzzy</b></td><td>top-level topic on which user 'xyzzy' should publish</td></tr>
<tr><td><b>xyzzy/status</b></td><td>a sub-topic on which user 'xyzzy' could publish</td></tr>
<tr><td><b>xyzzy/sensor</b></td><td>another sub-topic on which user 'xyzzy' could publish</td></tr>
<tr><td><b>xyzzy/sensor/1</b></td><td>a possible sub-sub-topic</td></tr>
</table>
</p>

<p>The message subscription field specifies topics to receive.  The subscription may include a # character as a wildcard, as per the following examples.</p>
<p><table>
<tr><td><b>#</b></td><td>subscribe to all messages (typical for this application)</td></tr>
<tr><td><b>xyzzy</b></td><td>subscribe to the top-level published messages for user xyzzy</td></tr>
<tr><td><b>xyzzy/#</b></td><td>subscribe to all published messages for user xyzzy, including subtopics</td></tr>
</table>
</p>
<p>Changing the subscription field immediately changes what is received; the monitor unsubscribes from the previous pattern and subscribes to the new one.  Entering an empty field defaults to the global pattern '#'.</p>

<p>The large text field is the console area which shows both debugging and status log messages as well as received messages.</p>

<h2>Data Format</h2>

<p>Each message received is processed as plain text integer numbers separated by spaces.  Either two or five value messages are supported: with two values, they are interpreted as the X and Y location ("X Y"), with five values as a position and an RGB color ("X Y R G B").  All values may range from 0 to 100 inclusive.  The topic name is used to identify the point, so multiple messages on the same topic will dynamically move a plot point.  Some sample messages follow.</p>

<p><table>
<tr><td><b>0 0</b></td><td>move the point to the lower left corner</td></tr>
<tr><td><b>50 50</b></td><td>move the point to the center</td></tr>
<tr><td><b>100 50 100 0 0</b></td><td>move to right edge and paint it red</td></tr>
<tr><td><b>75 75 0 0 100</b></td><td>move near upper right and paint it blue</td></tr>
</table>
</p>

<h2>More Information</h2>

<p>The IDeATE server has more detailed information on the server help page at <b>https://mqtt.ideate.cmu.edu</b></p>

""")
        text.scrollToAnchor("top")
        text.setReadOnly(True)
        return

    # --- logging to screen -------------------------------------------------------------
    def enable_console_logging(self):
        # get the root logger to receive all logging traffic
        logger = logging.getLogger()
        # create a logging handler which writes to the console window via self.write
        handler = logging.StreamHandler(self)
        handler.setFormatter(logging.Formatter('%(levelname)s:%(name)s: %(message)s'))
        logger.addHandler(handler)
        # logger.setLevel(logging.NOTSET)
        logger.setLevel(logging.DEBUG)
        # logger.setLevel(logging.WARNING)
        handler.setLevel(logging.NOTSET)
        self._handler = handler
        log.info("Enabled logging in console window.")
        return

    def disable_console_logging(self):
        if self._handler is not None:
            logging.getLogger().removeHandler(self._handler)
            self._handler = None

    # --- window and qt event processing -------------------------------------------------------------
    def show_status(self, string):
        self.statusbar.showMessage(string)

    def _poll_console_queue(self):
        """Write any queued console text to the console text area from the main thread."""
        while not self.console_queue.empty():
            string = str(self.console_queue.get())
            stripped = string.rstrip()
            if stripped != "":
                self.consoleOutput.appendPlainText(stripped)
        return

    def write(self, string):
        """Write output to the console text area in a thread-safe way.  Qt only allows
        calls from the main thread, but the service routines run on separate threads."""
        self.console_queue.put(string)
        return

    def quitSelected(self):
        self.write("User selected quit.")
        self.close()

    def closeEvent(self, event):
        self.write("Received window close event.")
        self.main.app_is_exiting()
        self.disable_console_logging()
        super(MainGUI,self).closeEvent(event)

    def set_connected_state(self, flag):
        if flag is True:
            self.connected.setText("  Connected   ")
            self.connected.setStyleSheet("color: white; background-color: green;")
        else:
            self.connected.setText(" Not Connected ")
            self.connected.setStyleSheet("color: white; background-color: blue;")


