1.- (void)applicationWillEnterForeground:(UIApplication *)application
//当前状态:UIApplicationStateBackground
2.- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo
//当前状态:UIApplicationStateInactive !! 已经不是后台了
3.- (void)applicationDidBecomeActive:(UIApplication *)application
//当前状态:UIApplicationStateActive
测试设备:ios6 和 ios9