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: ^

            {

                [selfrequestServerHowManyUnreadMessages];

            }

            ]

           )

        {

            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 = [NSDatedateWithTimeIntervalSinceNow:15];

            alarm.timeZone = [NSTimeZonedefaultTimeZone];

            alarm.repeatInterval = 0;

            alarm.soundName = UILocalNotificationDefaultSoundName;

            alarm.alertBody = @"Time to request MOA2 Server!";

            [app scheduleLocalNotification:alarm];

        }

    }

    elseif([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。