i3-companion: display notifications for Bluetooth

This is a good example of using PropertiesChanged. I don't know if
it's possible to filter the interface we want to receive.

bin/i3-companion

@@ -404,6 +404,37 @@ async def output_update(i3, event):
     output_update.running = asyncio.create_task(output_update_now())
 
 
+@on(
+    DBusSignal(
+        path="/org/bluez",
+        interface="org.freedesktop.DBus.Properties",
+        member="PropertiesChanged",
+        signature="sa{sv}as",
+    )
+)
+async def bluetooth_notifications(
+    i3, event, path, interface, changed, invalid
+):
+    """Display notifications related to Bluetooth state."""
+    if interface != "org.bluez.Device1":
+        return
+    if not "Connected" in changed:
+        return
+    logger.info(changed["Connected"])
+    peer = i3.system_bus["org.bluez"][path]
+    obd = await peer.get_async_interface(interface)
+    name = await obd.Name
+    icon = await obd.Icon
+    state = await obd.Connected
+    state = "connected" if state else "disconnected"
+    await notify(
+        i3,
+        app_icon=icon,
+        summary=name,
+        body=f"Bluetooth device {state}",
+    )
+
+
 @on(
     DBusSignal(
         path="/",
@@ -546,7 +577,7 @@ async def network_manager_status(i3, event, *args):
             if vpn:
                 state = await nma.State
                 if state == NM_ACTIVE_CONNECTION_STATE_ACTIVATED:
-                    status.append(icons['vpn'])
+                    status.append(icons["vpn"])
                     status.append(await nma.Id)
 
         # Final status line