private void GetSystemConnectedOrPairedDevices()
        {
            try
            {
                //heart rate
                var guid = Guid.Parse("0000180d-0000-1000-8000-00805f9b34fb");

                // SystemDevices = Adapter.GetSystemConnectedOrPairedDevices(new[] { guid }).Select(d => new DeviceListItemViewModel(d)).ToList();
                // remove the GUID filter for test
                // Avoid to loose already IDevice with a connection, otherwise you can't close it
                // Keep the reference of already known devices and drop all not in returned list.
                var pairedOrConnectedDeviceWithNullGatt = Adapter.GetSystemConnectedOrPairedDevices();
                SystemDevices.RemoveAll(sd => !pairedOrConnectedDeviceWithNullGatt.Any(p => p.Id == sd.Id));
                SystemDevices.AddRange(pairedOrConnectedDeviceWithNullGatt.Where(d => !SystemDevices.Any(sd => sd.Id == d.Id)).Select(d => new DeviceListItemViewModel(d)));
                RaisePropertyChanged(() => SystemDevices);
            }
            catch (Exception ex)
            {
                Trace.Message("Failed to retreive system connected devices. {0}", ex.Message);
            }
        }