umqtt.robust: Add sub example which can survive server restart.

Author: pfalcon

umqtt.robust/example_sub_robust.py

@@ -0,0 +1,41 @@
+import time
+from umqtt.robust import MQTTClient
+
+
+def sub_cb(topic, msg):
+    print((topic, msg))
+
+
+c = MQTTClient("umqtt_client", "localhost")
+# Print diagnostic messages when retries/reconnects happens
+c.DEBUG = True
+c.set_callback(sub_cb)
+# Connect to server, requesting not to clean session for this
+# client. If there was no existing session (False return value
+# from connect() method), we perform the initial setup of client
+# session - subscribe to needed topics. Afterwards, these
+# subscriptions will be stored server-side, and will be persistent,
+# (as we use clean_session=False).
+#
+# There can be a problem when a session for a given client exists,
+# but doesn't have subscriptions a particular application expects.
+# In this case, a session needs to be cleaned first. See
+# example_reset_session.py for an obvious way how to do that.
+#
+# In an actual application, it's up to its developer how to
+# manage these issues. One extreme is to have external "provisioning"
+# phase, where initial session setup, and any further management of
+# a session, is done by external tools. This allows to save resources
+# on a small embedded device. Another extreme is to have an application
+# to perform auto-setup (e.g., clean session, then re-create session
+# on each restart). This example shows mid-line between these 2
+# approaches, where initial setup of session is done by application,
+# but if anything goes wrong, there's an external tool to clean session.
+if not c.connect(clean_session=False):
+    print("New session being set up")
+    c.subscribe(b"foo_topic")
+
+while 1:
+    c.wait_msg()
+
+c.disconnect()