    # --- GUI widget event processing ----------------------------------------------------------------------

    def mqtt_server_name_entered(self):
        name = self.mqtt_server_name.text()
        self.write("Server name changed: %s" % name)
        self.main.set_server_name(name)

    def decode_port_selection(self):
        title = self.port_selector.currentText()
        if title == "":
            return None
        else:
            return int(title.split()[0])  # convert the first token to a number

    def mqtt_port_selected(self, title):
        portnum  = self.decode_port_selection()
        self.write("Port selection changed: %s" % title)
        self.main.set_server_port(portnum)

    def mqtt_username_entered(self):
        name = self.mqtt_username.text()
        self.write("User name changed: %s" % name)
        self.main.set_username(name)

    def mqtt_password_entered(self):
        name = self.mqtt_password.text()
        self.write("Password changed: %s" % name)
        self.main.set_password(name)

    def connection_requested(self):
        # When the connect button is pressed, make sure all fields are up to
        # date.  It is otherwise possible to leave a text field selected with
        # unreceived changes while pressing Connect.
        hostname = self.mqtt_server_name.text()
        portnum  = self.decode_port_selection()
        username = self.mqtt_username.text()
        password = self.mqtt_password.text()

        self.main.set_server_name(hostname)
        self.main.set_server_port(portnum)
        self.main.set_username(username)
        self.main.set_password(password)

        self.main.connect_to_mqtt_server()

    def mqtt_sub_entered(self):
        sub = self.mqtt_sub.text()
        if sub == '':
            self.mqtt_sub.setText("#")
            sub = "#"

        self.write("Subscription changed to: %s" % sub)
        self.main.set_subscription(sub)

################################################################
class MainApp(QtCore.QObject):
    """Main application object holding any non-GUI related state."""

    # class variable with Qt signal used to communicate between network thread and main thread
    _messageReceived = QtCore.pyqtSignal(str, bytes, name='_messageReceived')

    def __init__(self):

        super(MainApp,self).__init__()
        # Attach a handler to the keyboard interrupt (control-C).
        signal.signal(signal.SIGINT, self._sigint_handler)

        # load any available persistent application settings
        QtCore.QCoreApplication.setOrganizationName("IDeATe")
        QtCore.QCoreApplication.setOrganizationDomain("ideate.cmu.edu")
        QtCore.QCoreApplication.setApplicationName('mqtt_plotter')
        self.settings = QtCore.QSettings()

        # uncomment to restore 'factory defaults'
        # self.settings.clear()

        # MQTT server settings
        self.hostname = self.settings.value('mqtt_host', 'mqtt.ideate.cmu.edu')
        self.portnum  = self.settings.value('mqtt_port', None)
        self.username = self.settings.value('mqtt_user', 'students')
        self.password = self.settings.value('mqtt_password', '(not yet entered)')

        # Create a default subscription based on the username.  The hash mark is a wildcard.
        username = getpass.getuser()

        # self.subscription = self.settings.value('mqtt_subscription', username + '/#')
        self.subscription = self.settings.value('mqtt_subscription', '#')

        # create the interface window
        self.window = MainGUI(self)

        # Initialize the MQTT client system
        self.client = mqtt.Client()
        self.client.enable_logger(mqtt_log)
        self.client.on_log = self.on_log
        self.client.on_connect = self.on_connect
        self.client.on_disconnect = self.on_disconnect
        self.client.on_message = self.on_message
        self.client.tls_set()

        # Connect the signal used to transfer received messages from the network server thread to the main thread.
        self._messageReceived.connect(self.process_message)

        self.window.show_status("Please set the MQTT server address and select Connect.")
        return

    ################################################################
    def app_is_exiting(self):
        if self.client.is_connected():
            self.client.disconnect()
            self.client.loop_stop()

    def _sigint_handler(self, signal, frame):
        print("Keyboard interrupt caught, running close handlers...")
        self.app_is_exiting()
        sys.exit(0)

