解析微信服务器推送的 XML 消息

    :param xml: XML 消息
    :return: 解析成功返回对应的消息或事件，否则返回 ``UnknownMessage``

def parse_message(xml):
    """
    解析微信服务器推送的 XML 消息

    :param xml: XML 消息
    :return: 解析成功返回对应的消息或事件，否则返回 ``UnknownMessage``
    """
    if not xml:
        return
    message = xmltodict.parse(to_text(xml))['xml']
    message_type = message['MsgType'].lower()
    event_type = None
    if message_type == 'event' or message_type.startswith('device_'):
        if 'Event' in message:
            event_type = message['Event'].lower()
        # special event type for device_event
        if event_type is None and message_type.startswith('device_'):
            event_type = message_type
        elif message_type.startswith('device_'):
            event_type = 'device_{event}'.format(event=event_type)

        if event_type == 'subscribe' and message.get('EventKey'):
            event_key = message['EventKey']
            if event_key.startswith(('scanbarcode|', 'scanimage|')):
                event_type = 'subscribe_scan_product'
                message['Event'] = event_type
            elif event_key.startswith('qrscene_'):
                # Scan to subscribe with scene id event
                event_type = 'subscribe_scan'
                message['Event'] = event_type
                message['EventKey'] = event_key[len('qrscene_'):]
        message_class = EVENT_TYPES.get(event_type, UnknownMessage)
    else:
        message_class = MESSAGE_TYPES.get(message_type, UnknownMessage)
    return message_class(message)