Reconfigure MQTT binary_sensor component if discovery info is changed (#18169)

* Recreate component if discovery info is changed

* Update component instead of remove+add

* Set name and unique_id in __init__

* Update unit test

* Cleanup

* More cleanup

* Refactor according to review comments

* Change discovery_hash

* Review comments, add tests

* Fix handling of value_template

homeassistant/components/mqtt/subscription.py

@@ -0,0 +1,54 @@
+"""
+Helper to handle a set of topics to subscribe to.
+
+For more details about this component, please refer to the documentation at
+https://home-assistant.io/components/mqtt/
+"""
+import logging
+
+from homeassistant.components import mqtt
+from homeassistant.components.mqtt import DEFAULT_QOS
+from homeassistant.loader import bind_hass
+from homeassistant.helpers.typing import HomeAssistantType
+
+_LOGGER = logging.getLogger(__name__)
+
+
+@bind_hass
+async def async_subscribe_topics(hass: HomeAssistantType, sub_state: dict,
+                                 topics: dict):
+    """(Re)Subscribe to a set of MQTT topics.
+
+    State is kept in sub_state.
+    """
+    cur_state = sub_state if sub_state is not None else {}
+    sub_state = {}
+    for key in topics:
+        topic = topics[key].get('topic', None)
+        msg_callback = topics[key].get('msg_callback', None)
+        qos = topics[key].get('qos', DEFAULT_QOS)
+        encoding = topics[key].get('encoding', 'utf-8')
+        topic = (topic, msg_callback, qos, encoding)
+        (cur_topic, unsub) = cur_state.pop(
+            key, ((None, None, None, None), None))
+
+        if topic != cur_topic and topic[0] is not None:
+            if unsub is not None:
+                unsub()
+            unsub = await mqtt.async_subscribe(
+                hass, topic[0], topic[1], topic[2], topic[3])
+        sub_state[key] = (topic, unsub)
+
+    for key, (topic, unsub) in list(cur_state.items()):
+        if unsub is not None:
+            unsub()
+
+    return sub_state
+
+
+@bind_hass
+async def async_unsubscribe_topics(hass: HomeAssistantType, sub_state: dict):
+    """Unsubscribe from all MQTT topics managed by async_subscribe_topics."""
+    await async_subscribe_topics(hass, sub_state, {})
+
+    return sub_state