Detect whether devices connected.

def _detect_devices(self) -> None:
        '''Detect whether devices connected.'''
        devices_num = len(self.devices_list)
        if devices_num == 0:
            raise DeviceConnectionException(
                'No devices are connected. Please connect the device with USB or via WLAN and turn on the USB debugging option.')
        elif not self.device_sn and devices_num > 1:
            raise DeviceConnectionException(
                f"Multiple devices detected: {' | '.join(self.devices_list)}, please specify device serial number or host.")
        else:
            self.device_sn = self.devices_list[0]
        if self.get_state() == 'offline':
            raise DeviceConnectionException(
                'The device is offline. Please reconnect.')