Add a disconnect callback for BlueZ

The callback will be called only on unsolicited disconnect event from
peer. An initiated disconnect from the host will not call it.

Other implementations (Core Bluetooth and .NET) will fail with a
NotImplementedError exception.

bleak/backends/bluezdbus/client.py

@@ -40,6 +40,8 @@ def __init__(self, address, loop=None, **kwargs):
         self._bus = None
         self._rules = {}
 
+        self._disconnected_callback = None
+
         self._char_path_to_uuid = {}
 
         # We need to know BlueZ version since battery level characteristic
@@ -51,6 +53,20 @@ def __init__(self, address, loop=None, **kwargs):
 
     # Connectivity methods
 
+    def set_disconnected_callback(
+            self, callback: Callable[[BaseBleakClient], None], **kwargs
+    ) -> None:
+        """Set the disconnected callback.
+        The callback will be called on DBus PropChanged event with
+        the 'Connected' key set to False.
+
+        Args:
+            callback: callback to be called on disconnection.
+
+        """
+
+        self._disconnected_callback = callback
+
     async def connect(self, **kwargs) -> bool:
         """Connect to the specified GATT server.
 
@@ -531,7 +547,17 @@ def _properties_changed_callback(self, message):
                 self._notification_callbacks[message.path](
                     message.path, message.body[1]
                 )
-
+        elif message.body[0] == defs.DEVICE_INTERFACE:
+            device_path = '/org/bluez/%s/dev_%s' % (self.device,
+                                        self.address.replace(':', '_'))
+            if message.path == device_path:
+                message_body_map = message.body[1]
+                if 'Connected' in message_body_map and \
+                   not message_body_map['Connected']:
+                    logger.debug("Device {} disconnected."
+                                 .format(self.address))
+                    if self._disconnected_callback is not None:
+                        self._disconnected_callback(self)
 
 def _data_notification_wrapper(func, char_map):
     @wraps(func)

bleak/backends/client.py

@@ -46,6 +46,30 @@ async def __aexit__(self, exc_type, exc_val, exc_tb):
 
     # Connectivity methods
 
+    @abc.abstractmethod
+    async def set_disconnected_callback(
+            self, callback: Callable[['BaseBleakClient'], None], **kwargs
+    ) -> None:
+        """Set the disconnect callback.
+        The callback will only be called on unsolicited disconnect event.
+
+        Callbacks must accept one input which is the client object itself.
+
+        .. code-block:: python
+
+            def callback(client):
+                print("Client with address {} got disconnected!".format(client.address))
+
+            client.set_disconnected_callback(callback)
+            client.connect()
+
+        Args:
+            callback: callback to be called on disconnection.
+
+        """
+
+        raise NotImplementedError()
+
     @abc.abstractmethod
     async def connect(self, **kwargs) -> bool:
         """Connect to the specified GATT server.