如果你的应用程序需要在设备休眠的时候还能够收到服务器端发送的消息，那我们就可以借助VOIP的模式来实现这一需求。但是如果的应用程序并不是正真的VOIP应用，那当你把你的应用提交到AppStore的时候基本上会被苹果Reject. 但是如果你的应用是企业内部发布的或者你只想了解其中的原理，那该文也许对您会有所帮助。
一、在iOS中如何应用VOIP
大多数VOIP应用需要设置后台audio 应用去传递音频，因此你应该设置audio 和voip两个键值。如果只是想通过VOIP来达到socket在休眠状态下维持长连接那只需要象上图一样设置VOIP即可。
接口
1、NSInputStream 和NSOutputStream
NSStreamNetworkServiceType
stream.
改属性的值设为
NSStreamNetworkServiceTypeVoIP.
2、NSURLRequest
method of your NSMutableURLRequest object to set the network service
type of the request. The service type should be set to
NSURLNetworkServiceTypeVoIP.
For Core Foundation streams, use the CFReadStreamSetProperty or
CFWriteStreamSetProperty function to add the kCFStreamNetwork-
ServiceType property to the stream. The value for this property should be
set to kCFStreamNetworkServiceTypeVoIP.
由于，VOIP应用程序需要一直运行以确保收到来电，所以如果程 序通过一个非零的exit code退出，系统将自动重启这个应用程序（这种退出方式可以发生在内存压力大时终止程序运行）。尽管如此，中断应用程序会release所有的
sockets，包括那个用于连接voip 服务的socket。因此，当程序运行时，它需要一直从头创建socket。
sockets，包括那个用于连接voip 服务的socket。因此，当程序运行时，它需要一直从头创建socket。
当你建立了handler之后，确定应用程序所需的最大超时。系统保证会在最大超时之前调用handler，但是这个时间是不确定的，所以你的handler必须在你申明的超时之前做好执行程序的准备。
二、通过GCDAsyncSocket来实现休眠状态下的长连接：
1、在创建socket并且成功连接上远程服务器的delegate中执行下述代码
- - (void)socket:(GCDAsyncSocket *)sock didConnectToHost:(NSString *)host port:(UInt16)connectedPort{
- [socket performBlock:^{
- [socket enableBackgroundingOnSocket];
- }];
- }
2、在ApplicationDidEnterBackground方法中调用setKeepAliveTimeOut以保持长连接
- BOOL backgroundAccepted = [[UIApplication sharedApplication] setKeepAliveTimeout:600 handler:^{ [self backgroundHandler]; }];
- if (backgroundAccepted)
- {
- NSLog(@"VOIP backgrounding accepted");
- }
参考：
1、XMPP中长连接问答
2、GCDAsyncSocket实现后台长连接的VOIP问答
3、后台运行（内容比较杂，但还是很有参考价值）
4、有一篇国内的关于VOIP以及后台程序的博文（内容还是比较杂－－还是英文的，估计只能由博主自己看懂，仅做参考）
http://blog.csdn.net/gnicky/article/details/7452418
5、iOS socket编程基础
6、一篇*.com中的问答，其中对iOS中对VOIP的解释比较详细
If
you want to let your VOIP application run in background , except those
base settings in plist file, you need a TCP socket who's property is set
to VOIP, than the iOS system will
take care this socket for you, when your application enter background ,
every thing was 'sleep' except that tcp socket. and if VOIP server send
some data thought that TCP socket, your application will be awake up
for 10 secs. during this time, you can post
a local notification.
Only
Tcp socket can be set as VOIP Socket. But from i know , mostly VOIP
application are based on UDP socket. if you do not want to separate the
control socket from the data socket.
you should create another tcp socket which is focus on 'awake' your
application , and from my personal experience , it's very hard to keep
this 'awake' signal and the real sip control signal synchronize, the
application always miss the sip invite request.
Tcp socket can be set as VOIP Socket. But from i know , mostly VOIP
application are based on UDP socket. if you do not want to separate the
control socket from the data socket.
you should create another tcp socket which is focus on 'awake' your
application , and from my personal experience , it's very hard to keep
this 'awake' signal and the real sip control signal synchronize, the
application always miss the sip invite request.
So,the
best way is separating the sip control single from the UDP data socket ,
make it as a tcp socket , this is the best solution , but never use tcp
socket to transfer voice data.
best way is separating the sip control single from the UDP data socket ,
make it as a tcp socket , this is the best solution , but never use tcp
socket to transfer voice data.
Another
dirty way: keep the application awake all the time. As i said , each
TCP single the application received thought that 'VOIP' tcp socket ,
will keep application awake for
10 seconds, so at the end of this duration(after 9 secs) , you can send
a response to the server to ask for another signal , when the next
signal arrived, the application will be awake again,after 9 secs , send
response again. keep doing this, your application
will awake forever.
dirty way: keep the application awake all the time. As i said , each
TCP single the application received thought that 'VOIP' tcp socket ,
will keep application awake for
10 seconds, so at the end of this duration(after 9 secs) , you can send
a response to the server to ask for another signal , when the next
signal arrived, the application will be awake again,after 9 secs , send
response again. keep doing this, your application
will awake forever.
7.CocoaAsyncSocket