问UILocalNotification在后台10分钟后不会开火

EN

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        [application endBackgroundTask: bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    dispatch_async(dispatch_get_main_queue(), ^{
        while ([application backgroundTimeRemaining] > 1.0) {
            NSString *friend = [self checkForIncomingChat];
            if ([friend length]>0) {
                [[UIApplication sharedApplication] cancelAllLocalNotifications];
                UILocalNotification *localNotif = [[UILocalNotification alloc] init];
                if (localNotif) {
                    localNotif.alertBody = [NSString stringWithFormat: NSLocalizedString(@"%@", nil), friend];
                    localNotif.alertAction = NSLocalizedString(@"Receive", nil);
                    localNotif.soundName = @"alarmsound.caf";
                    localNotif.applicationIconBadgeNumber = 1;
                    [application presentLocalNotificationNow:localNotif];
                    friend = nil;
                }
            }
            sleep(1);
        }
        [application endBackgroundTask:self->bgTask];
        self->bgTask = UIBackgroundTaskInvalid;
    });
 }

 - (NSString *) checkForIncomingChat {

    NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
    NSString *incall = [prefs objectForKey:@"incall"];
    if ([incall length]>0) {
        [prefs setObject:@"" forKey:@"incall"];
        return incall;
    }
    return @"";
};