IOS在后台每隔一段时间执行一下

步骤：

1.在info.plist里加入UIBackgroundModes键，其值为数组，数组之一为voip字符串：

<key>UIBackgroundModes</key><array><string>voip</string></array>

2.在程序启动的时候调用- (void)setupBackgroundHandler函数，函数体如下：

#pragma mark - VoIP

- ( void )setupBackgroundHandler
{
    if ( UIUDeviceIsBackgroundSupported() )

        if (
           [[UIApplication sharedApplication] setKeepAliveTimeout:600 handler: ^
            {
                [ self  requestServerHowManyUnreadMessages];
            }
            ]
           )
        {
            UDLog(@ "Set Background handler successed!" );
        }
        else
        { //failed
            UDLog(@ "Set Background handler failed!" );
        }
    }
    else
    {
        UDLog(@ "This Deviece is not Background supported." );
    }
}

- ( void )requestServerHowManyUnreadMessages
{
    UIApplication* app = [UIApplication sharedApplication];

    if ([app applicationState] == UIApplicationStateBackground)
    {
        NSArray  * oldNotifications = [app scheduledLocalNotifications];
        if  ([oldNotifications count] > 0)
            [app cancelAllLocalNotifications];
        UILocalNotification* alarm = [[[UILocalNotification alloc] init] autorelease];
        if  (alarm)
        {
            alarm.fireDate = [ NSDate  dateWithTimeIntervalSinceNow:15];
            alarm.timeZone = [ NSTimeZone  defaultTimeZone];
            alarm.repeatInterval = 0;
            alarm.soundName = UILocalNotificationDefaultSoundName;
            alarm.alertBody = @ "Time to request MOA2 Server!" ;
            [app scheduleLocalNotification:alarm];
        }
    }
    else  if ([app applicationState] == UIApplicationStateActive)
    {
        UIAlertView *alertView =  [[[UIAlertView alloc] init] autorelease];
        [alertView setTitle:@ "alert" ];
        [alertView setMessage:@ "Time to request MOA2 Server!" ];
        [alertView addButtonWithTitle: NSLocalizedString (@ "cancel" , nil )];
        [alertView setDelegate: nil ];
        [alertView show];
    }
}

---

解说：

- (BOOL)setKeepAliveTimeout:(NSTimeInterval)timeout handler:(void (^)(void))keepAliveHandler

函数功能：app每隔timeout唤醒一次。

0.要成功调用该函数，就必须在Info.plist里设UIBackgroundModes键的array值之一voip字符串.

1.timeout必须>=600

2.唤醒app的时间间隔是不精准的。

3.唤醒后只有10秒执行时间。即handler里的代码要在10秒类执行完。10秒后app再次被阻塞。

（可以用-backgroundTimeRemaining属性来返回剩余时间）

4.该函数成功调用后，在程序生命周期内有效。

该函数的效果在回到前台的状况下,依然有效。（因此可以把它当timer使.） 

5.clearKeepAliveTimeout函数用来清除handler。