后台提醒数字（应用程序applicationIcon上的数字)

@property(nonatomic) NSInteger applicationIconBadgeNumber;  // set to 0 to hide. default is 0. In iOS 8.0 and later, your application must register for user notifications using -[UIApplication registerUserNotificationSettings:] before being able to set the icon badge.

发现8.0以后，要先写-[UIApplication registerUserNotificationSettings:]这个方法

于是：

[[UIApplication sharedApplication] registerUserNotificationSettings:[UIUserNotificationSettings settingsForTypes:UIUserNotificationTypeBadge categories:nil]];

[UIApplication sharedApplication].applicationIconBadgeNumber = [responseObject[@"status"] intValue];

应用程序进入后台默认处于休眠状态，这样就无法及时读取未读消息数，那怎么办呢？

在后台开启任务让程序时刻保持运行状态：（能维持运行的时间是不确定的，