iOS xmpp如何获取离线消息

看网上代码说开启流模块

        _storage = [XMPPStreamManagementMemoryStorage new];
        _xmppStreamManagement = [[XMPPStreamManagement alloc] initWithStorage:_storage];
        _xmppStreamManagement.autoResume = YES;
        [_xmppStreamManagement addDelegate:self delegateQueue:dispatch_get_main_queue()];
        [_xmppStreamManagement activate:self.xmppStream];

//登陆成功
- (void)xmppStreamDidAuthenticate:(XMPPStream *)sender {
    LRLog(@"登陆成功%s", __func__);
    //发送在线通知给服务器，服务器才会将离线消息推送过来
    XMPPPresence *presence = [XMPPPresence presence]; //默认"available"
    [self.xmppStream sendElement:presence];
    //启用流管理
    [_xmppStreamManagement enableStreamManagementWithResumption:YES maxTimeout:0];
}

我调试中没有效果，获取不到离线消息。