    ################################################################
    def set_server_name(self, name):
        self.hostname = name
        self.settings.setValue('mqtt_host', name)

    def set_server_port(self, value):
        self.portnum = value
        self.settings.setValue('mqtt_port', self.portnum)

    def set_username(self, name):
        self.username = name
        self.settings.setValue('mqtt_user', name)

    def set_password(self, name):
        self.password = name
        self.settings.setValue('mqtt_password', name)

    def connect_to_mqtt_server(self):
        if self.client.is_connected():
            self.window.write("Already connected.")
        else:
            if self.portnum is None:
                log.warning("Please specify the server port before attempting connection.")
            else:
                log.debug("Initiating MQTT connection to %s:%d" % (self.hostname, self.portnum))
                self.window.write("Attempting connection.")
                self.client.username_pw_set(self.username, self.password)
                self.client.connect_async(self.hostname, self.portnum)
                self.client.loop_start()

    def disconnect_from_mqtt_server(self):
        if self.client.is_connected():
            self.client.disconnect()
        else:
            self.window.write("Not connected.")
        self.client.loop_stop()

    ################################################################
    # The callback for when the broker responds to our connection request.
    def on_connect(self, client, userdata, flags, rc):
        self.window.write("Connected to server with with flags: %s, result code: %s" % (flags, rc))

        if rc == 0:
            log.info("Connection succeeded.")

        elif rc > 0:
            if rc < len(mqtt_rc_codes):
                log.warning("Connection failed with error: %s", mqtt_rc_codes[rc])
            else:
                log.warning("Connection failed with unknown error %d", rc)

        # Subscribing in on_connect() means that if we lose the connection and reconnect then subscriptions will be renewed.
        client.subscribe(self.subscription)
        self.window.show_status("Connected.")
        self.window.set_connected_state(True)
        return

    # The callback for when the broker responds with error messages.
    def on_log(client, userdata, level, buf):
        log.debug("on_log level %s: %s", level, userdata)
        return

    def on_disconnect(self, client, userdata, rc):
        log.debug("disconnected")
        self.window.write("Disconnected from server.")
        self.window.show_status("Disconnected.")
        self.window.set_connected_state(False)

    # The callback for when a message has been received on a topic to which this
    # client is subscribed.  The message variable is a MQTTMessage that describes
    # all of the message parameters.

    # Some useful MQTTMessage fields: topic, payload, qos, retain, mid, properties.
    #   The payload is a binary string (bytes).
    #   qos is an integer quality of service indicator (0,1, or 2)
    #   mid is an integer message ID.

    # N.B. this function is called from the network server thread, but the actual
    # processing needs to happen on the main thread for graphic output.
    def on_message(self, client, userdata, msg):
        self.window.write("{%s} %s" % (msg.topic, msg.payload))
        self._messageReceived.emit(msg.topic, msg.payload)

    @QtCore.pyqtSlot(str, bytes)
    def process_message(self, topic, payload):
        # Update the particle plotter.  The name of the particle is the topic
        # name, and the data updated depends on the message format.
        name = topic

        # Parse the message text by attempting to convert all tokens to integers.  The values
        # are scaled so that the default range is 0 to 100.
        tokens = payload.split()
        try:
            values = [(0.01 * int(x)) for x in tokens]
            if len(values) > 1:
                self.window.plot.update_particle_position(name, values[0:2])
            if len(values) > 4:
                self.window.plot.update_particle_color(name, values[2:5])

        except ValueError:
            log.warning("Error parsing message into numbers: %s %s", topic, payload)

        return

    ################################################################
    def set_subscription(self, sub):
        if self.client.is_connected():
            self.client.unsubscribe(self.subscription)
            try:
                self.client.subscribe(sub)
                self.subscription = sub
                self.settings.setValue('mqtt_subscription', sub)
            except ValueError:
                self.window.write("Invalid subscription string, not changed.")
                self.client.subscribe(self.subscription)
        else:
            self.subscription = sub
            self.settings.setValue('mqtt_subscription', sub)

    ################################################################

def main():
    # Optionally add an additional root log handler to stream messages to the terminal console.
    if False:
        console_handler = logging.StreamHandler()
        console_handler.setLevel(logging.DEBUG)
        console_handler.setFormatter(logging.Formatter('%(levelname)s:%(name)s: %(message)s'))
        logging.getLogger().addHandler(console_handler)

    # initialize the Qt system itself
    app = QtWidgets.QApplication(sys.argv)

    # create the main application controller
    main = MainApp()

    # run the event loop until the user is done
    log.info("Starting event loop.")
    sys.exit(app.exec_())

################################################################
# Main script follows.  This sequence is executed when the script is initiated from the command line.

if __name__ == "__main__":
    main()