Add connected ivar to MessageBus

fixes #74

dbus_next/aio/message_bus.py

@@ -110,6 +110,9 @@ class MessageBus(BaseMessageBus):
     :ivar unique_name: The unique name of the message bus connection. It will
         be :class:`None` until the message bus connects.
     :vartype unique_name: str
+    :ivar connected: True if this message bus is expected to be able to send
+        and receive messages.
+    :vartype connected: bool
     """
     def __init__(self,
                  bus_address: str = None,

dbus_next/glib/message_bus.py

@@ -139,6 +139,9 @@ class MessageBus(BaseMessageBus):
         :class:`AuthExternal <dbus_next.auth.AuthExternal>`.
     :type auth: :class:`Authenticator <dbus_next.auth.Authenticator>`
 
+    :ivar connected: True if this message bus is expected to be able to send
+        and receive messages.
+    :vartype connected: bool
     :ivar unique_name: The unique name of the message bus connection. It will
         be :class:`None` until the message bus connects.
     :vartype unique_name: str

dbus_next/message_bus.py

@@ -44,6 +44,9 @@ class BaseMessageBus:
     :ivar unique_name: The unique name of the message bus connection. It will
         be :class:`None` until the message bus connects.
     :vartype unique_name: str
+    :ivar connected: True if this message bus is expected to be able to send
+        and receive messages.
+    :vartype connected: bool
     """
     def __init__(self,
                  bus_address: Optional[str] = None,
@@ -82,6 +85,12 @@ def __init__(self,
 
         self._setup_socket()
 
+    @property
+    def connected(self):
+        if self.unique_name is None or self._disconnected or self._user_disconnect:
+            return False
+        return True
+
     def export(self, path: str, interface: ServiceInterface):
         """Export the service interface on this message bus to make it available
         to other clients.

test/test_disconnect.py

@@ -10,8 +10,10 @@
 async def test_bus_disconnect_before_reply(event_loop):
     '''In this test, the bus disconnects before the reply comes in. Make sure
     the caller receives a reply with the error instead of hanging.'''
-
-    bus = await MessageBus().connect()
+    bus = MessageBus()
+    assert not bus.connected
+    await bus.connect()
+    assert bus.connected
 
     ping = bus.call(
         Message(destination='org.freedesktop.DBus',
@@ -25,12 +27,16 @@ async def test_bus_disconnect_before_reply(event_loop):
         await ping
 
     assert bus._disconnected
+    assert not bus.connected
     assert (await bus.wait_for_disconnect()) is None
 
 
 @pytest.mark.asyncio
 async def test_unexpected_disconnect(event_loop):
-    bus = await MessageBus().connect()
+    bus = MessageBus()
+    assert not bus.connected
+    await bus.connect()
+    assert bus.connected
 
     ping = bus.call(
         Message(destination='org.freedesktop.DBus',
@@ -44,6 +50,7 @@ async def test_unexpected_disconnect(event_loop):
         await ping
 
     assert bus._disconnected
+    assert not bus.connected
 
     with pytest.raises(OSError):
         await bus.wait_for_disconnect()