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。