Added a mosquitto example.

examples/mosquitto/mosquitto.conf

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+user root

examples/mosquitto/publish.py

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+#!/usr/bin/env python3
+
+import argparse
+
+from paho.mqtt import client as mqtt_client
+
+
+DEFAULT_HOST = 'localhost'
+DEFAULT_PORT = 1883
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        'host', nargs='?', default=DEFAULT_HOST,
+        help=('The IP or hostname of the MQTT server. '
+              'Defaults to {}.'.format(DEFAULT_HOST)))
+    parser.add_argument(
+        'port', type=int, nargs='?', default=DEFAULT_PORT,
+        help=('The port of the MQTT server. '
+              'Defaults to {}.'.format(DEFAULT_PORT)))
+    parser.add_argument('topic', help=('The topic to publish to.'))
+    parser.add_argument('payload', help=('The payload to send to the topic.'))
+    args = parser.parse_args()
+
+    client = mqtt_client.Client()
+    client.on_connect = on_connect
+    client.on_message = on_message
+
+    client.connect(args.host, args.port)
+    try:
+        client.loop_start()
+        client.publish(args.topic, args.payload)
+    finally:
+        client.loop_stop()
+
+
+if __name__ == "__main__":
+    main()

examples/mosquitto/snapcraft.yaml

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+name: mosquitto
+version: 0.1
+summary: mosquitto server and client
+description: |
+  MQTT example with a server, a publisher and a subscriber.
+apps:
+  mosquitto:
+    command: usr/sbin/mosquitto -c $SNAP/mosquitto.conf
+    daemon: simple
+    caps:
+      - network-listener
+      - network-service
+  subscribe:
+    command: bin/subscribe
+    caps:
+      - network-listener
+    security-override:
+      read-paths:
+        - /etc/hosts.deny
+        - /etc/hosts.allow
+  publish:
+    command: bin/publish
+
+parts:
+    mosquitto:
+      plugin: copy
+      stage-packages: [mosquitto]
+      files:
+        mosquitto.conf: mosquitto.conf
+    mqtt-client:
+      plugin: copy
+      files:
+        subscribe.py: bin/subscribe
+        publish.py: bin/publish
+      after: [mqtt-paho-py3]

examples/mosquitto/subscribe.py

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+#!/usr/bin/env python3
+
+import argparse
+
+from paho.mqtt import client as mqtt_client
+
+
+DEFAULT_HOST = 'localhost'
+DEFAULT_PORT = 1883
+
+topic = ''
+
+
+def on_connect(client, userdata, unused1, unused2):
+    # Ignore the unused arguments.
+    del unused1, unused2
+    print('MQTT subscriber connected.')
+    client.subscribe(topic)
+
+
+def on_message(unused1, unused2, message):
+    # Ignore the unused arguments.
+    del unused1, unused2
+    print(message.topic + ' ' + str(message.payload))
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        'host', nargs='?', default=DEFAULT_HOST,
+        help=('The IP or hostname of the MQTT server. '
+              'Defaults to {}.'.format(DEFAULT_HOST)))
+    parser.add_argument(
+        'port', type=int, nargs='?', default=DEFAULT_PORT,
+        help=('The port of the MQTT server. '
+              'Defaults to {}.'.format(DEFAULT_PORT)))
+    parser.add_argument('topic', help=('The topic to subscribe to.'))
+    args = parser.parse_args()
+
+    client = mqtt_client.Client()
+    client.on_connect = on_connect
+    client.on_message = on_message
+
+    global topic
+    topic = args.topic
+    client.connect(args.host, args.port)
+    client.loop_forever()
+
+
+if __name__ == "__main__":
+    main()